From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: Wen Yang <wenyang@linux.alibaba.com>
Cc: Sasha Levin <sashal@kernel.org>,
Xunlei Pang <xlpang@linux.alibaba.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 4.9 00/10] fix a race in release_task when flushing the dentry
Date: Thu, 7 Jan 2021 13:17:55 +0100 [thread overview]
Message-ID: <X/b781Kwn48xq8aS@kroah.com> (raw)
In-Reply-To: <20210107075222.62623-1-wenyang@linux.alibaba.com>
On Thu, Jan 07, 2021 at 03:52:12PM +0800, Wen Yang wrote:
> The dentries such as /proc/<pid>/ns/ have the DCACHE_OP_DELETE flag, they
> should be deleted when the process exits.
>
> Suppose the following race appears：
>
> release_task dput
> -> proc_flush_task
> -> dentry->d_op->d_delete(dentry)
> -> __exit_signal
> -> dentry->d_lockref.count-- and return.
>
> In the proc_flush_task(), if another process is using this dentry, it will
> not be deleted. At the same time, in dput(), d_op->d_delete() can be executed
> before __exit_signal(pid has not been hashed), d_delete returns false, so
> this dentry still cannot be deleted.
>
> This dentry will always be cached (although its count is 0 and the
> DCACHE_OP_DELETE flag is set), its parent denry will also be cached too, and
> these dentries can only be deleted when drop_caches is manually triggered.
>
> This will result in wasted memory. What's more troublesome is that these
> dentries reference pid, according to the commit f333c700c610 ("pidns: Add a
> limit on the number of pid namespaces"), if the pid cannot be released, it
> may result in the inability to create a new pid_ns.
>
> This issue was introduced by 60347f6716aa ("pid namespaces: prepare
> proc_flust_task() to flush entries from multiple proc trees"), exposed by
> f333c700c610 ("pidns: Add a limit on the number of pid namespaces"), and then
> fixed by 7bc3e6e55acf ("proc: Use a list of inodes to flush from proc").
Why are you just submitting a series for 4.9 and 4.19, what about 4.14?
We can't have users move to a newer kernel and then experience old bugs,
right?
But the larger question is why are you backporting a whole new feature
here? Why is CLONE_PIDFD needed? That feels really wrong...
thanks,
greg k-h
next prev parent reply other threads:[~2021-01-07 12:17 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-07 7:52 [PATCH v2 4.9 00/10] fix a race in release_task when flushing the dentry Wen Yang
2021-01-07 7:52 ` [PATCH v2 4.9 01/10] clone: add CLONE_PIDFD Wen Yang
2021-01-07 7:52 ` [PATCH v2 4.9 02/10] pidfd: add polling support Wen Yang
2021-01-07 7:52 ` [PATCH v2 4.9 03/10] proc: Pass file mode to proc_pid_make_inode Wen Yang
2021-01-07 7:52 ` [PATCH v2 4.9 04/10] proc: Better ownership of files for non-dumpable tasks in user namespaces Wen Yang
2021-01-07 7:52 ` [PATCH v2 4.9 05/10] proc: use %u for pid printing and slightly less stack Wen Yang
2021-01-07 7:52 ` [PATCH v2 4.9 06/10] proc: Rename in proc_inode rename sysctl_inodes sibling_inodes Wen Yang
2021-01-07 7:52 ` [PATCH v2 4.9 07/10] proc: Generalize proc_sys_prune_dcache into proc_prune_siblings_dcache Wen Yang
2021-01-07 7:52 ` [PATCH v2 4.9 08/10] proc: Clear the pieces of proc_inode that proc_evict_inode cares about Wen Yang
2021-01-07 7:52 ` [PATCH v2 4.9 09/10] proc: Use d_invalidate in proc_prune_siblings_dcache Wen Yang
2021-01-07 7:52 ` [PATCH v2 4.9 10/10] proc: Use a list of inodes to flush from proc Wen Yang
2021-01-07 12:17 ` Greg Kroah-Hartman [this message]
2021-01-07 16:21 ` [PATCH v2 4.9 00/10] fix a race in release_task when flushing the dentry Wen Yang
2021-01-07 18:28 ` Greg Kroah-Hartman
2021-01-08 2:42 ` Wen Yang
2021-01-09 12:36 ` Greg Kroah-Hartman
-- strict thread matches above, loose matches on Subject: below --
2021-01-07 7:34 Wen Yang
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