[PATCH] daemon.c and path.enter_repo(): revamp path validation.

The whitelist of git-daemon is checked against return value from
enter_repo(), and enter_repo() used to return the value obtained
from getcwd() to avoid directory aliasing issues as discussed
earier (mid October 2005).

Unfortunately, it did not go well as we hoped.

For example, /pub on a kernel.org public machine is a symlink to
its real mountpoint, and it is understandable that the
administrator does not want to adjust the whitelist every time
/pub needs to point at a different partition for storage
allcation or whatever reasons.  Being able to keep using
/pub/scm as the whitelist is a desirable property.

So this version of enter_repo() reports what it used to chdir()
and validate, but does not use getcwd() to canonicalize the
directory name.  When it sees a user relative path ~user/path,
it internally resolves it to try chdir() there, but it still
reports ~user/path (possibly after appending .git if allowed to
do so, in which case it would report ~user/path.git).

What this means is that if a whitelist wants to allow a user
relative path, it needs to say "~" (for all users) or list user
home directories like "~alice" "~bob".  And no, you cannot say
/home if the advertised way to access user home directories are
~alice,~bob, etc.  The whole point of this is to avoid
unnecessary aliasing issues.

Anyway, because of this, daemon needs to do a bit more work to
guard itself.  Namely, it needs to make sure that the accessor
does not try to exploit its leading path match rule by inserting
/../ in the middle or hanging /.. at the end.  I resurrected the
belts and suspender paranoia code HPA did for this purpose.

This check cannot be done in the enter_repo() unconditionally,
because there are valid callers of enter_repo() that want to
honor /../; authorized users coming over ssh to run send-pack
and fetch-pack should be allowed to do so.

Signed-off-by: Junio C Hamano <junkio@cox.net>

1 files changed, 60 insertions, 4 deletions

diff --git a/daemon.c b/daemon.c
index 91b96569cd..539f6e87af 100644
--- a/daemon.c
+++ b/daemon.c
@@ -82,9 +82,63 @@ static void loginfo(const char *err, ...)
 	va_end(params);
 }
 
+static int avoid_alias(char *p)
+{
+	int sl, ndot;
+
+	/* 
+	 * This resurrects the belts and suspenders paranoia check by HPA
+	 * done in <435560F7.4080006@zytor.com> thread, now enter_repo()
+	 * does not do getcwd() based path canonicalizations.
+	 *
+	 * sl becomes true immediately after seeing '/' and continues to
+	 * be true as long as dots continue after that without intervening
+	 * non-dot character.
+	 */
+	if (!p || (*p != '/' && *p != '~'))
+		return -1;
+	sl = 1; ndot = 0;
+	p++;
+
+	while (1) {
+		char ch = *p++;
+		if (sl) {
+			if (ch == '.')
+				ndot++;
+			else if (ch == '/') {
+				if (ndot < 3)
+					/* reject //, /./ and /../ */
+					return -1;
+				ndot = 0;
+			}
+			else if (ch == 0) {
+				if (0 < ndot && ndot < 3)
+					/* reject /.$ and /..$ */
+					return -1;
+				return 0;
+			}
+			else
+				sl = ndot = 0;
+		}
+		else if (ch == 0)
+			return 0;
+		else if (ch == '/') {
+			sl = 1;
+			ndot = 0;
+		}
+	}
+}
+
 static char *path_ok(char *dir)
 {
-	char *path = enter_repo(dir, strict_paths);
+	char *path;
+
+	if (avoid_alias(dir)) {
+		logerror("'%s': aliased", dir);
+		return NULL;
+	}
+
+	path = enter_repo(dir, strict_paths);
 
 	if (!path) {
 		logerror("'%s': unable to chdir or not a git archive", dir);
@@ -96,9 +150,11 @@ static char *path_ok(char *dir)
 		int pathlen = strlen(path);
 
 		/* The validation is done on the paths after enter_repo
-		 * canonicalization, so whitelist should be written in
-		 * terms of real pathnames (i.e. after ~user is expanded
-		 * and symlinks resolved).
+		 * appends optional {.git,.git/.git} and friends, but 
+		 * it does not use getcwd().  So if your /pub is
+		 * a symlink to /mnt/pub, you can whitelist /pub and
+		 * do not have to say /mnt/pub.
+		 * Do not say /pub/.
 		 */
 		for ( pp = ok_paths ; *pp ; pp++ ) {
 			int len = strlen(*pp);