On Wednesday, February 26, 2014 4:22:00 pm Mateusz Guzik wrote: > On Wed, Feb 26, 2014 at 03:29:31PM -0500, John Baldwin wrote: > > On Wednesday, February 26, 2014 2:23:48 pm Mateusz Guzik wrote: > > > On Tue, Feb 25, 2014 at 01:08:43PM -0500, John Baldwin wrote: > > > > On Friday, February 21, 2014 5:29:09 pm Mateusz Guzik wrote: > > > > > Author: mjg > > > > > Date: Fri Feb 21 22:29:09 2014 > > > > > New Revision: 262309 > > > > > URL: http://svnweb.freebsd.org/changeset/base/262309 > > > > > > > > > > Log: > > > > > Fix a race between kern_proc_{o,}filedesc_out and fdescfree leading > > > > > to use-after-free. > > > > > > > > > > fdescfree proceeds to free file pointers once fd_refcnt reaches 0, but > > > > > kern_proc_{o,}filedesc_out only checked for hold count. > > > > > > > > Can you describe the race in more detail? The kern_* routines hold > > > > FILEDESC_SLOCK() while they read the file which should prevent > > > > fdescfree() from free'ing any files. Note that fdfree() (called > > > > under FILEDESC_XLOCK() clears the file pointer to NULL via the > > > > bzero(), so the sysctl handler should only see non-NULL pointers > > > > for files that are not yet free'd. > > > > > > > > > > oops, you are right. I somehow misread the code. > > > > > > Still, the change is harmless and matches other loop which iterates the > > > table (see sysctl_kern_file), so I think it can stay. > > > > Actually, I think that should be undone. That was a hacky workaround for > > a long time ago when the FILEDESC locking wasn't a simple sx lock but was > > a mtx + sleep lock combo thing that had fast and slow variants. > > > > > Other thing is that with that change in place we can get rid of > > > XLOCK/XUNLOCK around fdfree in fdescfree. > > > > I would rather remove the fd_refcnt checks, or do them differently (not > > in the loop). Right now a reader might be confused to think that > > fd_refcnt can change within the loop when it cannot and then worry about > > unhandled races that don't exist (i.e. if fd_refcnt can change within > > the loop, what prevents the individual file objects from being freed out > > from under the loop?) > > > > But it can change. > > kern_proc_filedesc_out calls export_fd_to_sb which drops the lock for > each fp and sysctl_kern_proc_ofiledesc drops the lock when dealing with > vnodes. > > As far as I can say all this is safe - either data is refed (vref on a > vnode) or the lock is still held while the data is being read, so by the > time fp can be freed it is no longer used.
Ugh, ok. Then the change is fine as-is, but I think we have to leave the locking in place around fdfree() still as a result. -- John Baldwin _______________________________________________ svn-src-all@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/svn-src-all To unsubscribe, send any mail to "svn-src-all-unsubscr...@freebsd.org"
