btrfs-progs: prevent incorrect use of subvol_strip_mountpoint

Add additional bound checks to prevent memory corruption on incorrect
usage of subvol_strip_mountpoint. Assert sane return value by properly
comparing the mount point to the full_path before stripping it off.

Mitigates issue: "btrfs send -p" fails if source and parent subvolumes
are on different mountpoints (memory corruption):

    https://github.com/kdave/btrfs-progs/issues/96

Note that this does not properly fix this bug, but prevents a possible
security issue by unexpected usage of "btrfs send -p".

Issue: #96
Pull-request: #98
Signed-off-by: Axel Burri <axel@tty0.ch>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/utils.c b/utils.c
index e9cb3a82..f867e5a7 100644
--- a/utils.c
+++ b/utils.c
@@ -2484,6 +2484,11 @@ const char *subvol_strip_mountpoint(const char *mnt, const char *full_path)
 	if (!len)
 		return full_path;
 
+	if ((strncmp(mnt, full_path, len) != 0) || (full_path[len] != '/')) {
+		error("not on mount point: %s", mnt);
+		exit(1);
+	}
+
 	if (mnt[len - 1] != '/')
 		len += 1;