svn commit: r262309 - head/sys/kern
John Baldwin
jhb at freebsd.org
Wed Feb 26 20:29:51 UTC 2014
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?)
I originally thought just that when I saw this commit and was going to
note that your change was not a true fix and still left the race open
until I found upon further investigation that your change was just
noise since there wasn't a race to begin with. In the interest of
clarity, please either remove the fd_refcnt checks from these loops
(including sysctl_kern_file) or fix all three of them to do a single
check after the SLOCK() but before the for loop itself.
--
John Baldwin
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