From: Al Viro <viro@zeniv.linux.org.uk>
To: Amir Goldstein <amir73il@gmail.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>,
Dmitry Vyukov <dvyukov@google.com>,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
syzkaller-bugs@googlegroups.com
Subject: Re: [PATCH v2] acct: fix possible deadlock in acct_pin_kill
Date: Thu, 4 Apr 2019 19:44:48 +0100 [thread overview]
Message-ID: <20190404184448.GC2217@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20190404105255.12189-1-amir73il@gmail.com>
On Thu, Apr 04, 2019 at 01:52:55PM +0300, Amir Goldstein wrote:
> This looks like an old bug, pre-dating the "Fixes" commit, but the
> "Fixes" commit did not handle it properly.
>
> The bug recently surfaced as a lockdep possible deadlock warning
> with commit d1d04ef8572b ("ovl: stack file ops").
>
> When acct_on() replaces one acct file with another, it takes sb_writers
> lock on new file sb and calls acct_pin_kill(old) before releasing the
> sb_writers lock.
>
> If new file is on the same fs as old file, acct_pin_kill(old) fail to
> file_start_write_trylock() and skip writing the old file, because
> sb_writers (of new) is already taken by acct_on().
>
> If new file is not on same fs as old file, this ordering violation
> creates an unneeded dependency between new sb_writers and old sb_writers,
> which may later be reported as possible deadlock.
>
> This could result in an actual deadlock if acct file is replaced from
> an old file in overlayfs over "real fs" to a new file in "real fs".
> acct_on() takes freeze protection on "real fs" and tries to write to
> overlayfs file. overlayfs is not freeze protected so do_acct_process()
> can carry on with __kernel_write() to overlayfs file, which would
> try to take freeze protection on "real fs" and deadlock.
Huh? sb_writers is taken when we *open* the new file. Then we replace
its ->path.mnt with a clone and transfer the write count from the original
to new one. And close the old file while we are at it.
From sb_writers POV mainline has
sb_start_write(new_sb) // in open
sb_start_write(new_sb) // mnt_want_write() on clone
last write to old_sb, then sb_end_write(old_sb) // acct_pin_kill()
sb_end_write(new_sb) // mnt_drop_write(mnt)
and you flip the order of the last two lines.
Could you explain how exactly does your patch help whatever
problem overlayfs has?
next prev parent reply other threads:[~2019-04-04 18:44 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-04 10:52 [PATCH v2] acct: fix possible deadlock in acct_pin_kill Amir Goldstein
2019-04-04 18:44 ` Al Viro [this message]
2019-04-04 18:49 ` Al Viro
2019-04-04 19:05 ` Al Viro
2019-04-04 19:33 ` Amir Goldstein
2019-04-11 19:10 ` Amir Goldstein
2019-04-04 19:19 ` Al Viro
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