avoid sending pointer values in struct passwd

[Damien Miller]

On Wed, 25 Nov 2020, Yuichiro NAITO wrote:

> Thanks for reviewing my patch.
> 
> > 2020/11/20 23:45、Peter Stuge <peter at stuge.se>のメール:
> > 
> > Yuichiro NAITO wrote:
> >> Take a look at my GitHub pull request to see my patch.
> >> 
> >> https://github.com/openssh/openssh-portable/pull/216
> > 
> > I think the length at the beginning should be tied to the (number of?)
> > members that are sent instead of the struct passwd size on either side.
> 
> OK.
> I fixed to send number of struct passwd members at first in sshbuf_put_passwd().
> And sshbuf_get_passwd() checks it.

Thanks for reminding me about this.

IMO sshbuf-*.c isn't the right place for this. Sending/receiving password
structs is only done in one place in OpenSSH, so I'd prefer to leave it
where it is.

I think that its easier to always send the values as 64 bit quantities,
but POSIX doesn't guarantee that uid_t, gid_t and time_t are unsigned
so I think it is safest to explicitly encode the sign of these values.

(I'm not worried about signed/unsigned overflow upon decoding: the
process doing the decoding is unprivileged, sandboxed and already
completely trusts the privileged process to send it good data.)

So something like this perhaps (against OpenBSD):

diff --git a/monitor.c b/monitor.c
index d71520b..ec484ca 100644
--- a/monitor.c
+++ b/monitor.c
@@ -639,8 +639,15 @@ mm_answer_sign(struct ssh *ssh, int sock, struct sshbuf *m)
 	return (0);
 }
 
+#define PUTPW(b, id) \
+	do { \
+		if ((r = sshbuf_put_u8(b, pwent->id < 0)) != 0 || \
+		    (r = sshbuf_put_u64(b, pwent->id < 0 ? \
+		    -pwent->id : pwent->id)) != 0) \
+			fatal_fr(r, "assemble pw %s", #id); \
+	} while (0)
+
 /* Retrieves the password entry and also checks if the user is permitted */
-
 int
 mm_answer_pwnamallow(struct ssh *ssh, int sock, struct sshbuf *m)
 {
@@ -676,10 +683,14 @@ mm_answer_pwnamallow(struct ssh *ssh, int sock, struct sshbuf *m)
 	authctxt->pw = pwent;
 	authctxt->valid = 1;
 
-	/* XXX don't sent pwent to unpriv; send fake class/dir/shell too */
-	if ((r = sshbuf_put_u8(m, 1)) != 0 ||
-	    (r = sshbuf_put_string(m, pwent, sizeof(*pwent))) != 0 ||
-	    (r = sshbuf_put_cstring(m, pwent->pw_name)) != 0 ||
+	/* XXX send fake class/dir/shell, etc. */
+	if ((r = sshbuf_put_u8(m, 1)) != 0)
+		fatal_fr(r, "assemble ok");
+	PUTPW(m, pw_uid);
+	PUTPW(m, pw_gid);
+	PUTPW(m, pw_change);
+	PUTPW(m, pw_expire);
+	if ((r = sshbuf_put_cstring(m, pwent->pw_name)) != 0 ||
 	    (r = sshbuf_put_cstring(m, "*")) != 0 ||
 	    (r = sshbuf_put_cstring(m, pwent->pw_gecos)) != 0 ||
 	    (r = sshbuf_put_cstring(m, pwent->pw_class)) != 0 ||
diff --git a/monitor_wrap.c b/monitor_wrap.c
index d4ab862..a6193be 100644
--- a/monitor_wrap.c
+++ b/monitor_wrap.c
@@ -242,6 +242,16 @@ mm_sshkey_sign(struct ssh *ssh, struct sshkey *key, u_char **sigp, size_t *lenp,
 	return (0);
 }
 
+#define GETPW(b, id) \
+	do { \
+		u_char _neg = 0; \
+		int64_t _i = 0; \
+		if ((r = sshbuf_get_u8(b, &_neg)) != 0 || \
+		    (r = sshbuf_get_u64(b, &_i)) != 0) \
+			fatal_fr(r, "parse pw %s", #id); \
+		pw->id = _neg ? -_i : _i; \
+	} while (0)
+
 struct passwd *
 mm_getpwnamallow(struct ssh *ssh, const char *username)
 {
@@ -273,14 +283,11 @@ mm_getpwnamallow(struct ssh *ssh, const char *username)
 		goto out;
 	}
 
-	/* XXX don't like passing struct passwd like this */
 	pw = xcalloc(sizeof(*pw), 1);
-	if ((r = sshbuf_get_string_direct(m, &p, &len)) != 0)
-		fatal_fr(r, "parse");
-	if (len != sizeof(*pw))
-		fatal_f("struct passwd size mismatch");
-	memcpy(pw, p, sizeof(*pw));
-
+	GETPW(m, pw_uid);
+	GETPW(m, pw_gid);
+	GETPW(m, pw_change);
+	GETPW(m, pw_expire);
 	if ((r = sshbuf_get_cstring(m, &pw->pw_name, NULL)) != 0 ||
 	    (r = sshbuf_get_cstring(m, &pw->pw_passwd, NULL)) != 0 ||
 	    (r = sshbuf_get_cstring(m, &pw->pw_gecos, NULL)) != 0 ||