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Date:   Tue, 19 May 2020 07:49:39 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Pavel Machek <pavel@...x.de>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org,
        syzbot+c8a8197c8852f566b9d9@...kaller.appspotmail.com,
        syzbot+40b71e145e73f78f81ad@...kaller.appspotmail.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 4.19 02/80] shmem: fix possible deadlocks on
 shmlock_user_lock

On Mon, May 18, 2020 at 06:10:58PM -0700, Hugh Dickins wrote:
> Hi Pavel,
> 
> On Mon, 18 May 2020, Pavel Machek wrote:
> 
> > Hi!
> > 
> > > This may not risk an actual deadlock, since shmem inodes do not take
> > > part in writeback accounting, but there are several easy ways to avoid
> > > it.
> > 
> > ...
> > 
> > > Take info->lock out of the chain and the possibility of deadlock or
> > > lockdep warning goes away.
> > 
> > It is unclear to me if actual possibility of deadlock exists or not,
> > but anyway:
> > 
> > >  	int retval = -ENOMEM;
> > >  
> > > -	spin_lock_irq(&info->lock);
> > > +	/*
> > > +	 * What serializes the accesses to info->flags?
> > > +	 * ipc_lock_object() when called from shmctl_do_lock(),
> > > +	 * no serialization needed when called from shm_destroy().
> > > +	 */
> > >  	if (lock && !(info->flags & VM_LOCKED)) {
> > >  		if (!user_shm_lock(inode->i_size, user))
> > >  			goto out_nomem;
> > 
> > Should we have READ_ONCE() here? If it is okay, are concurency
> > sanitizers smart enough to realize that it is okay? Replacing warning
> > with different one would not be exactly a win...
> 
> If a sanitizer comes to question this change, I don't see how a
> READ_ONCE() anywhere near here (on info->flags?) is likely to be
> enough to satisfy it - it would be asking for a locking scheme that
> it understands (being unable to read the comment) - and might then
> ask for that same locking in the numerous other places that read
> info->flags (and a few that write it).  Add data_race()s all over?
> 
> (Or are you concerned about that inode->i_size, which I suppose ought
> really to be i_size_read(inode) on some 32-bit configurations; though
> that's of very long standing, and has never caused any concern before.)
> 
> I am not at all willing to add annotations speculatively, in case this
> or that tool turns out to want help later.  So far I've not heard of
> any such complaint on 5.7-rc[3456] or linux-next: but maybe this is
> too soon to hear a complaint, and you feel this should not be rushed
> into -stable?
> 
> This was an AUTOSEL selection, to which I have no objection, but it
> isn't something we were desperate to push into -stable: so I've also
> no objection if Greg shares your concern, and prefers to withdraw it.
> (That choice may depend on to what extent he expects to be keeping
> -stable clean against upcoming sanitizers in future.)

Sanitizers run on stable trees all the time as that's the releases that
ends up on products, where people run them.  That's why I like to take
those types of fixes, especially when tools report them.

thanks,

greg k-h

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