Hook into vmalloc and vmap, and dynamically allocate real shadow
memory to back the mappings.
Most mappings in vmalloc space are small, requiring less than a full
page of shadow space. Allocating a full shadow page per mapping would
therefore be wasteful. Furthermore, to ensure that different mappings
use different shadow pages, mappings would have to be aligned to
KASAN_SHADOW_SCALE_SIZE * PAGE_SIZE.
Instead, share backing space across multiple mappings. Allocate
a backing page the first time a mapping in vmalloc space uses a
particular page of the shadow region. Keep this page around
regardless of whether the mapping is later freed - in the mean time
the page could have become shared by another vmalloc mapping.
This can in theory lead to unbounded memory growth, but the vmalloc
allocator is pretty good at reusing addresses, so the practical memory
usage grows at first but then stays fairly stable.
This requires architecture support to actually use: arches must stop
mapping the read-only zero page over portion of the shadow region that
covers the vmalloc space and instead leave it unmapped.
This allows KASAN with VMAP_STACK, and will be needed for architectures
that do not have a separate module space (e.g. powerpc64, which I am
currently working on). It also allows relaxing the module alignment
back to PAGE_SIZE.
Link: https://bugzilla.kernel.org/show_bug.cgi?id=202009
Acked-by: Vasily Gorbik <[email protected]>
Signed-off-by: Daniel Axtens <[email protected]>
[Mark: rework shadow allocation]
Signed-off-by: Mark Rutland <[email protected]>
--
v2: let kasan_unpoison_shadow deal with ranges that do not use a
full shadow byte.
v3: relax module alignment
rename to kasan_populate_vmalloc which is a much better name
deal with concurrency correctly
v4: Integrate Mark's rework
Poision pages on vfree
Handle allocation failures. I've tested this by inserting artificial
failures and using test_vmalloc to stress it. I haven't handled the
per-cpu case: it looked like it would require a messy hacking-up of
the function to deal with an OOM failure case in a debug feature.
---
Documentation/dev-tools/kasan.rst | 60 +++++++++++++++++++++++++++
include/linux/kasan.h | 24 +++++++++++
include/linux/moduleloader.h | 2 +-
include/linux/vmalloc.h | 12 ++++++
lib/Kconfig.kasan | 16 ++++++++
lib/test_kasan.c | 26 ++++++++++++
mm/kasan/common.c | 67 +++++++++++++++++++++++++++++++
mm/kasan/generic_report.c | 3 ++
mm/kasan/kasan.h | 1 +
mm/vmalloc.c | 28 ++++++++++++-
10 files changed, 237 insertions(+), 2 deletions(-)
diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
index b72d07d70239..35fda484a672 100644
--- a/Documentation/dev-tools/kasan.rst
+++ b/Documentation/dev-tools/kasan.rst
@@ -215,3 +215,63 @@ brk handler is used to print bug reports.
A potential expansion of this mode is a hardware tag-based mode, which would
use hardware memory tagging support instead of compiler instrumentation and
manual shadow memory manipulation.
+
+What memory accesses are sanitised by KASAN?
+--------------------------------------------
+
+The kernel maps memory in a number of different parts of the address
+space. This poses something of a problem for KASAN, which requires
+that all addresses accessed by instrumented code have a valid shadow
+region.
+
+The range of kernel virtual addresses is large: there is not enough
+real memory to support a real shadow region for every address that
+could be accessed by the kernel.
+
+By default
+~~~~~~~~~~
+
+By default, architectures only map real memory over the shadow region
+for the linear mapping (and potentially other small areas). For all
+other areas - such as vmalloc and vmemmap space - a single read-only
+page is mapped over the shadow area. This read-only shadow page
+declares all memory accesses as permitted.
+
+This presents a problem for modules: they do not live in the linear
+mapping, but in a dedicated module space. By hooking in to the module
+allocator, KASAN can temporarily map real shadow memory to cover
+them. This allows detection of invalid accesses to module globals, for
+example.
+
+This also creates an incompatibility with ``VMAP_STACK``: if the stack
+lives in vmalloc space, it will be shadowed by the read-only page, and
+the kernel will fault when trying to set up the shadow data for stack
+variables.
+
+CONFIG_KASAN_VMALLOC
+~~~~~~~~~~~~~~~~~~~~
+
+With ``CONFIG_KASAN_VMALLOC``, KASAN can cover vmalloc space at the
+cost of greater memory usage. Currently this is only supported on x86.
+
+This works by hooking into vmalloc and vmap, and dynamically
+allocating real shadow memory to back the mappings.
+
+Most mappings in vmalloc space are small, requiring less than a full
+page of shadow space. Allocating a full shadow page per mapping would
+therefore be wasteful. Furthermore, to ensure that different mappings
+use different shadow pages, mappings would have to be aligned to
+``KASAN_SHADOW_SCALE_SIZE * PAGE_SIZE``.
+
+Instead, we share backing space across multiple mappings. We allocate
+a backing page the first time a mapping in vmalloc space uses a
+particular page of the shadow region. We keep this page around
+regardless of whether the mapping is later freed - in the mean time
+this page could have become shared by another vmalloc mapping.
+
+This can in theory lead to unbounded memory growth, but the vmalloc
+allocator is pretty good at reusing addresses, so the practical memory
+usage grows at first but then stays fairly stable.
+
+This allows ``VMAP_STACK`` support on x86, and enables support of
+architectures that do not have a fixed module region.
diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index cc8a03cc9674..d666748cd378 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h
@@ -70,8 +70,18 @@ struct kasan_cache {
int free_meta_offset;
};
+/*
+ * These functions provide a special case to support backing module
+ * allocations with real shadow memory. With KASAN vmalloc, the special
+ * case is unnecessary, as the work is handled in the generic case.
+ */
+#ifndef CONFIG_KASAN_VMALLOC
int kasan_module_alloc(void *addr, size_t size);
void kasan_free_shadow(const struct vm_struct *vm);
+#else
+static inline int kasan_module_alloc(void *addr, size_t size) { return 0; }
+static inline void kasan_free_shadow(const struct vm_struct *vm) {}
+#endif
int kasan_add_zero_shadow(void *start, unsigned long size);
void kasan_remove_zero_shadow(void *start, unsigned long size);
@@ -194,4 +204,18 @@ static inline void *kasan_reset_tag(const void *addr)
#endif /* CONFIG_KASAN_SW_TAGS */
+#ifdef CONFIG_KASAN_VMALLOC
+int kasan_populate_vmalloc(unsigned long requested_size,
+ struct vm_struct *area);
+void kasan_free_vmalloc(void *start, unsigned long size);
+#else
+static inline int kasan_populate_vmalloc(unsigned long requested_size,
+ struct vm_struct *area)
+{
+ return 0;
+}
+
+static inline void kasan_free_vmalloc(void *start, unsigned long size) {}
+#endif
+
#endif /* LINUX_KASAN_H */
diff --git a/include/linux/moduleloader.h b/include/linux/moduleloader.h
index 5229c18025e9..ca92aea8a6bd 100644
--- a/include/linux/moduleloader.h
+++ b/include/linux/moduleloader.h
@@ -91,7 +91,7 @@ void module_arch_cleanup(struct module *mod);
/* Any cleanup before freeing mod->module_init */
void module_arch_freeing_init(struct module *mod);
-#ifdef CONFIG_KASAN
+#if defined(CONFIG_KASAN) && !defined(CONFIG_KASAN_VMALLOC)
#include <linux/kasan.h>
#define MODULE_ALIGN (PAGE_SIZE << KASAN_SHADOW_SCALE_SHIFT)
#else
diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
index 9b21d0047710..cdc7a60f7d81 100644
--- a/include/linux/vmalloc.h
+++ b/include/linux/vmalloc.h
@@ -21,6 +21,18 @@ struct notifier_block; /* in notifier.h */
#define VM_UNINITIALIZED 0x00000020 /* vm_struct is not fully initialized */
#define VM_NO_GUARD 0x00000040 /* don't add guard page */
#define VM_KASAN 0x00000080 /* has allocated kasan shadow memory */
+
+/*
+ * VM_KASAN is used slighly differently depending on CONFIG_KASAN_VMALLOC.
+ *
+ * If IS_ENABLED(CONFIG_KASAN_VMALLOC), VM_KASAN is set on a vm_struct after
+ * shadow memory has been mapped. It's used to handle allocation errors so that
+ * we don't try to poision shadow on free if it was never allocated.
+ *
+ * Otherwise, VM_KASAN is set for kasan_module_alloc() allocations and used to
+ * determine which allocations need the module shadow freed.
+ */
+
/*
* Memory with VM_FLUSH_RESET_PERMS cannot be freed in an interrupt or with
* vfree_atomic().
diff --git a/lib/Kconfig.kasan b/lib/Kconfig.kasan
index 4fafba1a923b..a320dc2e9317 100644
--- a/lib/Kconfig.kasan
+++ b/lib/Kconfig.kasan
@@ -6,6 +6,9 @@ config HAVE_ARCH_KASAN
config HAVE_ARCH_KASAN_SW_TAGS
bool
+config HAVE_ARCH_KASAN_VMALLOC
+ bool
+
config CC_HAS_KASAN_GENERIC
def_bool $(cc-option, -fsanitize=kernel-address)
@@ -135,6 +138,19 @@ config KASAN_S390_4_LEVEL_PAGING
to 3TB of RAM with KASan enabled). This options allows to force
4-level paging instead.
+config KASAN_VMALLOC
+ bool "Back mappings in vmalloc space with real shadow memory"
+ depends on KASAN && HAVE_ARCH_KASAN_VMALLOC
+ help
+ By default, the shadow region for vmalloc space is the read-only
+ zero page. This means that KASAN cannot detect errors involving
+ vmalloc space.
+
+ Enabling this option will hook in to vmap/vmalloc and back those
+ mappings with real shadow memory allocated on demand. This allows
+ for KASAN to detect more sorts of errors (and to support vmapped
+ stacks), but at the cost of higher memory usage.
+
config TEST_KASAN
tristate "Module for testing KASAN for bug detection"
depends on m && KASAN
diff --git a/lib/test_kasan.c b/lib/test_kasan.c
index b63b367a94e8..d375246f5f96 100644
--- a/lib/test_kasan.c
+++ b/lib/test_kasan.c
@@ -18,6 +18,7 @@
#include <linux/slab.h>
#include <linux/string.h>
#include <linux/uaccess.h>
+#include <linux/vmalloc.h>
/*
* Note: test functions are marked noinline so that their names appear in
@@ -709,6 +710,30 @@ static noinline void __init kmalloc_double_kzfree(void)
kzfree(ptr);
}
+#ifdef CONFIG_KASAN_VMALLOC
+static noinline void __init vmalloc_oob(void)
+{
+ void *area;
+
+ pr_info("vmalloc out-of-bounds\n");
+
+ /*
+ * We have to be careful not to hit the guard page.
+ * The MMU will catch that and crash us.
+ */
+ area = vmalloc(3000);
+ if (!area) {
+ pr_err("Allocation failed\n");
+ return;
+ }
+
+ ((volatile char *)area)[3100];
+ vfree(area);
+}
+#else
+static void __init vmalloc_oob(void) {}
+#endif
+
static int __init kmalloc_tests_init(void)
{
/*
@@ -752,6 +777,7 @@ static int __init kmalloc_tests_init(void)
kasan_strings();
kasan_bitops();
kmalloc_double_kzfree();
+ vmalloc_oob();
kasan_restore_multi_shot(multishot);
diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index 2277b82902d8..b8374e3773cf 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -568,6 +568,7 @@ void kasan_kfree_large(void *ptr, unsigned long ip)
/* The object will be poisoned by page_alloc. */
}
+#ifndef CONFIG_KASAN_VMALLOC
int kasan_module_alloc(void *addr, size_t size)
{
void *ret;
@@ -603,6 +604,7 @@ void kasan_free_shadow(const struct vm_struct *vm)
if (vm->flags & VM_KASAN)
vfree(kasan_mem_to_shadow(vm->addr));
}
+#endif
extern void __kasan_report(unsigned long addr, size_t size, bool is_write, unsigned long ip);
@@ -722,3 +724,68 @@ static int __init kasan_memhotplug_init(void)
core_initcall(kasan_memhotplug_init);
#endif
+
+#ifdef CONFIG_KASAN_VMALLOC
+static int kasan_populate_vmalloc_pte(pte_t *ptep, unsigned long addr,
+ void *unused)
+{
+ unsigned long page;
+ pte_t pte;
+
+ if (likely(!pte_none(*ptep)))
+ return 0;
+
+ page = __get_free_page(GFP_KERNEL);
+ if (!page)
+ return -ENOMEM;
+
+ memset((void *)page, KASAN_VMALLOC_INVALID, PAGE_SIZE);
+ pte = pfn_pte(PFN_DOWN(__pa(page)), PAGE_KERNEL);
+
+ /*
+ * Ensure poisoning is visible before the shadow is made visible
+ * to other CPUs.
+ */
+ smp_wmb();
+
+ spin_lock(&init_mm.page_table_lock);
+ if (likely(pte_none(*ptep))) {
+ set_pte_at(&init_mm, addr, ptep, pte);
+ page = 0;
+ }
+ spin_unlock(&init_mm.page_table_lock);
+ if (page)
+ free_page(page);
+ return 0;
+}
+
+int kasan_populate_vmalloc(unsigned long requested_size, struct vm_struct *area)
+{
+ unsigned long shadow_start, shadow_end;
+ int ret;
+
+ shadow_start = (unsigned long)kasan_mem_to_shadow(area->addr);
+ shadow_start = ALIGN_DOWN(shadow_start, PAGE_SIZE);
+ shadow_end = (unsigned long)kasan_mem_to_shadow(
+ area->addr + area->size);
+ shadow_end = ALIGN(shadow_end, PAGE_SIZE);
+
+ ret = apply_to_page_range(&init_mm, shadow_start,
+ shadow_end - shadow_start,
+ kasan_populate_vmalloc_pte, NULL);
+ if (ret)
+ return ret;
+
+ kasan_unpoison_shadow(area->addr, requested_size);
+
+ area->flags |= VM_KASAN;
+
+ return 0;
+}
+
+void kasan_free_vmalloc(void *start, unsigned long size)
+{
+ size = round_up(size, KASAN_SHADOW_SCALE_SIZE);
+ kasan_poison_shadow(start, size, KASAN_VMALLOC_INVALID);
+}
+#endif
diff --git a/mm/kasan/generic_report.c b/mm/kasan/generic_report.c
index 36c645939bc9..2d97efd4954f 100644
--- a/mm/kasan/generic_report.c
+++ b/mm/kasan/generic_report.c
@@ -86,6 +86,9 @@ static const char *get_shadow_bug_type(struct kasan_access_info *info)
case KASAN_ALLOCA_RIGHT:
bug_type = "alloca-out-of-bounds";
break;
+ case KASAN_VMALLOC_INVALID:
+ bug_type = "vmalloc-out-of-bounds";
+ break;
}
return bug_type;
diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
index 014f19e76247..8b1f2fbc780b 100644
--- a/mm/kasan/kasan.h
+++ b/mm/kasan/kasan.h
@@ -25,6 +25,7 @@
#endif
#define KASAN_GLOBAL_REDZONE 0xFA /* redzone for global variable */
+#define KASAN_VMALLOC_INVALID 0xF9 /* unallocated space in vmapped page */
/*
* Stack redzone shadow values
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 4fa8d84599b0..c20a7e663004 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2056,6 +2056,22 @@ static struct vm_struct *__get_vm_area_node(unsigned long size,
setup_vmalloc_vm(area, va, flags, caller);
+ /*
+ * For KASAN, if we are in vmalloc space, we need to cover the shadow
+ * area with real memory. If we come here through VM_ALLOC, this is
+ * done by a higher level function that has access to the true size,
+ * which might not be a full page.
+ *
+ * We assume module space comes via VM_ALLOC path.
+ */
+ if (is_vmalloc_addr(area->addr) && !(area->flags & VM_ALLOC)) {
+ if (kasan_populate_vmalloc(area->size, area)) {
+ unmap_vmap_area(va);
+ kfree(area);
+ return NULL;
+ }
+ }
+
return area;
}
@@ -2233,6 +2249,9 @@ static void __vunmap(const void *addr, int deallocate_pages)
debug_check_no_locks_freed(area->addr, get_vm_area_size(area));
debug_check_no_obj_freed(area->addr, get_vm_area_size(area));
+ if (area->flags & VM_KASAN)
+ kasan_free_vmalloc(area->addr, area->size);
+
vm_remove_mappings(area, deallocate_pages);
if (deallocate_pages) {
@@ -2483,6 +2502,9 @@ void *__vmalloc_node_range(unsigned long size, unsigned long align,
if (!addr)
return NULL;
+ if (kasan_populate_vmalloc(real_size, area))
+ return NULL;
+
/*
* In this function, newly allocated vm_struct has VM_UNINITIALIZED
* flag. It means that vm_struct is not fully initialized.
@@ -3324,10 +3346,14 @@ struct vm_struct **pcpu_get_vm_areas(const unsigned long *offsets,
spin_unlock(&vmap_area_lock);
/* insert all vm's */
- for (area = 0; area < nr_vms; area++)
+ for (area = 0; area < nr_vms; area++) {
setup_vmalloc_vm(vms[area], vas[area], VM_ALLOC,
pcpu_get_vm_areas);
+ /* assume success here */
+ kasan_populate_vmalloc(sizes[area], vms[area]);
+ }
+
kfree(vas);
return vms;
--
2.20.1
Le 15/08/2019 à 02:16, Daniel Axtens a écrit :
> Hook into vmalloc and vmap, and dynamically allocate real shadow
> memory to back the mappings.
>
> Most mappings in vmalloc space are small, requiring less than a full
> page of shadow space. Allocating a full shadow page per mapping would
> therefore be wasteful. Furthermore, to ensure that different mappings
> use different shadow pages, mappings would have to be aligned to
> KASAN_SHADOW_SCALE_SIZE * PAGE_SIZE.
>
> Instead, share backing space across multiple mappings. Allocate
> a backing page the first time a mapping in vmalloc space uses a
> particular page of the shadow region. Keep this page around
> regardless of whether the mapping is later freed - in the mean time
> the page could have become shared by another vmalloc mapping.
>
> This can in theory lead to unbounded memory growth, but the vmalloc
> allocator is pretty good at reusing addresses, so the practical memory
> usage grows at first but then stays fairly stable.
I guess people having gigabytes of memory don't mind, but I'm concerned
about tiny targets with very little amount of memory. I have boards with
as little as 32Mbytes of RAM. The shadow region for the linear space
already takes one eighth of the RAM. I'd rather avoid keeping unused
shadow pages busy.
Each page of shadow memory represent 8 pages of real memory. Could we
use page_ref to count how many pieces of a shadow page are used so that
we can free it when the ref count decreases to 0.
>
> This requires architecture support to actually use: arches must stop
> mapping the read-only zero page over portion of the shadow region that
> covers the vmalloc space and instead leave it unmapped.
Why 'must' ? Couldn't we switch back and forth from the zero page to
real page on demand ?
If the zero page is not mapped for unused vmalloc space, bad memory
accesses will Oops on the shadow memory access instead of Oopsing on the
real bad access, making it more difficult to locate and identify the issue.
>
> This allows KASAN with VMAP_STACK, and will be needed for architectures
> that do not have a separate module space (e.g. powerpc64, which I am
> currently working on). It also allows relaxing the module alignment
> back to PAGE_SIZE.
Why 'needed' ? powerpc32 doesn't have a separate module space and
doesn't need that.
>
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=202009
> Acked-by: Vasily Gorbik <[email protected]>
> Signed-off-by: Daniel Axtens <[email protected]>
> [Mark: rework shadow allocation]
> Signed-off-by: Mark Rutland <[email protected]>
>
> --
>
> v2: let kasan_unpoison_shadow deal with ranges that do not use a
> full shadow byte.
>
> v3: relax module alignment
> rename to kasan_populate_vmalloc which is a much better name
> deal with concurrency correctly
>
> v4: Integrate Mark's rework
> Poision pages on vfree
> Handle allocation failures. I've tested this by inserting artificial
> failures and using test_vmalloc to stress it. I haven't handled the
> per-cpu case: it looked like it would require a messy hacking-up of
> the function to deal with an OOM failure case in a debug feature.
>
> ---
> Documentation/dev-tools/kasan.rst | 60 +++++++++++++++++++++++++++
> include/linux/kasan.h | 24 +++++++++++
> include/linux/moduleloader.h | 2 +-
> include/linux/vmalloc.h | 12 ++++++
> lib/Kconfig.kasan | 16 ++++++++
> lib/test_kasan.c | 26 ++++++++++++
> mm/kasan/common.c | 67 +++++++++++++++++++++++++++++++
> mm/kasan/generic_report.c | 3 ++
> mm/kasan/kasan.h | 1 +
> mm/vmalloc.c | 28 ++++++++++++-
> 10 files changed, 237 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
> index b72d07d70239..35fda484a672 100644
> --- a/Documentation/dev-tools/kasan.rst
> +++ b/Documentation/dev-tools/kasan.rst
> @@ -215,3 +215,63 @@ brk handler is used to print bug reports.
> A potential expansion of this mode is a hardware tag-based mode, which would
> use hardware memory tagging support instead of compiler instrumentation and
> manual shadow memory manipulation.
> +
> +What memory accesses are sanitised by KASAN?
> +--------------------------------------------
> +
> +The kernel maps memory in a number of different parts of the address
> +space. This poses something of a problem for KASAN, which requires
> +that all addresses accessed by instrumented code have a valid shadow
> +region.
> +
> +The range of kernel virtual addresses is large: there is not enough
> +real memory to support a real shadow region for every address that
> +could be accessed by the kernel.
> +
> +By default
> +~~~~~~~~~~
> +
> +By default, architectures only map real memory over the shadow region
> +for the linear mapping (and potentially other small areas). For all
> +other areas - such as vmalloc and vmemmap space - a single read-only
> +page is mapped over the shadow area. This read-only shadow page
> +declares all memory accesses as permitted.
> +
> +This presents a problem for modules: they do not live in the linear
> +mapping, but in a dedicated module space. By hooking in to the module
> +allocator, KASAN can temporarily map real shadow memory to cover
> +them. This allows detection of invalid accesses to module globals, for
> +example.
> +
> +This also creates an incompatibility with ``VMAP_STACK``: if the stack
> +lives in vmalloc space, it will be shadowed by the read-only page, and
> +the kernel will fault when trying to set up the shadow data for stack
> +variables.
> +
> +CONFIG_KASAN_VMALLOC
> +~~~~~~~~~~~~~~~~~~~~
> +
> +With ``CONFIG_KASAN_VMALLOC``, KASAN can cover vmalloc space at the
> +cost of greater memory usage. Currently this is only supported on x86.
> +
> +This works by hooking into vmalloc and vmap, and dynamically
> +allocating real shadow memory to back the mappings.
> +
> +Most mappings in vmalloc space are small, requiring less than a full
> +page of shadow space. Allocating a full shadow page per mapping would
> +therefore be wasteful. Furthermore, to ensure that different mappings
> +use different shadow pages, mappings would have to be aligned to
> +``KASAN_SHADOW_SCALE_SIZE * PAGE_SIZE``.
> +
> +Instead, we share backing space across multiple mappings. We allocate
> +a backing page the first time a mapping in vmalloc space uses a
> +particular page of the shadow region. We keep this page around
> +regardless of whether the mapping is later freed - in the mean time
> +this page could have become shared by another vmalloc mapping.
> +
> +This can in theory lead to unbounded memory growth, but the vmalloc
> +allocator is pretty good at reusing addresses, so the practical memory
> +usage grows at first but then stays fairly stable.
> +
> +This allows ``VMAP_STACK`` support on x86, and enables support of
> +architectures that do not have a fixed module region.
That's wrong, powerpc32 doesn't have a fixed module region and is
already supported.
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index cc8a03cc9674..d666748cd378 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -70,8 +70,18 @@ struct kasan_cache {
> int free_meta_offset;
> };
>
> +/*
> + * These functions provide a special case to support backing module
> + * allocations with real shadow memory. With KASAN vmalloc, the special
> + * case is unnecessary, as the work is handled in the generic case.
> + */
> +#ifndef CONFIG_KASAN_VMALLOC
> int kasan_module_alloc(void *addr, size_t size);
> void kasan_free_shadow(const struct vm_struct *vm);
> +#else
> +static inline int kasan_module_alloc(void *addr, size_t size) { return 0; }
> +static inline void kasan_free_shadow(const struct vm_struct *vm) {}
> +#endif
>
> int kasan_add_zero_shadow(void *start, unsigned long size);
> void kasan_remove_zero_shadow(void *start, unsigned long size);
> @@ -194,4 +204,18 @@ static inline void *kasan_reset_tag(const void *addr)
>
> #endif /* CONFIG_KASAN_SW_TAGS */
>
> +#ifdef CONFIG_KASAN_VMALLOC
> +int kasan_populate_vmalloc(unsigned long requested_size,
> + struct vm_struct *area);
> +void kasan_free_vmalloc(void *start, unsigned long size);
> +#else
> +static inline int kasan_populate_vmalloc(unsigned long requested_size,
> + struct vm_struct *area)
> +{
> + return 0;
> +}
> +
> +static inline void kasan_free_vmalloc(void *start, unsigned long size) {}
> +#endif
> +
> #endif /* LINUX_KASAN_H */
> diff --git a/include/linux/moduleloader.h b/include/linux/moduleloader.h
> index 5229c18025e9..ca92aea8a6bd 100644
> --- a/include/linux/moduleloader.h
> +++ b/include/linux/moduleloader.h
> @@ -91,7 +91,7 @@ void module_arch_cleanup(struct module *mod);
> /* Any cleanup before freeing mod->module_init */
> void module_arch_freeing_init(struct module *mod);
>
> -#ifdef CONFIG_KASAN
> +#if defined(CONFIG_KASAN) && !defined(CONFIG_KASAN_VMALLOC)
> #include <linux/kasan.h>
> #define MODULE_ALIGN (PAGE_SIZE << KASAN_SHADOW_SCALE_SHIFT)
> #else
> diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> index 9b21d0047710..cdc7a60f7d81 100644
> --- a/include/linux/vmalloc.h
> +++ b/include/linux/vmalloc.h
> @@ -21,6 +21,18 @@ struct notifier_block; /* in notifier.h */
> #define VM_UNINITIALIZED 0x00000020 /* vm_struct is not fully initialized */
> #define VM_NO_GUARD 0x00000040 /* don't add guard page */
> #define VM_KASAN 0x00000080 /* has allocated kasan shadow memory */
> +
> +/*
> + * VM_KASAN is used slighly differently depending on CONFIG_KASAN_VMALLOC.
> + *
> + * If IS_ENABLED(CONFIG_KASAN_VMALLOC), VM_KASAN is set on a vm_struct after
> + * shadow memory has been mapped. It's used to handle allocation errors so that
> + * we don't try to poision shadow on free if it was never allocated.
> + *
> + * Otherwise, VM_KASAN is set for kasan_module_alloc() allocations and used to
> + * determine which allocations need the module shadow freed.
> + */
> +
> /*
> * Memory with VM_FLUSH_RESET_PERMS cannot be freed in an interrupt or with
> * vfree_atomic().
> diff --git a/lib/Kconfig.kasan b/lib/Kconfig.kasan
> index 4fafba1a923b..a320dc2e9317 100644
> --- a/lib/Kconfig.kasan
> +++ b/lib/Kconfig.kasan
> @@ -6,6 +6,9 @@ config HAVE_ARCH_KASAN
> config HAVE_ARCH_KASAN_SW_TAGS
> bool
>
> +config HAVE_ARCH_KASAN_VMALLOC
> + bool
> +
> config CC_HAS_KASAN_GENERIC
> def_bool $(cc-option, -fsanitize=kernel-address)
>
> @@ -135,6 +138,19 @@ config KASAN_S390_4_LEVEL_PAGING
> to 3TB of RAM with KASan enabled). This options allows to force
> 4-level paging instead.
>
> +config KASAN_VMALLOC
> + bool "Back mappings in vmalloc space with real shadow memory"
> + depends on KASAN && HAVE_ARCH_KASAN_VMALLOC
> + help
> + By default, the shadow region for vmalloc space is the read-only
> + zero page. This means that KASAN cannot detect errors involving
> + vmalloc space.
> +
> + Enabling this option will hook in to vmap/vmalloc and back those
> + mappings with real shadow memory allocated on demand. This allows
> + for KASAN to detect more sorts of errors (and to support vmapped
> + stacks), but at the cost of higher memory usage.
> +
> config TEST_KASAN
> tristate "Module for testing KASAN for bug detection"
> depends on m && KASAN
> diff --git a/lib/test_kasan.c b/lib/test_kasan.c
> index b63b367a94e8..d375246f5f96 100644
> --- a/lib/test_kasan.c
> +++ b/lib/test_kasan.c
Could we put the testing part in a separate patch ?
> @@ -18,6 +18,7 @@
> #include <linux/slab.h>
> #include <linux/string.h>
> #include <linux/uaccess.h>
> +#include <linux/vmalloc.h>
>
> /*
> * Note: test functions are marked noinline so that their names appear in
> @@ -709,6 +710,30 @@ static noinline void __init kmalloc_double_kzfree(void)
> kzfree(ptr);
> }
>
> +#ifdef CONFIG_KASAN_VMALLOC
> +static noinline void __init vmalloc_oob(void)
> +{
> + void *area;
> +
> + pr_info("vmalloc out-of-bounds\n");
> +
> + /*
> + * We have to be careful not to hit the guard page.
> + * The MMU will catch that and crash us.
> + */
> + area = vmalloc(3000);
> + if (!area) {
> + pr_err("Allocation failed\n");
> + return;
> + }
> +
> + ((volatile char *)area)[3100];
> + vfree(area);
> +}
> +#else
> +static void __init vmalloc_oob(void) {}
> +#endif
> +
> static int __init kmalloc_tests_init(void)
> {
> /*
> @@ -752,6 +777,7 @@ static int __init kmalloc_tests_init(void)
> kasan_strings();
> kasan_bitops();
> kmalloc_double_kzfree();
> + vmalloc_oob();
>
> kasan_restore_multi_shot(multishot);
>
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index 2277b82902d8..b8374e3773cf 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -568,6 +568,7 @@ void kasan_kfree_large(void *ptr, unsigned long ip)
> /* The object will be poisoned by page_alloc. */
> }
>
> +#ifndef CONFIG_KASAN_VMALLOC
> int kasan_module_alloc(void *addr, size_t size)
> {
> void *ret;
> @@ -603,6 +604,7 @@ void kasan_free_shadow(const struct vm_struct *vm)
> if (vm->flags & VM_KASAN)
> vfree(kasan_mem_to_shadow(vm->addr));
> }
> +#endif
>
> extern void __kasan_report(unsigned long addr, size_t size, bool is_write, unsigned long ip);
>
> @@ -722,3 +724,68 @@ static int __init kasan_memhotplug_init(void)
>
> core_initcall(kasan_memhotplug_init);
> #endif
> +
> +#ifdef CONFIG_KASAN_VMALLOC
> +static int kasan_populate_vmalloc_pte(pte_t *ptep, unsigned long addr,
> + void *unused)
> +{
> + unsigned long page;
> + pte_t pte;
> +
> + if (likely(!pte_none(*ptep)))
> + return 0;
Prior to this, the zero shadow area should be mapped, and the test
should be:
if (likely(pte_pfn(*ptep) != PHYS_PFN(__pa(kasan_early_shadow_page))))
return 0;
> +
> + page = __get_free_page(GFP_KERNEL);
> + if (!page)
> + return -ENOMEM;
> +
> + memset((void *)page, KASAN_VMALLOC_INVALID, PAGE_SIZE);
> + pte = pfn_pte(PFN_DOWN(__pa(page)), PAGE_KERNEL);
> +
> + /*
> + * Ensure poisoning is visible before the shadow is made visible
> + * to other CPUs.
> + */
> + smp_wmb();
> +
> + spin_lock(&init_mm.page_table_lock);
> + if (likely(pte_none(*ptep))) {
> + set_pte_at(&init_mm, addr, ptep, pte);
> + page = 0;
> + }
> + spin_unlock(&init_mm.page_table_lock);
> + if (page)
> + free_page(page);
> + return 0;
> +}
> +
> +int kasan_populate_vmalloc(unsigned long requested_size, struct vm_struct *area)
> +{
> + unsigned long shadow_start, shadow_end;
> + int ret;
> +
> + shadow_start = (unsigned long)kasan_mem_to_shadow(area->addr);
> + shadow_start = ALIGN_DOWN(shadow_start, PAGE_SIZE);
> + shadow_end = (unsigned long)kasan_mem_to_shadow(
> + area->addr + area->size);
> + shadow_end = ALIGN(shadow_end, PAGE_SIZE);
> +
> + ret = apply_to_page_range(&init_mm, shadow_start,
> + shadow_end - shadow_start,
> + kasan_populate_vmalloc_pte, NULL);
> + if (ret)
> + return ret;
> +
> + kasan_unpoison_shadow(area->addr, requested_size);
> +
> + area->flags |= VM_KASAN;
> +
> + return 0;
> +}
> +
> +void kasan_free_vmalloc(void *start, unsigned long size)
> +{
> + size = round_up(size, KASAN_SHADOW_SCALE_SIZE);
> + kasan_poison_shadow(start, size, KASAN_VMALLOC_INVALID);
> +}
> +#endif
> diff --git a/mm/kasan/generic_report.c b/mm/kasan/generic_report.c
> index 36c645939bc9..2d97efd4954f 100644
> --- a/mm/kasan/generic_report.c
> +++ b/mm/kasan/generic_report.c
> @@ -86,6 +86,9 @@ static const char *get_shadow_bug_type(struct kasan_access_info *info)
> case KASAN_ALLOCA_RIGHT:
> bug_type = "alloca-out-of-bounds";
> break;
> + case KASAN_VMALLOC_INVALID:
> + bug_type = "vmalloc-out-of-bounds";
> + break;
> }
>
> return bug_type;
> diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> index 014f19e76247..8b1f2fbc780b 100644
> --- a/mm/kasan/kasan.h
> +++ b/mm/kasan/kasan.h
> @@ -25,6 +25,7 @@
> #endif
>
> #define KASAN_GLOBAL_REDZONE 0xFA /* redzone for global variable */
> +#define KASAN_VMALLOC_INVALID 0xF9 /* unallocated space in vmapped page */
>
> /*
> * Stack redzone shadow values
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 4fa8d84599b0..c20a7e663004 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -2056,6 +2056,22 @@ static struct vm_struct *__get_vm_area_node(unsigned long size,
>
> setup_vmalloc_vm(area, va, flags, caller);
>
> + /*
> + * For KASAN, if we are in vmalloc space, we need to cover the shadow
> + * area with real memory. If we come here through VM_ALLOC, this is
> + * done by a higher level function that has access to the true size,
> + * which might not be a full page.
> + *
> + * We assume module space comes via VM_ALLOC path.
> + */
> + if (is_vmalloc_addr(area->addr) && !(area->flags & VM_ALLOC)) {
> + if (kasan_populate_vmalloc(area->size, area)) {
> + unmap_vmap_area(va);
> + kfree(area);
> + return NULL;
> + }
> + }
> +
> return area;
> }
>
> @@ -2233,6 +2249,9 @@ static void __vunmap(const void *addr, int deallocate_pages)
> debug_check_no_locks_freed(area->addr, get_vm_area_size(area));
> debug_check_no_obj_freed(area->addr, get_vm_area_size(area));
>
> + if (area->flags & VM_KASAN)
> + kasan_free_vmalloc(area->addr, area->size);
> +
> vm_remove_mappings(area, deallocate_pages);
>
> if (deallocate_pages) {
> @@ -2483,6 +2502,9 @@ void *__vmalloc_node_range(unsigned long size, unsigned long align,
> if (!addr)
> return NULL;
>
> + if (kasan_populate_vmalloc(real_size, area))
> + return NULL;
> +
> /*
> * In this function, newly allocated vm_struct has VM_UNINITIALIZED
> * flag. It means that vm_struct is not fully initialized.
> @@ -3324,10 +3346,14 @@ struct vm_struct **pcpu_get_vm_areas(const unsigned long *offsets,
> spin_unlock(&vmap_area_lock);
>
> /* insert all vm's */
> - for (area = 0; area < nr_vms; area++)
> + for (area = 0; area < nr_vms; area++) {
> setup_vmalloc_vm(vms[area], vas[area], VM_ALLOC,
> pcpu_get_vm_areas);
>
> + /* assume success here */
> + kasan_populate_vmalloc(sizes[area], vms[area]);
> + }
> +
> kfree(vas);
> return vms;
>
>
Christophe
Hi Christophe,
On Fri, Aug 16, 2019 at 09:47:00AM +0200, Christophe Leroy wrote:
> Le 15/08/2019 à 02:16, Daniel Axtens a écrit :
> > Hook into vmalloc and vmap, and dynamically allocate real shadow
> > memory to back the mappings.
> >
> > Most mappings in vmalloc space are small, requiring less than a full
> > page of shadow space. Allocating a full shadow page per mapping would
> > therefore be wasteful. Furthermore, to ensure that different mappings
> > use different shadow pages, mappings would have to be aligned to
> > KASAN_SHADOW_SCALE_SIZE * PAGE_SIZE.
> >
> > Instead, share backing space across multiple mappings. Allocate
> > a backing page the first time a mapping in vmalloc space uses a
> > particular page of the shadow region. Keep this page around
> > regardless of whether the mapping is later freed - in the mean time
> > the page could have become shared by another vmalloc mapping.
> >
> > This can in theory lead to unbounded memory growth, but the vmalloc
> > allocator is pretty good at reusing addresses, so the practical memory
> > usage grows at first but then stays fairly stable.
>
> I guess people having gigabytes of memory don't mind, but I'm concerned
> about tiny targets with very little amount of memory. I have boards with as
> little as 32Mbytes of RAM. The shadow region for the linear space already
> takes one eighth of the RAM. I'd rather avoid keeping unused shadow pages
> busy.
I think this depends on how much shadow would be in constant use vs what
would get left unused. If the amount in constant use is sufficiently
large (or the residue is sufficiently small), then it may not be
worthwhile to support KASAN_VMALLOC on such small systems.
> Each page of shadow memory represent 8 pages of real memory. Could we use
> page_ref to count how many pieces of a shadow page are used so that we can
> free it when the ref count decreases to 0.
>
> > This requires architecture support to actually use: arches must stop
> > mapping the read-only zero page over portion of the shadow region that
> > covers the vmalloc space and instead leave it unmapped.
>
> Why 'must' ? Couldn't we switch back and forth from the zero page to real
> page on demand ?
>
> If the zero page is not mapped for unused vmalloc space, bad memory accesses
> will Oops on the shadow memory access instead of Oopsing on the real bad
> access, making it more difficult to locate and identify the issue.
I agree this isn't nice, though FWIW this can already happen today for
bad addresses that fall outside of the usual kernel address space. We
could make the !KASAN_INLINE checks resilient to this by using
probe_kernel_read() to check the shadow, and treating unmapped shadow as
poison.
It's also worth noting that flipping back and forth isn't generally safe
unless going via an invalid table entry, so there'd still be windows
where a bad access might not have shadow mapped.
We'd need to reuse the common p4d/pud/pmd/pte tables for unallocated
regions, or the tables alone would consume significant amounts of memory
(e..g ~32GiB for arm64 defconfig), and thus we'd need to be able to
switch all levels between pgd and pte, which is much more complicated.
I strongly suspect that the additional complexity will outweigh the
benefit.
[...]
> > +#ifdef CONFIG_KASAN_VMALLOC
> > +static int kasan_populate_vmalloc_pte(pte_t *ptep, unsigned long addr,
> > + void *unused)
> > +{
> > + unsigned long page;
> > + pte_t pte;
> > +
> > + if (likely(!pte_none(*ptep)))
> > + return 0;
>
> Prior to this, the zero shadow area should be mapped, and the test should
> be:
>
> if (likely(pte_pfn(*ptep) != PHYS_PFN(__pa(kasan_early_shadow_page))))
> return 0;
As above, this would need a more comprehensive redesign, so I don't
think it's worth going into that level of nit here. :)
If we do try to use common shadow for unallocate VA ranges, it probably
makes sense to have a common poison page that we can use, so that we can
report vmalloc-out-of-bounfds.
Thanks,
Mark.
On Fri, Aug 16, 2019 at 10:08 AM Mark Rutland <[email protected]> wrote:
>
> Hi Christophe,
>
> On Fri, Aug 16, 2019 at 09:47:00AM +0200, Christophe Leroy wrote:
> > Le 15/08/2019 à 02:16, Daniel Axtens a écrit :
> > > Hook into vmalloc and vmap, and dynamically allocate real shadow
> > > memory to back the mappings.
> > >
> > > Most mappings in vmalloc space are small, requiring less than a full
> > > page of shadow space. Allocating a full shadow page per mapping would
> > > therefore be wasteful. Furthermore, to ensure that different mappings
> > > use different shadow pages, mappings would have to be aligned to
> > > KASAN_SHADOW_SCALE_SIZE * PAGE_SIZE.
> > >
> > > Instead, share backing space across multiple mappings. Allocate
> > > a backing page the first time a mapping in vmalloc space uses a
> > > particular page of the shadow region. Keep this page around
> > > regardless of whether the mapping is later freed - in the mean time
> > > the page could have become shared by another vmalloc mapping.
> > >
> > > This can in theory lead to unbounded memory growth, but the vmalloc
> > > allocator is pretty good at reusing addresses, so the practical memory
> > > usage grows at first but then stays fairly stable.
> >
> > I guess people having gigabytes of memory don't mind, but I'm concerned
> > about tiny targets with very little amount of memory. I have boards with as
> > little as 32Mbytes of RAM. The shadow region for the linear space already
> > takes one eighth of the RAM. I'd rather avoid keeping unused shadow pages
> > busy.
>
> I think this depends on how much shadow would be in constant use vs what
> would get left unused. If the amount in constant use is sufficiently
> large (or the residue is sufficiently small), then it may not be
> worthwhile to support KASAN_VMALLOC on such small systems.
>
> > Each page of shadow memory represent 8 pages of real memory. Could we use
> > page_ref to count how many pieces of a shadow page are used so that we can
> > free it when the ref count decreases to 0.
> >
> > > This requires architecture support to actually use: arches must stop
> > > mapping the read-only zero page over portion of the shadow region that
> > > covers the vmalloc space and instead leave it unmapped.
> >
> > Why 'must' ? Couldn't we switch back and forth from the zero page to real
> > page on demand ?
> >
> > If the zero page is not mapped for unused vmalloc space, bad memory accesses
> > will Oops on the shadow memory access instead of Oopsing on the real bad
> > access, making it more difficult to locate and identify the issue.
>
> I agree this isn't nice, though FWIW this can already happen today for
> bad addresses that fall outside of the usual kernel address space. We
> could make the !KASAN_INLINE checks resilient to this by using
> probe_kernel_read() to check the shadow, and treating unmapped shadow as
> poison.
Could we instead modify the page fault handlers to detect this case
and print a useful message?
>> > Instead, share backing space across multiple mappings. Allocate
>> > a backing page the first time a mapping in vmalloc space uses a
>> > particular page of the shadow region. Keep this page around
>> > regardless of whether the mapping is later freed - in the mean time
>> > the page could have become shared by another vmalloc mapping.
>> >
>> > This can in theory lead to unbounded memory growth, but the vmalloc
>> > allocator is pretty good at reusing addresses, so the practical memory
>> > usage grows at first but then stays fairly stable.
>>
>> I guess people having gigabytes of memory don't mind, but I'm concerned
>> about tiny targets with very little amount of memory. I have boards with as
>> little as 32Mbytes of RAM. The shadow region for the linear space already
>> takes one eighth of the RAM. I'd rather avoid keeping unused shadow pages
>> busy.
>
> I think this depends on how much shadow would be in constant use vs what
> would get left unused. If the amount in constant use is sufficiently
> large (or the residue is sufficiently small), then it may not be
> worthwhile to support KASAN_VMALLOC on such small systems.
I'm not unsympathetic to the cause of small-memory systems, but this is
useful as-is for x86, especially for VMAP_STACK. arm64 and s390 have
already been able to make use of it as well. So unless the design is
going to make it difficult to extend to small-memory systems - if it
bakes in concepts or APIs that are going to make things harder - I think
it might be worth merging as is. (pending the fixes for documentation
nits etc that you point out.)
>> Each page of shadow memory represent 8 pages of real memory. Could we use
>> page_ref to count how many pieces of a shadow page are used so that we can
>> free it when the ref count decreases to 0.
I'm not sure how much of a difference it will make, but I'll have a look.
>> > This requires architecture support to actually use: arches must stop
>> > mapping the read-only zero page over portion of the shadow region that
>> > covers the vmalloc space and instead leave it unmapped.
>>
>> Why 'must' ? Couldn't we switch back and forth from the zero page to real
>> page on demand ?
This code as currently written will not work if the architecture maps
the zero page over the portion of the shadow region that covers the
vmalloc space. So it's an implementation 'must' rather than a laws of
the universe 'must'.
We could perhaps map the zero page, but:
- you have to be really careful to get it right. If you accidentally
map the zero page onto memory where you shouldn't, you may permit
memory accesses that you should catch.
We could ameliorate this by taking Mark's suggestion and mapping a
poision page over the vmalloc space instead.
- I'm not sure what benefit is provided by having something mapped vs
leaving a hole, other than making the fault addresses more obvious.
- This gets complex, especially to do swapping correctly with respect
to various architectures' quirks (see e.g. 56eecdb912b5 "mm: Use
ptep/pmdp_set_numa() for updating _PAGE_NUMA bit" - ppc64 at least
requires that set_pte_at is never called on a valid PTE).
>> If the zero page is not mapped for unused vmalloc space, bad memory accesses
>> will Oops on the shadow memory access instead of Oopsing on the real bad
>> access, making it more difficult to locate and identify the issue.
I suppose. It's pretty easy on at least x86 and my draft ppc64
implementation to identify when an access falls into the shadow region
and then to reverse engineer the memory access that was being checked
based on the offset. As Andy points out, the fault handler could do this
automatically.
> I agree this isn't nice, though FWIW this can already happen today for
> bad addresses that fall outside of the usual kernel address space. We
> could make the !KASAN_INLINE checks resilient to this by using
> probe_kernel_read() to check the shadow, and treating unmapped shadow as
> poison.
>
> It's also worth noting that flipping back and forth isn't generally safe
> unless going via an invalid table entry, so there'd still be windows
> where a bad access might not have shadow mapped.
>
> We'd need to reuse the common p4d/pud/pmd/pte tables for unallocated
> regions, or the tables alone would consume significant amounts of memory
> (e..g ~32GiB for arm64 defconfig), and thus we'd need to be able to
> switch all levels between pgd and pte, which is much more complicated.
>
> I strongly suspect that the additional complexity will outweigh the
> benefit.
>
I'm not opposed to this in principle but I am also concerned about the
complexity involved.
Regards,
Daniel
> [...]
>
>> > +#ifdef CONFIG_KASAN_VMALLOC
>> > +static int kasan_populate_vmalloc_pte(pte_t *ptep, unsigned long addr,
>> > + void *unused)
>> > +{
>> > + unsigned long page;
>> > + pte_t pte;
>> > +
>> > + if (likely(!pte_none(*ptep)))
>> > + return 0;
>>
>> Prior to this, the zero shadow area should be mapped, and the test should
>> be:
>>
>> if (likely(pte_pfn(*ptep) != PHYS_PFN(__pa(kasan_early_shadow_page))))
>> return 0;
>
> As above, this would need a more comprehensive redesign, so I don't
> think it's worth going into that level of nit here. :)
>
> If we do try to use common shadow for unallocate VA ranges, it probably
> makes sense to have a common poison page that we can use, so that we can
> report vmalloc-out-of-bounfds.
>
> Thanks,
> Mark.
On Fri, Aug 16, 2019 at 10:41:00AM -0700, Andy Lutomirski wrote:
> On Fri, Aug 16, 2019 at 10:08 AM Mark Rutland <[email protected]> wrote:
> >
> > Hi Christophe,
> >
> > On Fri, Aug 16, 2019 at 09:47:00AM +0200, Christophe Leroy wrote:
> > > Le 15/08/2019 à 02:16, Daniel Axtens a écrit :
> > > > Hook into vmalloc and vmap, and dynamically allocate real shadow
> > > > memory to back the mappings.
> > > >
> > > > Most mappings in vmalloc space are small, requiring less than a full
> > > > page of shadow space. Allocating a full shadow page per mapping would
> > > > therefore be wasteful. Furthermore, to ensure that different mappings
> > > > use different shadow pages, mappings would have to be aligned to
> > > > KASAN_SHADOW_SCALE_SIZE * PAGE_SIZE.
> > > >
> > > > Instead, share backing space across multiple mappings. Allocate
> > > > a backing page the first time a mapping in vmalloc space uses a
> > > > particular page of the shadow region. Keep this page around
> > > > regardless of whether the mapping is later freed - in the mean time
> > > > the page could have become shared by another vmalloc mapping.
> > > >
> > > > This can in theory lead to unbounded memory growth, but the vmalloc
> > > > allocator is pretty good at reusing addresses, so the practical memory
> > > > usage grows at first but then stays fairly stable.
> > >
> > > I guess people having gigabytes of memory don't mind, but I'm concerned
> > > about tiny targets with very little amount of memory. I have boards with as
> > > little as 32Mbytes of RAM. The shadow region for the linear space already
> > > takes one eighth of the RAM. I'd rather avoid keeping unused shadow pages
> > > busy.
> >
> > I think this depends on how much shadow would be in constant use vs what
> > would get left unused. If the amount in constant use is sufficiently
> > large (or the residue is sufficiently small), then it may not be
> > worthwhile to support KASAN_VMALLOC on such small systems.
> >
> > > Each page of shadow memory represent 8 pages of real memory. Could we use
> > > page_ref to count how many pieces of a shadow page are used so that we can
> > > free it when the ref count decreases to 0.
> > >
> > > > This requires architecture support to actually use: arches must stop
> > > > mapping the read-only zero page over portion of the shadow region that
> > > > covers the vmalloc space and instead leave it unmapped.
> > >
> > > Why 'must' ? Couldn't we switch back and forth from the zero page to real
> > > page on demand ?
> > >
> > > If the zero page is not mapped for unused vmalloc space, bad memory accesses
> > > will Oops on the shadow memory access instead of Oopsing on the real bad
> > > access, making it more difficult to locate and identify the issue.
> >
> > I agree this isn't nice, though FWIW this can already happen today for
> > bad addresses that fall outside of the usual kernel address space. We
> > could make the !KASAN_INLINE checks resilient to this by using
> > probe_kernel_read() to check the shadow, and treating unmapped shadow as
> > poison.
>
> Could we instead modify the page fault handlers to detect this case
> and print a useful message?
In general we can't know if a bad access was a KASAN shadow lookup (e.g.
since the shadow of NULL falls outside of the shadow region), but we
could always print a message using kasan_shadow_to_mem() for any
unhandled fault to suggeest what the "real" address might have been.
Thanks,
Mark.
> On Aug 18, 2019, at 8:58 PM, Daniel Axtens <[email protected]> wrote:
>
>>> Each page of shadow memory represent 8 pages of real memory. Could we use
>>> page_ref to count how many pieces of a shadow page are used so that we can
>>> free it when the ref count decreases to 0.
>
> I'm not sure how much of a difference it will make, but I'll have a look.
>
There are a grand total of eight possible pages that could require a
given shadow page. I would suggest that, instead of reference
counting, you just check all eight pages.
Or, better yet, look at the actual vm_area_struct and are where prev
and next point. That should tell you exactly which range can be freed.