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From: John Baldwin <jhb@freebsd.org>
To: Mateusz Guzik <mjg@freebsd.org>
Subject: Re: svn commit: r262309 - head/sys/kern
Date: Tue, 25 Feb 2014 13:08:43 -0500
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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.

-- 
John Baldwin