linux-kernel - [PATCH AUTOSEL 5.0 093/317] x86/uaccess: Dont leak the AC flag into __put

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Message-Id: <20190522192338.23715-93-sashal@kernel.org>
Date:   Wed, 22 May 2019 15:19:54 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>, luto@...nel.org,
        Ingo Molnar <mingo@...nel.org>, Sasha Levin <sashal@...nel.org>
Subject: [PATCH AUTOSEL 5.0 093/317] x86/uaccess: Dont leak the AC flag into __put_user() argument evaluation

From: Peter Zijlstra <peterz@...radead.org>

[ Upstream commit 6ae865615fc43d014da2fd1f1bba7e81ee622d1b ]

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:

  2a418cf3f5f1 ("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@...radead.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Acked-by: Randy Dunlap <rdunlap@...radead.org> # build-tested
Acked-by: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: luto@...nel.org
Fixes: 2a418cf3f5f1 ("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@...nel.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 arch/x86/include/asm/uaccess.h | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index f3aed639dccda..2b0dd1b9c2087 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -431,10 +431,11 @@ 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();					\
-- 
2.20.1