Re: [PATCH] arm64: uaccess: fix put_user() with TTBR0 PAN

[Will Deacon]

On Thu, Nov 18, 2021 at 05:34:17PM +0100, Vincent Whitchurch wrote:
> The value argument to put_user() must be evaluated before the TTBR0
> switch is done.  Otherwise, if it is a function and the function sleeps,
> the reserved TTBR0 will be restored when the process is switched in
> again and the process will end up in an infinite loop of faults.
> 
> This problem was seen with the put_user() in schedule_tail().  A similar
> fix was done for RISC-V in commit 285a76bb2cf51b0c74c634 ("riscv:
> evaluate put_user() arg before enabling user access").
> 
> Fixes: f253d827f33cb5a5990 ("arm64: uaccess: refactor __{get,put}_user")
> Signed-off-by: Vincent Whitchurch <vincent.whitchurch@xxxxxxxx>
> ---
>  arch/arm64/include/asm/uaccess.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
> index 6e2e0b7031ab..96b26fa9d3d0 100644
> --- a/arch/arm64/include/asm/uaccess.h
> +++ b/arch/arm64/include/asm/uaccess.h
> @@ -362,10 +362,11 @@ do {									\
>  #define __put_user_error(x, ptr, err)					\
>  do {									\
>  	__typeof__(*(ptr)) __user *__p = (ptr);				\
> +	__typeof__(*(__p)) __val = (x);					\
>  	might_fault();							\
>  	if (access_ok(__p, sizeof(*__p))) {				\
>  		__p = uaccess_mask_ptr(__p);				\
> -		__raw_put_user((x), __p, (err));			\
> +		__raw_put_user(__val, __p, (err));			\
>  	} else	{							\
>  		(err) = -EFAULT;					\
>  	}								\


Oh, nice spot! I hope you didn't lose too much time debugging if you
actually ran into this...

Although it seems a lot less likely to cause a problem, should we do
something similar for __get_user_error() and assign to (x) outside of
the uaccess-disabled section?

Will