Re: [PATCH; request for votes] snmplib: Avoid that sprint_realloc_octet_string() triggers a segmentation fault

On Sat, May 19, 2018 at 02:07:56PM -0700, Bart Van Assche wrote:
> strlcpy() implementations typically scan for the end of the source argument
> passed to strlcpy(). Hence avoid passing an unterminated string to strlcpy().
> 
> Reported-by: Stuart Henderson <[email protected]>
> Fixes: 7f05daa8e0e0 ("CHANGES: BUG: 3444939: BUG: 1796886: snmplib: Avoid that sprint_realloc_octet_string() embeds unprintable control characters or binary zeroes in its output. This behavior could cause truncated output in snmptrapd.")
> ---
>  snmplib/mib.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/snmplib/mib.c b/snmplib/mib.c
> index a16144065029..1c875c06464f 100644
> --- a/snmplib/mib.c
> +++ b/snmplib/mib.c
> @@ -585,9 +585,10 @@ sprint_realloc_octet_string(u_char ** buf, size_t * buf_len,
>                              return 0;
>                      }
>                      if (memchr(cp, '\0', cnt) == NULL) {
> -                        /* No embedded '\0' - use strlcpy() to preserve UTF-8 */
> -                        strlcpy((char *)(*buf + *out_len), (char *)cp, cnt + 1);
> +                        /* No embedded '\0' - use memcpy() to preserve UTF-8 */

The comment does not make any sense - what have UTF-8 got to do with this?

Anyway, I went and looked on the spec for the 't' display hint in rfc2579
§3.1 and it explicitly states that unterminated utf-8 characters at the end
of an t encoded octet string are to be discarded.

Looking at the code surrounding this chunk seems to suggest that some TLC for
the 't' display hint could be useful.

> +                        memcpy(*buf + *out_len, cp, cnt);

In any case I have to admit that the functional part of the patch (use memcpy
rather than strlcpy to copy possibly not null terminated data) is good so
+1 for this part

/MF

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