[PATCH] um: proc/exitcode: fix simple_strtol() out-of-bounds read

[David Laight]

On Thu, 23 Apr 2026 01:39:25 +0800
"Shengzhuo Wei" <me at cherr.cc> wrote:

> The stack buffer 'buf' is declared as char[sizeof("nnnnn\0")] (7 bytes)
> and the copy size is min(count, sizeof(buf)).  When a user writes 7 or
> more bytes, copy_from_user fills all 7 bytes without a NUL terminator.
> The subsequent call to simple_strtol() expects a NUL-terminated string
> and will read past the end of buf on the stack.

You should probably also mention that write(, "123", 3) will lead to
buf[3] being read - which is uninitialised stack.

	David

> 
> write(2) should report the number of bytes consumed. Returning the original
> count would claim success even when the input was truncated, so userspace
> cannot detect it.
> 
> Clamp the copy length to sizeof(buf)-1, add a terminator, and return the
> consumed length.
> 
> Fixes: 201f99f170df ("uml: check length in exitcode_proc_write()")
> Fixes: e16f5350d4cf ("uml: get declaration of simple_strtoul")
> Signed-off-by: Shengzhuo Wei <me at cherr.cc>
> ---
>  arch/um/kernel/exitcode.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/um/kernel/exitcode.c b/arch/um/kernel/exitcode.c
> index 43edc2aa57e4fbd4a3d24f96878c76f9f8fd4eaa..8de404ff21a213918c5351bc20a6e047bf1b93f5 100644
> --- a/arch/um/kernel/exitcode.c
> +++ b/arch/um/kernel/exitcode.c
> @@ -43,16 +43,17 @@ static ssize_t exitcode_proc_write(struct file *file,
>  	size_t size;
>  	int tmp;
>  
> -	size = min(count, sizeof(buf));
> +	size = min(count, sizeof(buf) - 1);
>  	if (copy_from_user(buf, buffer, size))
>  		return -EFAULT;
> +	buf[size] = '\0';
>  
>  	tmp = simple_strtol(buf, &end, 0);
>  	if ((*end != '\0') && !isspace(*end))
>  		return -EINVAL;
>  
>  	uml_exitcode = tmp;
> -	return count;
> +	return size;
>  }
>  
>  static const struct proc_ops exitcode_proc_ops = {
> 
> ---
> base-commit: 6596a02b207886e9e00bb0161c7fd59fea53c081
> change-id: 20260423-fix_exitcode-908061ece624
> 
> Best regards,