Messages in this thread | | | Date | Thu, 7 Jan 2021 13:17:55 +0100 | From | Greg Kroah-Hartman <> | Subject | Re: [PATCH v2 4.9 00/10] fix a race in release_task when flushing the dentry |
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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
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