openssh unix dev - Mar 2015 - Invalid memory access / read stack overflow when reading config with zero bytes

On Mon, 30 Mar 2015, Damien Miller wrote:
> On Mon, 30 Mar 2015, Hanno B?ck wrote:
> 
> > On Mon, 30 Mar 2015 09:19:02 +1100 (AEDT)
> > Damien Miller <djm at mindrot.org> wrote:
> > 
> > > What version of OpenSSH is this?
> > 
> > 6.8 portable on Linux.
> 
> That's strange - the line numbers in the valgrind stack trace don't
> match. E.g.
> 
> ==5578==    at 0x4C2CFCA: __GI_strchr (in
> /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
> ==5578==    by 0x117B6B: process_config_line (readconf.c:785)
> ==5578==    by 0x119DED: read_config_file (readconf.c:1633)
reproduced; the line numbers were wrong.

diff --git a/readconf.c b/readconf.c
index 42a2961..5130407 100644
--- a/readconf.c
+++ b/readconf.c
@@ -763,7 +763,9 @@ process_config_line(Options *options, struct passwd *pw,
const char *host,
 	}
 
 	/* Strip trailing whitespace */
-	for (len = strlen(line) - 1; len > 0; len--) {
+	if ((len = strlen(line)) == 0)
+		return 0;
+	for (len--; len > 0; len--) {
 		if (strchr(WHITESPACE, line[len]) == NULL)
 			break;
 		line[len] = '\0';

On Mon, 30 Mar 2015 10:43:18 +1100 (AEDT)
Damien Miller <djm at mindrot.org> wrote:
> reproduced; the line numbers were wrong.
Sorry for the line numbers, should've thought of that. I used the
standard Gentoo package and it seems it does patching on that file.

I can confirm your patch fixes the issue, thanks. Will now run another
fuzzing job with the patch applied, will inform you if it finds
anything.

-- 
Hanno B?ck
http://hboeck.de/

mail/jabber: hanno at hboeck.de
GPG: BBB51E42
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On Mon, 30 Mar 2015, Hanno B?ck wrote:
> On Mon, 30 Mar 2015 10:43:18 +1100 (AEDT)
> Damien Miller <djm at mindrot.org> wrote:
> 
> > reproduced; the line numbers were wrong.
> 
> Sorry for the line numbers, should've thought of that. I used the
> standard Gentoo package and it seems it does patching on that file.
> 
> I can confirm your patch fixes the issue, thanks. Will now run another
> fuzzing job with the patch applied, will inform you if it finds
> anything.
Thanks - we'll certainly fix bugs in config parsing, but they aren't
that interesting from a security perspective. Someone who can write
to ~/.ssh/config already has arbitrary code execution via ProxyCommand,
etc.

authorized_keys is a good thing to fuzz if you can set up a good test
enviornment. I've spent about a CPU-month fuzzing key parsing and KRLs,
so I have some confidence that they are clean.

cf. https://anongit.mindrot.org/openssh-fuzz-cases.git/

-d