[patch for 2.6.26 3/4] vfs: utimensat(): fix error checking for {UTIME_NOW,UTIME_OMIT} case

The POSIX.1 draft spec for utimensat() says:

    Only a process with the effective user ID equal to the
    user ID of the file or with appropriate privileges may use
    futimens() or utimensat() with a non-null times argument
    that does not have both tv_nsec fields set to UTIME_NOW
    and does not have both tv_nsec fields set to UTIME_OMIT.

If this condition is violated, then the error EPERM should result.
However, the current implementation does not generate EPERM if
one tv_nsec field is UTIME_NOW while the other is UTIME_OMIT.
It should give this error for that case.

This patch:

a) Repairs that problem.
b) Removes the now unneeded nsec_special() helper function.
c) Adds some comments to explain the checks that are being
   performed.

Thanks to Miklos, who provided comments on the previous iteration
of this patch.  As a result, this version is a little simpler and
and its logic is better structured.

Miklos suggested an alternative idea, migrating the
is_owner_or_cap() checks into fs/attr.c:inode_change_ok() via
the use of an ATTR_OWNER_CHECK flag.  Maybe we could do that
later, but for now I've gone with this version, which is
IMO simpler, and can be more easily read as being correct.

Acked-by: Miklos Szeredi <miklos@szeredi.hu>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Ulrich Drepper <drepper@redhat.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 21 insertions, 15 deletions

diff --git a/fs/utimes.c b/fs/utimes.c
index d466bc587e6e..118d1c3241be 100644
--- a/fs/utimes.c
+++ b/fs/utimes.c
@@ -40,14 +40,9 @@ asmlinkage long sys_utime(char __user *filename, struct utimbuf __user *times)
 
 #endif
 
-static bool nsec_special(long nsec)
-{
-        return nsec == UTIME_OMIT || nsec == UTIME_NOW;
-}
-
 static bool nsec_valid(long nsec)
 {
-        if (nsec_special(nsec))
+        if (nsec == UTIME_OMIT || nsec == UTIME_NOW)
                 return true;
 
         return nsec >= 0 && nsec <= 999999999;
@@ -106,7 +101,7 @@ long do_utimes(int dfd, char __user *filename, struct timespec *times, int flags
                      times[1].tv_nsec == UTIME_NOW)
                 times = NULL;
 
-        /* Don't worry, the checks are done in inode_change_ok() */
+        /* In most cases, the checks are done in inode_change_ok() */
         newattrs.ia_valid = ATTR_CTIME | ATTR_MTIME | ATTR_ATIME;
         if (times) {
                 error = -EPERM;
@@ -128,15 +123,26 @@ long do_utimes(int dfd, char __user *filename, struct timespec *times, int flags
                         newattrs.ia_mtime.tv_nsec = times[1].tv_nsec;
                         newattrs.ia_valid |= ATTR_MTIME_SET;
                 }
-        }
 
-        /*
-         * 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))) {
+                /*
+                 * For the UTIME_OMIT/UTIME_NOW and UTIME_NOW/UTIME_OMIT
+                 * cases, we need to make an extra check that is not done by
+                 * inode_change_ok().
+                 */
+                if (((times[0].tv_nsec == UTIME_NOW &&
+                            times[1].tv_nsec == UTIME_OMIT)
+                     ||
+                     (times[0].tv_nsec == UTIME_OMIT &&
+                            times[1].tv_nsec == UTIME_NOW))
+                    && !is_owner_or_cap(inode))
+                        goto mnt_drop_write_and_out;
+        } else {
+
+                /*
+                 * If times is NULL (or both times are UTIME_NOW),
+                 * then we need to check permissions, because
+                 * inode_change_ok() won't do it.
+                 */
                 error = -EACCES;
                 if (IS_IMMUTABLE(inode))
                         goto mnt_drop_write_and_out;