[PATCH] ARM: kasan: Fix __get_user_check failure with kasan

[Russell King (Oracle)]

On Wed, Aug 25, 2021 at 02:46:50PM +0800, Lexi Shao wrote:
> 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.
> 
> 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.

This description doesn't actually make it clear why this is failing in
this case, until one looks at get_futex_value_locked() and realises
that what is actually going on here is:

	*dest = (typeof(*(p))) __r2;

which is why we end up with the store there.

> 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 at huawei.com>
> ---
>  arch/arm/include/asm/uaccess.h | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm/include/asm/uaccess.h b/arch/arm/include/asm/uaccess.h
> index a13d90206472..a6eb9af74870 100644
> --- 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;							\
> +		__e = __tmp_e;						\

There is no need to re-assign __tmp_e back to __e - you can just return
__tmp_e here.

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