linux-kernel - [PATCH 1/2] x86/uaccess: Dont leak the AC flag into __put

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Message-Id: <20190424072208.695962771@infradead.org>
Date:   Wed, 24 Apr 2019 09:19:24 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     mingo@...nel.org, tglx@...utronix.de, x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, luto@...nel.org,
        peterz@...radead.org, Randy Dunlap <rdunlap@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
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@...radead.org>
Acked-by: Randy Dunlap <rdunlap@...radead.org> # build-tested
Acked-by: Linus Torvalds <torvalds@...ux-foundation.org>
Fixes: 2a418cf3f5f1 ("x86/uaccess: Don't leak the AC flag into __put_user() value evaluation")
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.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();					\