[PATCH] pretty: avoid buffer overflow in format_person_part

Jeff King· May 22, 2012, 05:45 UTC · lore

When we parse the name and email from a commit to pretty-print them, we usually can just put the result directly into our strbuf result. However, if we are going to use the mailmap, then we must first copy them into a NUL-terminated buffer to feed to the mailmap machinery.

We did so by using strlcpy into a static buffer, but we used it wrong. We fed it the length of the substring we wanted to copy, but never checked that that length was less than the size of the destination buffer.

The simplest fix is to just use snprintf to copy the substring properly while still respecting the destination buffer's size. It might seem like replacing the static buffer with a strbuf would help, but we need to feed a static buffer to the mailmap machinery anyway, so there's not much benefit to handling arbitrary sizes.

A more ideal solution would be for mailmap to grow an interface that:

  1. Takes a pointer and length combination, instead of
     assuming a NUL-terminated string.

  2. Returns a pointer to the mailmap's allocated string,
     rather than copying it into the buffer.

Then we could avoid the need for an extra buffer entirely. However, doing this would involve a lot of refactoring of mailmap and of string_list (which mailmap uses to store the map itself). For now, let's do the simplest thing to fix the bug.

Signed-off-by: Jeff King <peff@peff.net>
---
Current git will not generate commits with names long enough to overflow
here. With my recent series, that is no longer the case. Of course, that
is not really relevant; a malicious attacker is free to craft a bogus
commit directly with hash-object.

 pretty.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

Show changes to pretty.c +4 −2

diff --git a/pretty.c b/pretty.c
index 02a0a2b..986e114 100644
--- a/pretty.c
+++ b/pretty.c
@@ -547,8 +547,10 @@ static size_t format_person_part(struct strbuf *sb, char part,
 	mail_end = s.mail_end;
 
 	if (part == 'N' || part == 'E') { /* mailmap lookup */
-		strlcpy(person_name, name_start, name_end - name_start + 1);
-		strlcpy(person_mail, mail_start, mail_end - mail_start + 1);
+		snprintf(person_name, sizeof(person_name), "%.*s",
+			 (int)(name_end - name_start), name_start);
+		snprintf(person_mail, sizeof(person_mail), "%.*s",
+			 (int)(mail_end - mail_start), mail_start);
 		mailmap_name(person_mail, sizeof(person_mail), person_name, sizeof(person_name));
 		name_start = person_name;
 		name_end = name_start + strlen(person_name);

-- 
1.7.9.7.33.gc430a50

Junio C Hamano· May 22, 2012, 20:53 UTC · re: Jeff King · lore

Re: [PATCH] pretty: avoid buffer overflow in format_person_part

Jeff King <peff@peff.net> writes:

Show 8 quoted lines

> A more ideal solution would be for mailmap to grow an
> interface that:
>
>   1. Takes a pointer and length combination, instead of
>      assuming a NUL-terminated string.
>
>   2. Returns a pointer to the mailmap's allocated string,
>      rather than copying it into the buffer.

I was looking at René's patch to refs codepath that converted some "we allocate because callee wants to have NUL-terminated strings" functions to take counted strings (or whatever we want to call it---perhaps "string slices"?), and it appears that we may benefit if more callers that do not have to own strings that comes to them used them instead of NUL-terminated strings as their interface. At some deep point near the leaf of the callchain, they may need to be converted in order to call external API (read: POSIX) that takes NUL-terminated strings, but internal callchain like the mailmap one, if done carefully, should let us avoid extra allocations.

patchpretty: avoid buffer overflow in format_person_part