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Subject[PATCH 1/2] x86/uaccess: Dont leak the AC flag into __put_user() argument evaluation
The __put_user() macro evaluates it's @ptr argument inside the
__uaccess_begin() / __uaccess_end() region. While this would normally
not be expected to be an issue, an UBSAN bug (it ignored -fwrapv,
fixed in GCC 8+) would transform the @ptr evaluation for:

drivers/gpu/drm/i915/i915_gem_execbuffer.c: if (unlikely(__put_user(offset, &urelocs[r-stack].presumed_offset))) {

into a signed-overflow-UB check and trigger the objtool AC validation.
Finish commit 2a418cf3f5f1 ("x86/uaccess: Don't leak the AC flag into
__put_user() value evaluation") and explicitly evaluate all 3 argument
early.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Fixes: 2a418cf3f5f1 ("x86/uaccess: Don't leak the AC flag into __put_user() value evaluation")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
arch/x86/include/asm/uaccess.h | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -427,10 +427,11 @@ do { \
({ \
__label__ __pu_label; \
int __pu_err = -EFAULT; \
- __typeof__(*(ptr)) __pu_val; \
- __pu_val = x; \
+ __typeof__(*(ptr)) __pu_val = (x); \
+ __typeof__(ptr) __pu_ptr = (ptr); \
+ __typeof__(size) __pu_size = (size); \
__uaccess_begin(); \
- __put_user_size(__pu_val, (ptr), (size), __pu_label); \
+ __put_user_size(__pu_val, __pu_ptr, __pu_size, __pu_label); \
__pu_err = 0; \
__pu_label: \
__uaccess_end(); \

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