linux-kernel - Re: [PATCH] netfs: Add a check for NULL folioq in netfs_writeback_unlock

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Message-ID: <3951592.1729843553@warthog.procyon.org.uk>
Date: Fri, 25 Oct 2024 09:05:53 +0100
From: David Howells <dhowells@...hat.com>
To: Chang Yu <marcus.yu.56@...il.com>
Cc: dhowells@...hat.com, jlayton@...nel.org, netfs@...ts.linux.dev,
    linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
    syzbot+af5c06208fa71bf31b16@...kaller.appspotmail.com,
    skhan@...uxfoundation.org
Subject: Re: [PATCH] netfs: Add a check for NULL folioq in netfs_writeback_unlock_folios

Chang Yu <marcus.yu.56@...il.com> wrote:

> syzkaller reported a null-pointer dereference bug
> (https://syzkaller.appspot.com/bug?extid=af5c06208fa71bf31b16) in
> netfs_writeback_unlock_folios caused by passing a NULL folioq to
> folioq_folio. Fix by adding a check before entering the loop.

And, of course, the preceding:

	if (slot >= folioq_nr_slots(folioq)) {

doesn't oops because it doesn't actually dereference folioq.

However... if we get into this function, there absolutely *should* be at least
one folioq in the rolling buffer.  Part of the rolling buffer's method of
operation involves keeping at least one folioq around at all times so that we
don't need to use locks to add/remove from the queue.

Either the rolling buffer wasn't initialised yet (and it should be initialised
for all write requests by netfs_create_write_req()) or it has been destroyed
already.

Either way, your patch is, unfortunately, just covering up the symptoms rather
than fixing the root cause.  I suggest instead that we patch the function to
detect the empty rolling buffer up front, dump some information about the bad
request and return.

David