vfs: fix permission checking in sys_utimensat

If utimensat() is called with both times set to UTIME_NOW or one of them to
UTIME_NOW and the other to UTIME_OMIT, then it will update the file time
without any permission checking.

I don't think this can be used for anything other than a local DoS, but could
be quite bewildering at that (e.g. "Why was that large source tree rebuilt
when I didn't modify anything???")

This affects all kernels from 2.6.22, when the utimensat() syscall was
introduced.

Fix by doing the same permission checking as for the "times == NULL" case.

Thanks to Michael Kerrisk, whose utimensat-non-conformances-and-fixes.patch in
-mm also fixes this (and breaks other stuff), only he didn't realize the
security implications of this bug.

Signed-off-by: Miklos Szeredi <[e-mail]>
Cc: Ulrich Drepper <[e-mail]>
Cc: Michael Kerrisk <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/utimes.c

···
 
 #endif
 
+static bool nsec_special(long nsec)
+{
+	return nsec == UTIME_OMIT || nsec == UTIME_NOW;
+}
+
 static bool nsec_valid(long nsec)
 {
-	if (nsec == UTIME_OMIT || nsec == UTIME_NOW)
+	if (nsec_special(nsec))
 		return true;
 
 	return nsec >= 0 && nsec <= 999999999;
···
 			newattrs.ia_mtime.tv_nsec = times[1].tv_nsec;
 			newattrs.ia_valid |= ATTR_MTIME_SET;
 		}
-	} else {
+	}
+
+	/*
+	 * If times is NULL or both times are either UTIME_OMIT or
+	 * UTIME_NOW, then need to check permissions, because
+	 * inode_change_ok() won't do it.
+	 */
+	if (!times || (nsec_special(times[0].tv_nsec) &&
+		       nsec_special(times[1].tv_nsec))) {
 		error = -EACCES;
                 if (IS_IMMUTABLE(inode))
 			goto mnt_drop_write_and_out;