locks: fix unlock when fcntl_setlk races with a close
diff mbox

Message ID 1452219742-18507-1-git-send-email-jeff.layton@primarydata.com
State New
Headers show

Commit Message

Jeff Layton Jan. 8, 2016, 2:22 a.m. UTC
Dmitry reported that he was able to reproduce the WARN_ON_ONCE that
fires in locks_free_lock_context when the flc_posix list isn't empty.

The problem turns out to be that we're basically rebuilding the
file_lock from scratch in fcntl_setlk when we discover that the setlk
has raced with a close. If the l_whence field is SEEK_CUR or SEEK_END,
then we may end up with fl_start and fl_end values that differ from
when the lock was initially set, if the file position or length of the
file has changed in the interim.

Fix this by just reusing the same lock request structure, and simply
override fl_type value with F_UNLCK as appropriate. That ensures that
we really are unlocking the lock that was initially set.

While we're there, make sure that we do pop a WARN_ON_ONCE if the
removal ever fails. Also return -EBADF in this event, since that's
what we would have returned if the close had happened earlier.

Cc: <stable@vger.kernel.org>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>
---
 fs/locks.c | 19 ++++++++++---------
 1 file changed, 10 insertions(+), 9 deletions(-)

Comments

Jeff Layton Jan. 8, 2016, 12:48 p.m. UTC | #1
On Thu,  7 Jan 2016 21:22:22 -0500
Jeff Layton <jlayton@poochiereds.net> wrote:

> Dmitry reported that he was able to reproduce the WARN_ON_ONCE that
> fires in locks_free_lock_context when the flc_posix list isn't empty.
> 
> The problem turns out to be that we're basically rebuilding the
> file_lock from scratch in fcntl_setlk when we discover that the setlk
> has raced with a close. If the l_whence field is SEEK_CUR or SEEK_END,
> then we may end up with fl_start and fl_end values that differ from
> when the lock was initially set, if the file position or length of the
> file has changed in the interim.
> 
> Fix this by just reusing the same lock request structure, and simply
> override fl_type value with F_UNLCK as appropriate. That ensures that
> we really are unlocking the lock that was initially set.
> 
> While we're there, make sure that we do pop a WARN_ON_ONCE if the
> removal ever fails. Also return -EBADF in this event, since that's
> what we would have returned if the close had happened earlier.
> 
> Cc: <stable@vger.kernel.org>
> Reported-by: Dmitry Vyukov <dvyukov@google.com>
> Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>
> ---
>  fs/locks.c | 19 ++++++++++---------
>  1 file changed, 10 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/locks.c b/fs/locks.c
> index 593dca300b29..0db640e4ced4 100644
> --- a/fs/locks.c
> +++ b/fs/locks.c
> @@ -2181,7 +2181,6 @@ int fcntl_setlk(unsigned int fd, struct file *filp, unsigned int cmd,
>  		goto out;
>  	}
>  
> -again:
>  	error = flock_to_posix_lock(filp, file_lock, &flock);
>  	if (error)
>  		goto out;
> @@ -2231,9 +2230,11 @@ again:
>  	spin_lock(&current->files->file_lock);
>  	f = fcheck(fd);
>  	spin_unlock(&current->files->file_lock);
> -	if (!error && f != filp && flock.l_type != F_UNLCK) {
> -		flock.l_type = F_UNLCK;
> -		goto again;
> +	if (!error && f != filp && file_lock->fl_type != F_UNLCK) {
> +		file_lock->fl_type = F_UNLCK;
> +		error = do_lock_file_wait(filp, cmd, file_lock);
> +		WARN_ON_ONCE(error);
> +		error = -EBADF;
>  	}
>  
>  out:
> @@ -2321,7 +2322,6 @@ int fcntl_setlk64(unsigned int fd, struct file *filp, unsigned int cmd,
>  		goto out;
>  	}
>  
> -again:
>  	error = flock64_to_posix_lock(filp, file_lock, &flock);
>  	if (error)
>  		goto out;
> @@ -2366,11 +2366,12 @@ again:
>  	spin_lock(&current->files->file_lock);
>  	f = fcheck(fd);
>  	spin_unlock(&current->files->file_lock);
> -	if (!error && f != filp && flock.l_type != F_UNLCK) {
> -		flock.l_type = F_UNLCK;
> -		goto again;
> +	if (!error && f != filp && file_lock->fl_type != F_UNLCK) {
> +		file_lock->fl_type = F_UNLCK;
> +		error = do_lock_file_wait(filp, cmd, file_lock);
> +		WARN_ON_ONCE(error);
> +		error = -EBADF;
>  	}
> -
>  out:
>  	locks_free_lock(file_lock);
>  	return error;

While this does fix Dmitri's reproducer, I think the basic concept of
removing locks like this after they are set is racy. Consider where we
have two threads:

Thread1				Thread2
----------------------------------------------------------------------------
fd1 = memfd_create(...);
fd2 = dup(fd1);
				fcntl(fd2, F_SETLK);
				(Here we call fcntl, and lock is set, but
				 task gets scheduled out before fcheck)
close(fd2)
fcntl(fd1, F_SETLK...);

				Task scheduled back in, does fcheck for fd2
				and finds that it's gone. Removes the lock
				that Thread1 just set.

So that seems wrong...in the face of the race above we can end up with
no lock set on the file, even though Thread1 thinks it has one. It is a
pretty unlikely race, but I don't see anything that prevents it.

The fix for filesystems that do not define their own ->lock op would be
pretty simple. We could do a fcheck after taking the flc_lock, but
before setting the lock on the file. The flc_lock should be enough to
prevent that race (though we may need to revisit some of the lockless
checks in locks_remove_posix). That wouldn't work for filesystems that
do set ->lock though, and I think we really do need a more general
solution there.

The good news is that OFD locks should be exempt from that fcheck
altogether. I'll spin up another patch for that, so we can at least
ensure that they aren't subject to that race.

Any thoughts on how to fix the above for traditional POSIX locks though?
Jeff Layton Jan. 8, 2016, 1:50 p.m. UTC | #2
The first patch in this patchset fixes a very long-standing bug in the
handling of races between setlk and close. That one should be appropriate
for all stable kernels and should apply to most kernels as-is.

The second patch exempts OFD locks from setlk/close race handling since
they shouldn't need it anyway. The rest of the patches add some better
debugging for these problems and do a little function name cleanup.

I'm planning to go ahead and put these into linux-next today and send
them to Linus for the 4.5 merge, unless there are any objections...

Jeff Layton (6):
  locks: fix unlock when fcntl_setlk races with a close
  locks: don't check for race with close when setting OFD lock
  locks: sprinkle some tracepoints around the file locking code
  locks: pass inode pointer to locks_free_lock_context
  locks: prink more detail when there are leaked locks
  locks: rename __posix_lock_file to posix_lock_inode

 fs/inode.c                      |   2 +-
 fs/locks.c                      | 123 +++++++++++++++++++++++++++-------------
 include/linux/fs.h              |   4 +-
 include/trace/events/filelock.h |  77 +++++++++++++++++++++++++
 4 files changed, 165 insertions(+), 41 deletions(-)
Bruce Fields Jan. 8, 2016, 4:16 p.m. UTC | #3
On Fri, Jan 08, 2016 at 07:48:04AM -0500, Jeff Layton wrote:
> On Thu,  7 Jan 2016 21:22:22 -0500
> Jeff Layton <jlayton@poochiereds.net> wrote:
> 
> > Dmitry reported that he was able to reproduce the WARN_ON_ONCE that
> > fires in locks_free_lock_context when the flc_posix list isn't empty.
> > 
> > The problem turns out to be that we're basically rebuilding the
> > file_lock from scratch in fcntl_setlk when we discover that the setlk
> > has raced with a close. If the l_whence field is SEEK_CUR or SEEK_END,
> > then we may end up with fl_start and fl_end values that differ from
> > when the lock was initially set, if the file position or length of the
> > file has changed in the interim.
> > 
> > Fix this by just reusing the same lock request structure, and simply
> > override fl_type value with F_UNLCK as appropriate. That ensures that
> > we really are unlocking the lock that was initially set.
> > 
> > While we're there, make sure that we do pop a WARN_ON_ONCE if the
> > removal ever fails. Also return -EBADF in this event, since that's
> > what we would have returned if the close had happened earlier.
> > 
> > Cc: <stable@vger.kernel.org>
> > Reported-by: Dmitry Vyukov <dvyukov@google.com>
> > Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>
> > ---
> >  fs/locks.c | 19 ++++++++++---------
> >  1 file changed, 10 insertions(+), 9 deletions(-)
> > 
> > diff --git a/fs/locks.c b/fs/locks.c
> > index 593dca300b29..0db640e4ced4 100644
> > --- a/fs/locks.c
> > +++ b/fs/locks.c
> > @@ -2181,7 +2181,6 @@ int fcntl_setlk(unsigned int fd, struct file *filp, unsigned int cmd,
> >  		goto out;
> >  	}
> >  
> > -again:
> >  	error = flock_to_posix_lock(filp, file_lock, &flock);
> >  	if (error)
> >  		goto out;
> > @@ -2231,9 +2230,11 @@ again:
> >  	spin_lock(&current->files->file_lock);
> >  	f = fcheck(fd);
> >  	spin_unlock(&current->files->file_lock);
> > -	if (!error && f != filp && flock.l_type != F_UNLCK) {
> > -		flock.l_type = F_UNLCK;
> > -		goto again;
> > +	if (!error && f != filp && file_lock->fl_type != F_UNLCK) {
> > +		file_lock->fl_type = F_UNLCK;
> > +		error = do_lock_file_wait(filp, cmd, file_lock);
> > +		WARN_ON_ONCE(error);
> > +		error = -EBADF;
> >  	}
> >  
> >  out:
> > @@ -2321,7 +2322,6 @@ int fcntl_setlk64(unsigned int fd, struct file *filp, unsigned int cmd,
> >  		goto out;
> >  	}
> >  
> > -again:
> >  	error = flock64_to_posix_lock(filp, file_lock, &flock);
> >  	if (error)
> >  		goto out;
> > @@ -2366,11 +2366,12 @@ again:
> >  	spin_lock(&current->files->file_lock);
> >  	f = fcheck(fd);
> >  	spin_unlock(&current->files->file_lock);
> > -	if (!error && f != filp && flock.l_type != F_UNLCK) {
> > -		flock.l_type = F_UNLCK;
> > -		goto again;
> > +	if (!error && f != filp && file_lock->fl_type != F_UNLCK) {
> > +		file_lock->fl_type = F_UNLCK;
> > +		error = do_lock_file_wait(filp, cmd, file_lock);
> > +		WARN_ON_ONCE(error);
> > +		error = -EBADF;
> >  	}
> > -
> >  out:
> >  	locks_free_lock(file_lock);
> >  	return error;
> 
> While this does fix Dmitri's reproducer, I think the basic concept of
> removing locks like this after they are set is racy. Consider where we
> have two threads:
> 
> Thread1				Thread2
> ----------------------------------------------------------------------------
> fd1 = memfd_create(...);
> fd2 = dup(fd1);
> 				fcntl(fd2, F_SETLK);
> 				(Here we call fcntl, and lock is set, but
> 				 task gets scheduled out before fcheck)
> close(fd2)
> fcntl(fd1, F_SETLK...);
> 
> 				Task scheduled back in, does fcheck for fd2
> 				and finds that it's gone. Removes the lock
> 				that Thread1 just set.
> 
> So that seems wrong...in the face of the race above we can end up with
> no lock set on the file, even though Thread1 thinks it has one. It is a
> pretty unlikely race, but I don't see anything that prevents it.
> 
> The fix for filesystems that do not define their own ->lock op would be
> pretty simple. We could do a fcheck after taking the flc_lock, but
> before setting the lock on the file. The flc_lock should be enough to
> prevent that race (though we may need to revisit some of the lockless
> checks in locks_remove_posix). That wouldn't work for filesystems that
> do set ->lock though, and I think we really do need a more general
> solution there.
> 
> The good news is that OFD locks should be exempt from that fcheck
> altogether. I'll spin up another patch for that, so we can at least
> ensure that they aren't subject to that race.
> 
> Any thoughts on how to fix the above for traditional POSIX locks though?

This logic seems to fall into a common trap by assuming that the result
of a posix lock followed by an unlock is a no-op.  The assumption is
false because the region, or parts of it, may have been locked by the
same owner before the initial lock.  You might think you're free of that
logic since closing is a scorched-earth "remove every lock owned by this
owner" event, except that as you point out the lock+unlock isn't atomic
here....

OK, I'm just repeating what you've said really.  I don't know how to fix
it.

--b.
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Patch
diff mbox

diff --git a/fs/locks.c b/fs/locks.c
index 593dca300b29..0db640e4ced4 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -2181,7 +2181,6 @@  int fcntl_setlk(unsigned int fd, struct file *filp, unsigned int cmd,
 		goto out;
 	}
 
-again:
 	error = flock_to_posix_lock(filp, file_lock, &flock);
 	if (error)
 		goto out;
@@ -2231,9 +2230,11 @@  again:
 	spin_lock(&current->files->file_lock);
 	f = fcheck(fd);
 	spin_unlock(&current->files->file_lock);
-	if (!error && f != filp && flock.l_type != F_UNLCK) {
-		flock.l_type = F_UNLCK;
-		goto again;
+	if (!error && f != filp && file_lock->fl_type != F_UNLCK) {
+		file_lock->fl_type = F_UNLCK;
+		error = do_lock_file_wait(filp, cmd, file_lock);
+		WARN_ON_ONCE(error);
+		error = -EBADF;
 	}
 
 out:
@@ -2321,7 +2322,6 @@  int fcntl_setlk64(unsigned int fd, struct file *filp, unsigned int cmd,
 		goto out;
 	}
 
-again:
 	error = flock64_to_posix_lock(filp, file_lock, &flock);
 	if (error)
 		goto out;
@@ -2366,11 +2366,12 @@  again:
 	spin_lock(&current->files->file_lock);
 	f = fcheck(fd);
 	spin_unlock(&current->files->file_lock);
-	if (!error && f != filp && flock.l_type != F_UNLCK) {
-		flock.l_type = F_UNLCK;
-		goto again;
+	if (!error && f != filp && file_lock->fl_type != F_UNLCK) {
+		file_lock->fl_type = F_UNLCK;
+		error = do_lock_file_wait(filp, cmd, file_lock);
+		WARN_ON_ONCE(error);
+		error = -EBADF;
 	}
-
 out:
 	locks_free_lock(file_lock);
 	return error;