linux-kernel - [PATCH] ARM: fix get_user() broken with veneer

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Message-Id: <20230926160903.62924-1-masahiroy@kernel.org>
Date:   Wed, 27 Sep 2023 01:09:03 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     patches@...linux.org.uk
Cc:     linux-kernel@...r.kernel.org, Russell King <linux@...linux.org.uk>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: [PATCH] ARM: fix get_user() broken with veneer

The 32-bit ARM kernel stops working if the kernel grows to the point
where veneers for __get_user_* are created.

AAPCS32 [1] states, "Register r12 (IP) may be used by a linker as a
scratch register between a routine and any subroutine it calls. It
can also be used within a routine to hold intermediate values between
subroutine calls."

However, bl instructions buried within the inline asm are unpredictable
for compilers; hence, "ip" must be added to the clobber list.

This becomes critical when veneers for __get_user_* are created because
veneers use the ip register since commit 02e541db0540 ("ARM: 8323/1:
force linker to use PIC veneers").

[1]: https://github.com/ARM-software/abi-aa/blob/2023Q1/aapcs32/aapcs32.rst

Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
Reviewed-by: Ard Biesheuvel <ardb@...nel.org>
---

KernelVersion: v6.6-rc1

 arch/arm/include/asm/uaccess.h | 14 ++------------
 1 file changed, 2 insertions(+), 12 deletions(-)

diff --git a/arch/arm/include/asm/uaccess.h b/arch/arm/include/asm/uaccess.h
index bb5c81823117..c28f5ec21e41 100644
--- a/arch/arm/include/asm/uaccess.h
+++ b/arch/arm/include/asm/uaccess.h
@@ -109,16 +109,6 @@ extern int __get_user_64t_1(void *);
 extern int __get_user_64t_2(void *);
 extern int __get_user_64t_4(void *);
 
-#define __GUP_CLOBBER_1	"lr", "cc"
-#ifdef CONFIG_CPU_USE_DOMAINS
-#define __GUP_CLOBBER_2	"ip", "lr", "cc"
-#else
-#define __GUP_CLOBBER_2 "lr", "cc"
-#endif
-#define __GUP_CLOBBER_4	"lr", "cc"
-#define __GUP_CLOBBER_32t_8 "lr", "cc"
-#define __GUP_CLOBBER_8	"lr", "cc"
-
 #define __get_user_x(__r2, __p, __e, __l, __s)				\
 	   __asm__ __volatile__ (					\
 		__asmeq("%0", "r0") __asmeq("%1", "r2")			\
@@ -126,7 +116,7 @@ extern int __get_user_64t_4(void *);
 		"bl	__get_user_" #__s				\
 		: "=&r" (__e), "=r" (__r2)				\
 		: "0" (__p), "r" (__l)					\
-		: __GUP_CLOBBER_##__s)
+		: "ip", "lr", "cc")
 
 /* narrowing a double-word get into a single 32bit word register: */
 #ifdef __ARMEB__
@@ -148,7 +138,7 @@ extern int __get_user_64t_4(void *);
 		"bl	__get_user_64t_" #__s				\
 		: "=&r" (__e), "=r" (__r2)				\
 		: "0" (__p), "r" (__l)					\
-		: __GUP_CLOBBER_##__s)
+		: "ip", "lr", "cc")
 #else
 #define __get_user_x_64t __get_user_x
 #endif
-- 
2.39.2