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ARM: 8813/1: Make aligned 2-byte getuser()/putuser() atomic on ARMv6+

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getuser() and putuser() (and there underscored variants) use two
strb[t]/ldrb[t] instructions when they are asked to get/put 16-bits.
This means that the read/write is not atomic even when performed to a
16-bit-aligned address.

This leads to problems with vhost: vhost uses __getuser() to read the
vring's 16-bit avail.index field, and if it happens to observe a partial
update of the index, wrong descriptors will be used which will lead to a
breakdown of the virtio communication.  A similar problem exists for
__putuser() which is used to write to the vring's used.index field.

The reason these functions use strb[t]/ldrb[t] is because strht/ldrht
instructions did not exist until ARMv6T2/ARMv7.  So we should be easily
able to fix this on ARMv7.  Also, since all ARMv6 processors also don't
actually use the unprivileged instructions anymore for uaccess (since
CONFIG_CPU_USE_DOMAINS is not used) we can easily fix them too.

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
This commit is contained in:
Vincent Whitchurch
2018-11-09 10:12:30 +01:00
committed by Russell King
parent f441882a52
commit 344eb5539a
3 changed files with 39 additions and 10 deletions
Download Patch File Download Diff File Expand all files Collapse all files

18
arch/arm/include/asm/uaccess.h
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@@ -349,6 +349,13 @@ do { \
#define __get_user_asm_byte(x, addr, err) \
__get_user_asm(x, addr, err, ldrb)
#if __LINUX_ARM_ARCH__ >= 6
#define __get_user_asm_half(x, addr, err) \
__get_user_asm(x, addr, err, ldrh)
#else
#ifndef __ARMEB__
#define __get_user_asm_half(x, __gu_addr, err) \
({ \
@@ -367,6 +374,8 @@ do { \
})
#endif
#endif /* __LINUX_ARM_ARCH__ >= 6 */
#define __get_user_asm_word(x, addr, err) \
__get_user_asm(x, addr, err, ldr)
#endif
@@ -442,6 +451,13 @@ do { \
#define __put_user_asm_byte(x, __pu_addr, err) \
__put_user_asm(x, __pu_addr, err, strb)
#if __LINUX_ARM_ARCH__ >= 6
#define __put_user_asm_half(x, __pu_addr, err) \
__put_user_asm(x, __pu_addr, err, strh)
#else
#ifndef __ARMEB__
#define __put_user_asm_half(x, __pu_addr, err) \
({ \
@@ -458,6 +474,8 @@ do { \
})
#endif
#endif /* __LINUX_ARM_ARCH__ >= 6 */
#define __put_user_asm_word(x, __pu_addr, err) \
__put_user_asm(x, __pu_addr, err, str)