Hi,
This is a refresh of my earlier attempt to fix READ_IMPLIES_EXEC. I think
it incorporates the feedback from v2, and I've now added a selftest. This
series is for x86, arm, and arm64; I'd like it to go via -tip, though,
just to keep this change together with the selftest. To that end, I'd like
to collect Acks from the respective architecture maintainers. (Note that
most other architectures don't suffer from this problem. e.g. powerpc's
behavior appears to already be correct. MIPS may need adjusting but the
history of CPU features and toolchain behavior is very unclear to me.)
Repeating the commit log from later in the series:
The READ_IMPLIES_EXEC work-around was designed for old toolchains that
lacked the ELF PT_GNU_STACK marking under the assumption that toolchains
that couldn't specify executable permission flags for the stack may not
know how to do it correctly for any memory region.
This logic is sensible for having ancient binaries coexist in a system
with possibly NX memory, but was implemented in a way that equated having
a PT_GNU_STACK marked executable as being as "broken" as lacking the
PT_GNU_STACK marking entirely. Things like unmarked assembly and stack
trampolines may cause PT_GNU_STACK to need an executable bit, but they
do not imply all mappings must be executable.
This confusion has led to situations where modern programs with explicitly
marked executable stack are forced into the READ_IMPLIES_EXEC state when
no such thing is needed. (And leads to unexpected failures when mmap()ing
regions of device driver memory that wish to disallow VM_EXEC[1].)
In looking for other reasons for the READ_IMPLIES_EXEC behavior, Jann
Horn noted that glibc thread stacks have always been marked RWX (until
2003 when they started tracking the PT_GNU_STACK flag instead[2]). And
musl doesn't support executable stacks at all[3]. As such, no breakage
for multithreaded applications is expected from this change.
[1] https://lkml.kernel.org/r/[email protected]
[2] https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=54ee14b3882
[3] https://lkml.kernel.org/r/[email protected]
-Kees
v3:
- split steps in to distinct patches
- include arm32 and arm64/compat
- add selftests to validate behavior
v2: https://lore.kernel.org/lkml/20190424203408.GA11386@beast/
v1: https://lore.kernel.org/lkml/20190423181210.GA2443@beast/
Kees Cook (7):
x86/elf: Add table to document READ_IMPLIES_EXEC
x86/elf: Split READ_IMPLIES_EXEC from executable GNU_STACK
x86/elf: Disable automatic READ_IMPLIES_EXEC for 64-bit address spaces
arm32/64, elf: Add tables to document READ_IMPLIES_EXEC
arm32/64, elf: Split READ_IMPLIES_EXEC from executable GNU_STACK
arm64, elf: Disable automatic READ_IMPLIES_EXEC for 64-bit address
spaces
selftests/exec: Add READ_IMPLIES_EXEC tests
arch/arm/kernel/elf.c | 27 +++-
arch/arm64/include/asm/elf.h | 23 +++-
arch/x86/include/asm/elf.h | 22 +++-
fs/compat_binfmt_elf.c | 5 +
tools/testing/selftests/exec/Makefile | 42 +++++-
.../selftests/exec/read_implies_exec.c | 121 ++++++++++++++++++
.../selftests/exec/strip-gnu-stack-bits.c | 34 +++++
.../testing/selftests/exec/strip-gnu-stack.c | 69 ++++++++++
8 files changed, 336 insertions(+), 7 deletions(-)
create mode 100644 tools/testing/selftests/exec/read_implies_exec.c
create mode 100644 tools/testing/selftests/exec/strip-gnu-stack-bits.c
create mode 100644 tools/testing/selftests/exec/strip-gnu-stack.c
--
2.20.1
Add tables to document the current behavior of READ_IMPLIES_EXEC in
preparation for changing the behavior for both arm64 and arm.
Signed-off-by: Kees Cook <[email protected]>
---
arch/arm/kernel/elf.c | 24 +++++++++++++++++++++---
arch/arm64/include/asm/elf.h | 20 ++++++++++++++++++++
2 files changed, 41 insertions(+), 3 deletions(-)
diff --git a/arch/arm/kernel/elf.c b/arch/arm/kernel/elf.c
index 182422981386..2f69cf978fe3 100644
--- a/arch/arm/kernel/elf.c
+++ b/arch/arm/kernel/elf.c
@@ -78,9 +78,27 @@ void elf_set_personality(const struct elf32_hdr *x)
EXPORT_SYMBOL(elf_set_personality);
/*
- * Set READ_IMPLIES_EXEC if:
- * - the binary requires an executable stack
- * - we're running on a CPU which doesn't support NX.
+ * An executable for which elf_read_implies_exec() returns TRUE will
+ * have the READ_IMPLIES_EXEC personality flag set automatically.
+ *
+ * The decision process for determining the results are:
+ *
+ * CPU: | lacks NX* | has NX |
+ * ELF: | | |
+ * -------------------------------|------------|
+ * missing GNU_STACK | exec-all | exec-all |
+ * GNU_STACK == RWX | exec-all | exec-all |
+ * GNU_STACK == RW | exec-all | exec-none |
+ *
+ * exec-all : all PROT_READ user mappings are executable, except when
+ * backed by files on a noexec-filesystem.
+ * exec-none : only PROT_EXEC user mappings are executable.
+ *
+ * *this column has no architectural effect: NX markings are ignored by
+ * hardware, but may have behavioral effects when "wants X" collides with
+ * "cannot be X" constraints in memory permission flags, as in
+ * https://lkml.kernel.org/r/[email protected]
+ *
*/
int arm_elf_read_implies_exec(int executable_stack)
{
diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
index b618017205a3..7fc779e3f1ec 100644
--- a/arch/arm64/include/asm/elf.h
+++ b/arch/arm64/include/asm/elf.h
@@ -96,6 +96,26 @@
*/
#define elf_check_arch(x) ((x)->e_machine == EM_AARCH64)
+/*
+ * An executable for which elf_read_implies_exec() returns TRUE will
+ * have the READ_IMPLIES_EXEC personality flag set automatically.
+ *
+ * The decision process for determining the results are:
+ *
+ * CPU*: | arm32 | arm64 |
+ * ELF: | | |
+ * -------------------------------|------------|
+ * missing GNU_STACK | exec-all | exec-all |
+ * GNU_STACK == RWX | exec-all | exec-all |
+ * GNU_STACK == RW | exec-none | exec-none |
+ *
+ * exec-all : all PROT_READ user mappings are executable, except when
+ * backed by files on a noexec-filesystem.
+ * exec-none : only PROT_EXEC user mappings are executable.
+ *
+ * *all arm64 CPUs support NX, so there is no "lacks NX" column.
+ *
+ */
#define elf_read_implies_exec(ex,stk) (stk != EXSTACK_DISABLE_X)
#define CORE_DUMP_USE_REGSET
--
2.20.1
With modern x86 64-bit environments, there should never be a need for
automatic READ_IMPLIES_EXEC, as the architecture is intended to always
be execute-bit aware (as in, the default memory protection should be NX
unless a region explicitly requests to be executable).
There were very old x86_64 systems that lacked the NX bit, but for those,
the NX bit is, obviously, unenforceable, so these changes should have
no impact on them.
Suggested-by: Hector Marco-Gisbert <[email protected]>
Signed-off-by: Kees Cook <[email protected]>
---
arch/x86/include/asm/elf.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index a7035065377c..c9b7be0bcad3 100644
--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -287,7 +287,7 @@ extern u32 elf_hwcap2;
* CPU: | lacks NX* | has NX, ia32 | has NX, x86_64 |
* ELF: | | | |
* -------------------------------|------------------|----------------|
- * missing GNU_STACK | exec-all | exec-all | exec-all |
+ * missing GNU_STACK | exec-all | exec-all | exec-none |
* GNU_STACK == RWX | exec-stack | exec-stack | exec-stack |
* GNU_STACK == RW | exec-none | exec-none | exec-none |
*
@@ -303,7 +303,7 @@ extern u32 elf_hwcap2;
*
*/
#define elf_read_implies_exec(ex, executable_stack) \
- (executable_stack == EXSTACK_DEFAULT)
+ (mmap_is_ia32() && executable_stack == EXSTACK_DEFAULT)
struct task_struct;
--
2.20.1
On Mon, Feb 10, 2020 at 11:30:42AM -0800, Kees Cook wrote:
> Hi,
>
> This is a refresh of my earlier attempt to fix READ_IMPLIES_EXEC. I think
> it incorporates the feedback from v2, and I've now added a selftest. This
> series is for x86, arm, and arm64; I'd like it to go via -tip, though,
> just to keep this change together with the selftest. To that end, I'd like
> to collect Acks from the respective architecture maintainers. (Note that
> most other architectures don't suffer from this problem. e.g. powerpc's
> behavior appears to already be correct. MIPS may need adjusting but the
> history of CPU features and toolchain behavior is very unclear to me.)
>
> Repeating the commit log from later in the series:
>
>
> The READ_IMPLIES_EXEC work-around was designed for old toolchains that
> lacked the ELF PT_GNU_STACK marking under the assumption that toolchains
> that couldn't specify executable permission flags for the stack may not
> know how to do it correctly for any memory region.
>
> This logic is sensible for having ancient binaries coexist in a system
> with possibly NX memory, but was implemented in a way that equated having
> a PT_GNU_STACK marked executable as being as "broken" as lacking the
> PT_GNU_STACK marking entirely. Things like unmarked assembly and stack
> trampolines may cause PT_GNU_STACK to need an executable bit, but they
> do not imply all mappings must be executable.
>
> This confusion has led to situations where modern programs with explicitly
> marked executable stack are forced into the READ_IMPLIES_EXEC state when
> no such thing is needed. (And leads to unexpected failures when mmap()ing
> regions of device driver memory that wish to disallow VM_EXEC[1].)
>
> In looking for other reasons for the READ_IMPLIES_EXEC behavior, Jann
> Horn noted that glibc thread stacks have always been marked RWX (until
> 2003 when they started tracking the PT_GNU_STACK flag instead[2]). And
> musl doesn't support executable stacks at all[3]. As such, no breakage
> for multithreaded applications is expected from this change.
>
> [1] https://lkml.kernel.org/r/[email protected]
> [2] https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=54ee14b3882
> [3] https://lkml.kernel.org/r/[email protected]
I'm happy to see this, I think it will help the situation.
While I'm not well versed in all the historical details, the general
approach makes sense to me and I've looked through the patches.
I would like to follow this up with a patch to again block VM_EXEC
from the RDMA related mmap of BAR paths..
Reviewed-by: Jason Gunthorpe <[email protected]>
Jason
On Mon, Feb 10, 2020 at 11:30:46AM -0800, Kees Cook wrote:
> Add tables to document the current behavior of READ_IMPLIES_EXEC in
> preparation for changing the behavior for both arm64 and arm.
>
> Signed-off-by: Kees Cook <[email protected]>
Reviewed-by: Catalin Marinas <[email protected]>