From: Jann Horn <[email protected]>
Currently the SLUB code represents encoded freelist entries as "void*".
That's misleading, those things are encoded under
CONFIG_SLAB_FREELIST_HARDENED so that they're not actually dereferencable.
Give them their own type, and split freelist_ptr() into one function per
direction (one for encoding, one for decoding).
Signed-off-by: Jann Horn <[email protected]>
Co-developed-by: Matteo Rizzo <[email protected]>
Signed-off-by: Matteo Rizzo <[email protected]>
---
v2:
* Fix compilation error with SLUB_TINY
* Move the freeptr_t typedef to slub.c
mm/slub.c | 43 ++++++++++++++++++++++++++++++++-----------
1 file changed, 32 insertions(+), 11 deletions(-)
diff --git a/mm/slub.c b/mm/slub.c
index e3b5d5c0eb3a..f8cc47eff742 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -360,13 +360,19 @@ static struct workqueue_struct *flushwq;
* Core slab cache functions
*******************************************************************/
+/*
+ * freeptr_t represents a SLUB freelist pointer, which might be encoded
+ * and not dereferenceable if CONFIG_SLAB_FREELIST_HARDENED is enabled.
+ */
+typedef struct { unsigned long v; } freeptr_t;
+
/*
* Returns freelist pointer (ptr). With hardening, this is obfuscated
* with an XOR of the address where the pointer is held and a per-cache
* random number.
*/
-static inline void *freelist_ptr(const struct kmem_cache *s, void *ptr,
- unsigned long ptr_addr)
+static inline freeptr_t freelist_ptr_encode(const struct kmem_cache *s,
+ void *ptr, unsigned long ptr_addr)
{
#ifdef CONFIG_SLAB_FREELIST_HARDENED
/*
@@ -379,25 +385,40 @@ static inline void *freelist_ptr(const struct kmem_cache *s, void *ptr,
* calls get_freepointer() with an untagged pointer, which causes the
* freepointer to be restored incorrectly.
*/
- return (void *)((unsigned long)ptr ^ s->random ^
- swab((unsigned long)kasan_reset_tag((void *)ptr_addr)));
+ return (freeptr_t){.v = (unsigned long)ptr ^ s->random ^
+ swab((unsigned long)kasan_reset_tag((void *)ptr_addr))};
#else
- return ptr;
+ return (freeptr_t){.v = (unsigned long)ptr};
#endif
}
+static inline void *freelist_ptr_decode(const struct kmem_cache *s,
+ freeptr_t ptr, unsigned long ptr_addr)
+{
+ void *decoded;
+
+#ifdef CONFIG_SLAB_FREELIST_HARDENED
+ /* See the comment in freelist_ptr_encode */
+ decoded = (void *)(ptr.v ^ s->random ^
+ swab((unsigned long)kasan_reset_tag((void *)ptr_addr)));
+#else
+ decoded = (void *)ptr.v;
+#endif
+ return decoded;
+}
+
/* Returns the freelist pointer recorded at location ptr_addr. */
static inline void *freelist_dereference(const struct kmem_cache *s,
void *ptr_addr)
{
- return freelist_ptr(s, (void *)*(unsigned long *)(ptr_addr),
+ return freelist_ptr_decode(s, *(freeptr_t *)(ptr_addr),
(unsigned long)ptr_addr);
}
static inline void *get_freepointer(struct kmem_cache *s, void *object)
{
object = kasan_reset_tag(object);
- return freelist_dereference(s, object + s->offset);
+ return freelist_dereference(s, (freeptr_t *)(object + s->offset));
}
#ifndef CONFIG_SLUB_TINY
@@ -421,15 +442,15 @@ __no_kmsan_checks
static inline void *get_freepointer_safe(struct kmem_cache *s, void *object)
{
unsigned long freepointer_addr;
- void *p;
+ freeptr_t p;
if (!debug_pagealloc_enabled_static())
return get_freepointer(s, object);
object = kasan_reset_tag(object);
freepointer_addr = (unsigned long)object + s->offset;
- copy_from_kernel_nofault(&p, (void **)freepointer_addr, sizeof(p));
- return freelist_ptr(s, p, freepointer_addr);
+ copy_from_kernel_nofault(&p, (freeptr_t *)freepointer_addr, sizeof(p));
+ return freelist_ptr_decode(s, p, freepointer_addr);
}
static inline void set_freepointer(struct kmem_cache *s, void *object, void *fp)
@@ -441,7 +462,7 @@ static inline void set_freepointer(struct kmem_cache *s, void *object, void *fp)
#endif
freeptr_addr = (unsigned long)kasan_reset_tag((void *)freeptr_addr);
- *(void **)freeptr_addr = freelist_ptr(s, fp, freeptr_addr);
+ *(freeptr_t *)freeptr_addr = freelist_ptr_encode(s, fp, freeptr_addr);
}
/* Loop over all objects in a slab */
base-commit: 24be4d0b46bb0c3c1dc7bacd30957d6144a70dfc
--
2.41.0.255.g8b1d071c50-goog
On Tue, 4 Jul 2023, Matteo Rizzo wrote:
> From: Jann Horn <[email protected]>
>
> Currently the SLUB code represents encoded freelist entries as "void*".
> That's misleading, those things are encoded under
> CONFIG_SLAB_FREELIST_HARDENED so that they're not actually dereferencable.
>
> Give them their own type, and split freelist_ptr() into one function per
> direction (one for encoding, one for decoding).
>
> Signed-off-by: Jann Horn <[email protected]>
> Co-developed-by: Matteo Rizzo <[email protected]>
> Signed-off-by: Matteo Rizzo <[email protected]>
Acked-by: David Rientjes <[email protected]>
On 7/4/23 15:58, Matteo Rizzo wrote:
> From: Jann Horn <[email protected]>
>
> Currently the SLUB code represents encoded freelist entries as "void*".
> That's misleading, those things are encoded under
> CONFIG_SLAB_FREELIST_HARDENED so that they're not actually dereferencable.
>
> Give them their own type, and split freelist_ptr() into one function per
> direction (one for encoding, one for decoding).
>
> Signed-off-by: Jann Horn <[email protected]>
> Co-developed-by: Matteo Rizzo <[email protected]>
> Signed-off-by: Matteo Rizzo <[email protected]>
Thanks, merged:
https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/slab.git/log/?h=slab/for-6.6/cleanup
While reviewing this I think more cleanups are possible but I will post them
on top as it's better for bisection purposes than folding anyway, and should
involve kasan folks.