Re: [PATCH v2 1/7] kdb: Fix buffer overflow during tab-complete

[Justin Stitt]

Hi,

On Mon, Apr 22, 2024 at 05:35:54PM +0100, Daniel Thompson wrote:
> Currently, when the user attempts symbol completion with the Tab key, kdb
> will use strncpy() to insert the completed symbol into the command buffer.
> Unfortunately it passes the size of the source buffer rather than the
> destination to strncpy() with predictably horrible results. Most obviously
> if the command buffer is already full but cp, the cursor position, is in
> the middle of the buffer, then we will write past the end of the supplied
> buffer.
> 
> Fix this by replacing the dubious strncpy() calls with memmove()/memcpy()
> calls plus explicit boundary checks to make sure we have enough space
> before we start moving characters around.
> 
> Reported-by: Justin Stitt <justinstitt@xxxxxxxxxx>
> Closes: https://lore.kernel.org/all/CAFhGd8qESuuifuHsNjFPR-Va3P80bxrw+LqvC8deA8GziUJLpw@xxxxxxxxxxxxxx/
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>

Nice! This is better than the conversions I tried to make earlier.

Your patch helps with https://github.com/KSPP/linux/issues/90

Reviewed-by: Justin Stitt <justinstitt@xxxxxxxxxx>
> ---
>  kernel/debug/kdb/kdb_io.c | 21 +++++++++++++--------
>  1 file changed, 13 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
> index 9443bc63c5a24..06dfbccb10336 100644
> --- a/kernel/debug/kdb/kdb_io.c
> +++ b/kernel/debug/kdb/kdb_io.c
> @@ -367,14 +367,19 @@ static char *kdb_read(char *buffer, size_t bufsize)
>  			kdb_printf(kdb_prompt_str);
>  			kdb_printf("%s", buffer);
>  		} else if (tab != 2 && count > 0) {
> -			len_tmp = strlen(p_tmp);
> -			strncpy(p_tmp+len_tmp, cp, lastchar-cp+1);
> -			len_tmp = strlen(p_tmp);
> -			strncpy(cp, p_tmp+len, len_tmp-len + 1);
> -			len = len_tmp - len;
> -			kdb_printf("%s", cp);
> -			cp += len;
> -			lastchar += len;
> +			/* How many new characters do we want from tmpbuffer? */
> +			len_tmp = strlen(p_tmp) - len;
> +			if (lastchar + len_tmp >= bufend)
> +				len_tmp = bufend - lastchar;
> +
> +			if (len_tmp) {
> +				/* + 1 ensures the '\0' is memmove'd */
> +				memmove(cp+len_tmp, cp, (lastchar-cp) + 1);
> +				memcpy(cp, p_tmp+len, len_tmp);
> +				kdb_printf("%s", cp);
> +				cp += len_tmp;
> +				lastchar += len_tmp;
> +			}
>  		}
>  		kdb_nextline = 1; /* reset output line number */
>  		break;
> 
> -- 
> 2.43.0
> 
>

Thanks
Justin