Re: [PATCH] riscv: Fix spurious errors from __get/put_kernel_nofault

Alexandre Ghiti
2024-03-25T07:42:34Z

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@vger.kernel.org
> Fixes: d464118cdc41 ("riscv: implement __get_kernel_nofault and __put_user_nofault")
> Signed-off-by: Samuel Holland <samuel.holland@sifive.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@rivosinc.com>

Thanks,

Alex