linux-kernel - [PATCH AUTOSEL 5.2 04/12] x86/uaccess: Don't leak the AC flags into __get

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Date:   Mon,  9 Sep 2019 11:40:44 -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>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>, broonie@...nel.org,
        sfr@...b.auug.org.au, akpm@...ux-foundation.org, mhocko@...e.cz,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH AUTOSEL 5.2 04/12] x86/uaccess: Don't leak the AC flags into __get_user() argument evaluation

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

[ Upstream commit 9b8bd476e78e89c9ea26c3b435ad0201c3d7dbf5 ]

Identical to __put_user(); the __get_user() argument evalution will too
leak UBSAN crud into the __uaccess_begin() / __uaccess_end() region.
While uncommon this was observed to happen for:

  drivers/xen/gntdev.c: if (__get_user(old_status, batch->status[i]))

where UBSAN added array bound checking.

This complements commit:

  6ae865615fc4 ("x86/uaccess: Dont leak the AC flag into __put_user() argument evaluation")

Tested-by Sedat Dilek <sedat.dilek@...il.com>
Reported-by: Randy Dunlap <rdunlap@...radead.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Josh Poimboeuf <jpoimboe@...hat.com>
Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
Cc: broonie@...nel.org
Cc: sfr@...b.auug.org.au
Cc: akpm@...ux-foundation.org
Cc: Randy Dunlap <rdunlap@...radead.org>
Cc: mhocko@...e.cz
Cc: Josh Poimboeuf <jpoimboe@...hat.com>
Link: https://lkml.kernel.org/r/20190829082445.GM2369@hirez.programming.kicks-ass.net
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 arch/x86/include/asm/uaccess.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index c82abd6e4ca39..869794bd0fd98 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -442,8 +442,10 @@ __pu_label:							\
 ({									\
 	int __gu_err;							\
 	__inttype(*(ptr)) __gu_val;					\
+	__typeof__(ptr) __gu_ptr = (ptr);				\
+	__typeof__(size) __gu_size = (size);				\
 	__uaccess_begin_nospec();					\
-	__get_user_size(__gu_val, (ptr), (size), __gu_err, -EFAULT);	\
+	__get_user_size(__gu_val, __gu_ptr, __gu_size, __gu_err, -EFAULT);	\
 	__uaccess_end();						\
 	(x) = (__force __typeof__(*(ptr)))__gu_val;			\
 	__builtin_expect(__gu_err, 0);					\
-- 
2.20.1