linux-kernel - Re: fs/dcache.c - BUG: soft lockup - CPU#5 stuck for 22s! [systemd-udevd:1667]

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: <CA+55aFwNv2-9=uXxzYH9UpN_x209Cv_BBu-9j2prSh2xVmmsAg@mail.gmail.com>
Date:	Thu, 29 May 2014 12:14:51 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Miklos Szeredi <mszeredi@...e.cz>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: fs/dcache.c - BUG: soft lockup - CPU#5 stuck for 22s! [systemd-udevd:1667]

On Thu, May 29, 2014 at 11:52 AM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> Grrr...  Sadly, that's not good enough.  Leaking rcu_read_lock() on
> success is trivial, but there's more serious problem: suppose dentries
> involved get moved before we get to locking what we thought was parent.
> We end up taking ->d_lock on two dentries that might be nowhere near each
> other in the tree, with obvious nasty implications.  Would be _very_ hard
> to reproduce ;-/

Yeah, I don't think you can reproduce that, but I guess renaming
directories into each other (two renames needed) could trigger an ABBA
deadlock by changing the topological order of dentry/parent.

I suspect there's no way in hell that tiny race will ever happen in
practice, but let's not risk it.

And your solution (to re-check after just taking the parent lock)
seems sufficient and sane, since dentry_lock_for_move() will always
take the parent lock(s) before we move a dentry.

So that looks good to me.

              Linus
--
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/