linux-kernel - [PATCH] Revert "x86/uaccess: Add stack frame output operand in get

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Date:   Wed, 12 Jul 2017 14:27:44 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Chris J Arges <chris.j.arges@...onical.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Borislav Petkov <bp@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Douglas Anderson <dianders@...omium.org>,
        Michael Davidson <md@...gle.com>,
        Greg Hackmann <ghackmann@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Stephen Hines <srhines@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Arnd Bergmann <arnd@...db.de>,
        Bernhard.Rosenkranzer@...aro.org,
        Matthias Kaehlcke <mka@...omium.org>
Subject: [PATCH] Revert "x86/uaccess: Add stack frame output operand in get_user() inline asm"

Commit f05058c4d652 supposedly "forces a stack frame to be created before
the inline asm code if CONFIG_FRAME_POINTER is enabled by listing the
stack pointer as an output operand for the get_user() inline assembly
statement.". This doesn't work as intended, at least with gcc v4.9.2 and
x86-64 the generated code is exactly the same with and without the patch.
However clang adds an extra instruction that adjusts %rsp, which ends up
causing double faults all over the place.

Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
---
 arch/x86/include/asm/uaccess.h | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index 476ea27f490b..9ec2beab73df 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -161,11 +161,10 @@ __typeof__(__builtin_choose_expr(sizeof(x) > sizeof(0UL), 0ULL, 0UL))
 ({									\
 	int __ret_gu;							\
 	register __inttype(*(ptr)) __val_gu asm("%"_ASM_DX);		\
-	register void *__sp asm(_ASM_SP);				\
 	__chk_user_ptr(ptr);						\
 	might_fault();							\
-	asm volatile("call __get_user_%P4"				\
-		     : "=a" (__ret_gu), "=r" (__val_gu), "+r" (__sp)	\
+	asm volatile("call __get_user_%P3"				\
+		     : "=a" (__ret_gu), "=r" (__val_gu)			\
 		     : "0" (ptr), "i" (sizeof(*(ptr))));		\
 	(x) = (__force __typeof__(*(ptr))) __val_gu;			\
 	__builtin_expect(__ret_gu, 0);					\
-- 
2.13.2.932.g7449e964c-goog