linux-ext4 - Re: [PATCH 06/13] afs: fix __afs_break_callback() / afs_drop_open

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Message-ID: <20240205-haarausfall-wegbleiben-688f8b69552b@brauner>
Date: Mon, 5 Feb 2024 13:28:46 +0100
From: Christian Brauner <brauner@...nel.org>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: linux-fsdevel@...r.kernel.org, 
	Linus Torvalds <torvalds@...ux-foundation.org>, linux-ext4@...r.kernel.org, linux-nfs@...r.kernel.org, 
	Miklos Szeredi <miklos@...redi.hu>, linux-cifs@...r.kernel.org
Subject: Re: [PATCH 06/13] afs: fix __afs_break_callback() /
 afs_drop_open_mmap() race

On Sun, Feb 04, 2024 at 02:17:32AM +0000, Al Viro wrote:
> In __afs_break_callback() we might check ->cb_nr_mmap and if it's non-zero
> do queue_work(&vnode->cb_work).  In afs_drop_open_mmap() we decrement
> ->cb_nr_mmap and do flush_work(&vnode->cb_work) if it reaches zero.
> 
> The trouble is, there's nothing to prevent __afs_break_callback() from
> seeing ->cb_nr_mmap before the decrement and do queue_work() after both
> the decrement and flush_work().  If that happens, we might be in trouble -
> vnode might get freed before the queued work runs.
> 
> __afs_break_callback() is always done under ->cb_lock, so let's make
> sure that ->cb_nr_mmap can change from non-zero to zero while holding
> ->cb_lock (the spinlock component of it - it's a seqlock and we don't
> need to mess with the counter).
> 
> Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
> ---

Acked-by: Christian Brauner <brauner@...nel.org>