[tip: core/urgent] stack: Replace "o" output with "r" input constraint

[tip-bot2 for Nick Desaulniers]

The following commit has been merged into the core/urgent branch of tip:

Commit-ID:     2515dd6ce8e545b0b2eece84920048ef9ed846c4
Gitweb:        https://git.kernel.org/tip/2515dd6ce8e545b0b2eece84920048ef9ed846c4
Author:        Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
AuthorDate:    Mon, 19 Apr 2021 16:17:41 -07:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Tue, 11 May 2021 09:56:11 +02:00

stack: Replace "o" output with "r" input constraint

"o" isn't a common asm() constraint to use; it triggers an assertion in
assert-enabled builds of LLVM that it's not recognized when targeting
aarch64 (though it appears to fall back to "m"). It's fixed in LLVM 13 now,
but there isn't really a good reason to use "o" in particular here. To
avoid causing build issues for those using assert-enabled builds of earlier
LLVM versions, the constraint needs changing.

Instead, if the point is to retain the __builtin_alloca(), make ptr appear
to "escape" via being an input to an empty inline asm block. This is
preferable anyways, since otherwise this looks like a dead store.

While the use of "r" was considered in

  https://lore.kernel.org/lkml/202104011447.2E7F543@keescook/

it was only tested as an output (which looks like a dead store, and wasn't
sufficient).

Use "r" as an input constraint instead, which behaves correctly across
compilers and architectures.

Fixes: 39218ff4c625 ("stack: Optionally randomize kernel stack offset each syscall")
Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Tested-by: Kees Cook <keescook@xxxxxxxxxxxx>
Tested-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Link: https://reviews.llvm.org/D100412
Link: https://bugs.llvm.org/show_bug.cgi?id=49956
Link: https://lore.kernel.org/r/20210419231741.4084415-1-keescook@xxxxxxxxxxxx

---
 include/linux/randomize_kstack.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h
index fd80fab..bebc911 100644
--- a/include/linux/randomize_kstack.h
+++ b/include/linux/randomize_kstack.h
@@ -38,7 +38,7 @@ void *__builtin_alloca(size_t size);
 		u32 offset = raw_cpu_read(kstack_offset);		\
 		u8 *ptr = __builtin_alloca(KSTACK_OFFSET_MAX(offset));	\
 		/* Keep allocation even after "ptr" loses scope. */	\
-		asm volatile("" : "=o"(*ptr) :: "memory");		\
+		asm volatile("" :: "r"(ptr) : "memory");		\
 	}								\
 } while (0)