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SubjectRe: [PATCH v2 1/7] kdb: Fix buffer overflow during tab-complete
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@google.com>
> Closes: https://lore.kernel.org/all/CAFhGd8qESuuifuHsNjFPR-Va3P80bxrw+LqvC8deA8GziUJLpw@mail.gmail.com/
> Cc: stable@vger.kernel.org
> Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>

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@google.com>
> ---
> 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

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