Commit 6ae86561 authored Apr 24, 2019 by Peter Zijlstra Committed by Ingo Molnar Apr 24, 2019

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 this commit:

  2a418cf3 ("x86/uaccess: Don't leak the AC flag into __put_user() value evaluation")

and explicitly evaluate all 3 arguments early.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: luto@kernel.org
Fixes: 2a418cf3 ("x86/uaccess: Don't leak the AC flag into __put_user() value evaluation")
Link: http://lkml.kernel.org/r/20190424072208.695962771@infradead.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 64604d54

arch/x86/include/asm/uaccess.h

+4 −3

Original line number	Diff line number	Diff line
		@@ -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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