[PATCH 5.10 01/77] ARM: 9132/1: Fix __get_user_check failure with ARM KASAN images

Greg Kroah-Hartman
2021-11-01T09:34:10Z

From: Lexi Shao <shaolexi@huawei.com>

commit df909df0770779f1a5560c2bb641a2809655ef28 upstream.

ARM: kasan: Fix __get_user_check failure with kasan

In macro __get_user_check defined in arch/arm/include/asm/uaccess.h,
error code is store in register int __e(r0). When kasan is
enabled, assigning value to kernel address might trigger kasan check,
which unexpectedly overwrites r0 and causes undefined behavior on arm
kasan images.

One example is failure in do_futex and results in process soft lockup.
Log:
watchdog: BUG: soft lockup - CPU#0 stuck for 62946ms! [rs:main
Q:Reg:1151]
...
(__asan_store4) from (futex_wait_setup+0xf8/0x2b4)
(futex_wait_setup) from (futex_wait+0x138/0x394)
(futex_wait) from (do_futex+0x164/0xe40)
(do_futex) from (sys_futex_time32+0x178/0x230)
(sys_futex_time32) from (ret_fast_syscall+0x0/0x50)

The soft lockup happens in function futex_wait_setup. The reason is
function get_futex_value_locked always return EINVAL, thus pc jump
back to retry label and causes looping.

This line in function get_futex_value_locked
ret = __get_user(*dest, from);
is expanded to
*dest = (typeof(*(p))) __r2; ,
in macro __get_user_check. Writing to pointer dest triggers kasan check
and overwrites the return value of __get_user_x function.
The assembly code of get_futex_value_locked in kernel/futex.c:
...
c01f6dc8: eb0b020e bl c04b7608 <__get_user_4>
// "x = (typeof(*(p))) __r2;" triggers kasan check and r0 is overwritten
c01f6dCc: e1a00007 mov r0, r7
c01f6dd0: e1a05002 mov r5, r2
c01f6dd4: eb04f1e6 bl c0333574 <__asan_store4>
c01f6dd8: e5875000 str r5, [r7]
// save ret value of __get_user(*dest, from), which is dest address now
c01f6ddc: e1a05000 mov r5, r0
...
// checking return value of __get_user failed
c01f6e00: e3550000 cmp r5, #0
...
c01f6e0c: 01a00005 moveq r0, r5
// assign return value to EINVAL
c01f6e10: 13e0000d mvnne r0, #13

Return value is the destination address of get_user thus certainly
non-zero, so get_futex_value_locked always return EINVAL.

Fix it by using a tmp vairable to store the error code before the
assignment. This fix has no effects to non-kasan images thanks to compiler
optimization. It only affects cases that overwrite r0 due to kasan check.

This should fix bug discussed in Link:
[1] https://lore.kernel.org/linux-arm-kernel/0ef7c2a5-5d8b-c5e0-63fa-31693fd4495c@gmail.com/

Fixes: 421015713b30 ("ARM: 9017/2: Enable KASan for ARM")
Signed-off-by: Lexi Shao <shaolexi@huawei.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/arm/include/asm/uaccess.h | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

--- a/arch/arm/include/asm/uaccess.h
+++ b/arch/arm/include/asm/uaccess.h
@@ -200,6 +200,7 @@ extern int __get_user_64t_4(void *);
register unsigned long __l asm("r1") = __limit; \
register int __e asm("r0"); \
unsigned int __ua_flags = uaccess_save_and_enable(); \
+ int __tmp_e; \
switch (sizeof(*(__p))) { \
case 1: \
if (sizeof((x)) >= 8) \
@@ -227,9 +228,10 @@ extern int __get_user_64t_4(void *);
break; \
default: __e = __get_user_bad(); break; \
} \
+ __tmp_e = __e; \
uaccess_restore(__ua_flags); \
x = (typeof(*(p))) __r2; \
- __e; \
+ __tmp_e; \
})

#define get_user(x, p) \