[PATCH] lockfile: fix buffer overflow in path handling

[Michael Haggerty <mhagger@alum.mit.edu>]

The path of the file to be locked is held in lock_file::filename,
which is a fixed-length buffer of length PATH_MAX.  This buffer is
also (temporarily) used to hold the path of the lock file, which is
the path of the file being locked plus ".lock".  Because of this, the
path of the file being locked must be less than (PATH_MAX - 5)
characters long (5 chars are needed for ".lock" and one character for
the NUL terminator).

On entry into lock_file(), the path length was only verified to be
less than PATH_MAX characters, not less than (PATH_MAX - 5)
characters.

When and if resolve_symlink() is called, then that function is
correctly told to treat the buffer as (PATH_MAX - 5) characters long.
This part is correct.  However:

* If LOCK_NODEREF was specified, then resolve_symlink() is never
  called.

* If resolve_symlink() is called but the path is not a symlink, then
  the length check is never applied.

So it is possible for a path with length (PATH_MAX - 5 <= len <
PATH_MAX) to make it through the checks.  When ".lock" is strcat()ted
to such a path, the lock_file::filename buffer is overflowed.

Fix the problem by adding a check when entering lock_file() that the
original path is less than (PATH_MAX - 5) characters.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
---
This patch applies to maint.  I discovered it when working on
something else.  From a brief check of history, it looks like this
code has always been vulnerable to buffer overflows in one way or
another.

I haven't tried to evaluate whether this problem could be exploited
remotely.  Even if so, the ramifications are likely to be limited to
denial-of-service, because the data that are written past the end of
the buffer are not under the control of the user; they would be one of
the strings "lock\0", "ock\0", "ck\0", "k\0", or "\0", depending on
the length of the path to be locked.

I'm thinking of rewriting this code to use strbufs to prevent similar
problems in the future, but that is a more extensive change that
wouldn't be appropriate for maint.

 lockfile.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/lockfile.c b/lockfile.c
index c6fb77b..3ac49cb 100644
--- a/lockfile.c
+++ b/lockfile.c
@@ -124,13 +124,13 @@ static char *resolve_symlink(char *p, size_t s)
 
 static int lock_file(struct lock_file *lk, const char *path, int flags)
 {
-	if (strlen(path) >= sizeof(lk->filename))
-		return -1;
-	strcpy(lk->filename, path);
 	/*
 	 * subtract 5 from size to make sure there's room for adding
 	 * ".lock" for the lock file name
 	 */
+	if (strlen(path) >= sizeof(lk->filename)-5)
+		return -1;
+	strcpy(lk->filename, path);
 	if (!(flags & LOCK_NODEREF))
 		resolve_symlink(lk->filename, sizeof(lk->filename)-5);
 	strcat(lk->filename, ".lock");
-- 
1.8.3.2