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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
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20
arch/arm/lib/putuser.S
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@@ -41,16 +41,13 @@ ENDPROC(__put_user_1)
ENTRY(__put_user_2)
check_uaccess r0, 2, r1, ip, __put_user_bad
mov ip, r2, lsr #8
#ifdef CONFIG_THUMB2_KERNEL
#ifndef __ARMEB__
2: TUSER(strb) r2, [r0]
3: TUSER(strb) ip, [r0, #1]
#if __LINUX_ARM_ARCH__ >= 6
2: TUSER(strh) r2, [r0]
#else
2: TUSER(strb) ip, [r0]
3: TUSER(strb) r2, [r0, #1]
#endif
#else /* !CONFIG_THUMB2_KERNEL */
mov ip, r2, lsr #8
#ifndef __ARMEB__
2: TUSER(strb) r2, [r0], #1
3: TUSER(strb) ip, [r0]
@@ -58,7 +55,8 @@ ENTRY(__put_user_2)
2: TUSER(strb) ip, [r0], #1
3: TUSER(strb) r2, [r0]
#endif
#endif /* CONFIG_THUMB2_KERNEL */
#endif /* __LINUX_ARM_ARCH__ >= 6 */
mov r0, #0
ret lr
ENDPROC(__put_user_2)
@@ -91,7 +89,9 @@ ENDPROC(__put_user_bad)
.pushsection __ex_table, "a"
.long 1b, __put_user_bad
.long 2b, __put_user_bad
#if __LINUX_ARM_ARCH__ < 6
.long 3b, __put_user_bad
#endif
.long 4b, __put_user_bad
.long 5b, __put_user_bad
.long 6b, __put_user_bad