linux-kernel - Re: fs/namei.c: Misuse of sequence counts?

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20141012042925.GN7996@ZenIV.linux.org.uk>
Date:	Sun, 12 Oct 2014 05:29:25 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Eric Biggers <ebiggers3@...il.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: fs/namei.c: Misuse of sequence counts?

On Sat, Oct 11, 2014 at 10:55:10PM -0500, Eric Biggers wrote:
> On Sun, Oct 12, 2014 at 12:46:35AM +0100, Al Viro wrote:
> > 
> > Nope.  What we do is
> > 	* pick parent inode and seqcount (in whatever order)
> > 	* THEN check that child is still unchanged.
> > The second part guarantees that parent dentry had been the parent of
> > child all along, since the moment we'd first fetched _child's_ seqcount.
> > And since a pinned positive dentry can't have its ->d_inode changed,
> > we know that the value of parent's inode we'd fetched remained valid
> > at least until we'd checked the child's seqcount and found it unchanged.
> > Which means that we had it valid at some point after we'd fetched parent's
> > seqcount.
> 
> Ah, very tricky.  And I take it that the other two fetches of d_inode in
> follow_dotdot_rcu() can likewise be unordered with respect to
> read_seqcount_begin(), because the underlying dentries are pinned as either
> mnt_mountpoint or mnt_root ---  which in RCU mode, is only guaranteed because of
> the call to synchronize_rcu() in namespace_unlock() prior to dropping
> references?

The last one is actually covered by read_seqretry(&mount_lock, nd->m_seq) -
if it still matches, we know that whatever we got from __lookup_mnt() must
have been valid through fetching ->d_inode and ->d_seq of its mnt_root.
Which means that those two are consistent regardless of that synchronize_rcu().

The one before it would probably be better off with similar check on mount_lock
as well.  That code *is* correct for the reason you've mentioned, but I wonder
if explicit check of mount_lock would be better - right now it's more subtle
than I'd like it to be.  I don't think the cost would be noticable - it's
smp_rmb() + fetch + comparison when we cross a mountpoint while following ..
in lazy pathwalk, but that needs profiling - handwaving is not good enough...
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/