Re: [PATCH 1/2] http-backend: Fix access beyond end of string.

On Sun, Nov 15, 2009 at 10:12 PM, Junio C Hamano [off-list ref] wrote:

Jeff King [off-list ref] writes:

quoted

On Sun, Nov 15, 2009 at 05:36:54PM -0800, Shawn O. Pearce wrote:
...

quoted

Shouldn't this instead be:

diff --git a/http-backend.c b/http-backend.c
index 9021266..16ec635 100644
--- a/http-backend.c
+++ b/http-backend.c

@@ -626,7 +626,7 @@ int main(int argc, char **argv)

                     }

                     cmd = c;
-                    cmd_arg = xmalloc(n);
+                    cmd_arg = xmalloc(n + 1);
                     strncpy(cmd_arg, dir + out[0].rm_so + 1, n);
                     cmd_arg[n] = '\0';
                     dir[out[0].rm_so] = 0;

The cmd_arg string was simply allocated too small.  Your fix is
terminating the string one character too short which would cause
get_loose_object and get_pack_file to break.

Actually, from my reading, I think his fix is right, because you trim
the first character during the strncpy (using "out[0].rm_so + 1").

Your regexps all start with leading "/", and rm_so+1 points at the
character after the slash; the intention being that you would copy
the rest of the matched sequence without the leading "/".

So allocating n = rm_eo - rm_so is Ok.  It counts the space for
terminating NUL.  But copying "up to n bytes" using strncpy(), only to NUL
terminate immediately later, is dubious.  You would want to copy only n-1
bytes.  I.e.

       n = out[0].rm_eo - out[0].rm_so; /* allocation */
       ... validate and fail invalid method ...
       cmd_arg = xmalloc(n);
       memcpy(cmd_arg, dir + out[0].rm_so + 1, n-1);
       cmd_arg[n-1] = '\0';

I think the strncpy( , ,n) would not harm anything because we won't
overflow dir because it's NUL terminated in getdir(), and the '\0'
shouldn't match the regex. But I agree that strncpy( , , n-1) is
better and memcpy( , , n-1) is better still.

Better eyes than mine have now looked at this and see different things
each time.  I wonder if some parts could be made a little less subtle
(perhaps along with the dir[out[0].rm_so] = 0;)?

Thanks,
Tarmigan