linux-kernel - Re: [PATCH] riscv: Fix spurious errors from __get/put_kernel

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Date: Mon, 25 Mar 2024 08:42:16 +0100
From: Alexandre Ghiti <alex@...ti.fr>
To: Samuel Holland <samuel.holland@...ive.com>,
 Palmer Dabbelt <palmer@...belt.com>, linux-riscv@...ts.infradead.org
Cc: linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] riscv: Fix spurious errors from __get/put_kernel_nofault

Hi Samuel,

On 12/03/2024 03:19, Samuel Holland wrote:
> These macros did not initialize __kr_err, so they could fail even if
> the access did not fault.
>
> Cc: stable@...r.kernel.org
> Fixes: d464118cdc41 ("riscv: implement __get_kernel_nofault and __put_user_nofault")
> Signed-off-by: Samuel Holland <samuel.holland@...ive.com>
> ---
> Found while testing the unaligned access speed series[1]. The observed
> behavior was that with RISCV_EFFICIENT_UNALIGNED_ACCESS=y, the
> copy_from_kernel_nofault() in prepend_copy() failed every time when
> filling out /proc/self/mounts, so all of the mount points were "xxx".
>
> I'm surprised this hasn't been seen before. For reference, I'm compiling
> with clang 18.
>
> [1]: https://lore.kernel.org/linux-riscv/20240308-disable_misaligned_probe_config-v9-0-a388770ba0ce@rivosinc.com/
>
>   arch/riscv/include/asm/uaccess.h | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/riscv/include/asm/uaccess.h b/arch/riscv/include/asm/uaccess.h
> index ec0cab9fbddd..72ec1d9bd3f3 100644
> --- a/arch/riscv/include/asm/uaccess.h
> +++ b/arch/riscv/include/asm/uaccess.h
> @@ -319,7 +319,7 @@ unsigned long __must_check clear_user(void __user *to, unsigned long n)
>   
>   #define __get_kernel_nofault(dst, src, type, err_label)			\
>   do {									\
> -	long __kr_err;							\
> +	long __kr_err = 0;						\
>   									\
>   	__get_user_nocheck(*((type *)(dst)), (type *)(src), __kr_err);	\
>   	if (unlikely(__kr_err))						\
> @@ -328,7 +328,7 @@ do {									\
>   
>   #define __put_kernel_nofault(dst, src, type, err_label)			\
>   do {									\
> -	long __kr_err;							\
> +	long __kr_err = 0;						\
>   									\
>   	__put_user_nocheck(*((type *)(src)), (type *)(dst), __kr_err);	\
>   	if (unlikely(__kr_err))						\


That's a good a catch! Surprising indeed that we did not find this 
before, that's the second bug in the uaccess routines in the last 3 (or 
4) releases, maybe we should improve our coverage here, I'll see what we 
can do.

You can add:

Reviewed-by: Alexandre Ghiti <alexghiti@...osinc.com>

Thanks,

Alex