From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 4C6FFC83F1A for ; Mon, 21 Jul 2025 20:51:10 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1udxSG-0003MV-IL; Mon, 21 Jul 2025 16:49:48 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1udxQT-0001v4-86 for qemu-devel@nongnu.org; Mon, 21 Jul 2025 16:47:57 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1udxQR-000157-70 for qemu-devel@nongnu.org; Mon, 21 Jul 2025 16:47:56 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1753130874; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=QBSbbaAKyl5aY4HyJSJ+sf8E1dv6K+OA1Yu7iM9qnfg=; b=JuWLWr6e3DcsmIrc3vctBPsJKHmV5GIaNmguOjBw/B0u90qksVal3SLDCRVtZqyv+gydKd RnJTTQhgAFbd3to0iUWY1VjyarRARt88kbYItNrUQsLG5FExk1/83zUY23GdZwMxIVcHNQ pOSBFrJdHHdyFhcphaJTQfbcaoC+oas= Received: from mx-prod-mc-02.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-678-Gf1YD_ZAOgqCUUgwxzXHFg-1; Mon, 21 Jul 2025 16:47:50 -0400 X-MC-Unique: Gf1YD_ZAOgqCUUgwxzXHFg-1 X-Mimecast-MFC-AGG-ID: Gf1YD_ZAOgqCUUgwxzXHFg_1753130869 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-02.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id EBF58195608B; Mon, 21 Jul 2025 20:47:48 +0000 (UTC) Received: from localhost (unknown [10.2.16.31]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id C73CB1966650; Mon, 21 Jul 2025 20:47:47 +0000 (UTC) Date: Mon, 21 Jul 2025 16:47:46 -0400 From: Stefan Hajnoczi To: Kevin Wolf Cc: qemu-devel@nongnu.org, Hanna Reitz , Stefan Weil , Paolo Bonzini , Fam Zheng , eblake@redhat.com, Stefano Garzarella , qemu-block@nongnu.org, Aarushi Mehta , hibriansong@gmail.com Subject: Re: [PATCH v2 01/12] aio-posix: fix race between io_uring CQE and AioHandler deletion Message-ID: <20250721204746.GA84885@fedora> References: <20250620000829.1426291-1-stefanha@redhat.com> <20250620000829.1426291-2-stefanha@redhat.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="pq8ywfv3rKnbJJ9N" Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 Received-SPF: pass client-ip=170.10.133.124; envelope-from=stefanha@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -29 X-Spam_score: -3.0 X-Spam_bar: --- X-Spam_report: (-3.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.926, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org --pq8ywfv3rKnbJJ9N Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Wed, Jul 02, 2025 at 02:10:45PM +0200, Kevin Wolf wrote: > Am 20.06.2025 um 02:08 hat Stefan Hajnoczi geschrieben: > > When an AioHandler is enqueued on ctx->submit_list for removal, the > > fill_sq_ring() function will submit an io_uring POLL_REMOVE operation to > > cancel the in-flight POLL_ADD operation. > >=20 > > There is a race when another thread enqueues an AioHandler for deletion > > on ctx->submit_list when the POLL_ADD CQE has already appeared. In that > > case POLL_REMOVE is unnecessary. The code already handled this, but > > forgot that the AioHandler itself is still on ctx->submit_list when the > > POLL_ADD CQE is being processed. It's unsafe to delete the AioHandler at > > that point in time (use-after-free). > >=20 > > Solve this problem by keeping the AioHandler alive but setting a flag so > > that it will be deleted by fill_sq_ring() when it runs. > >=20 > > Signed-off-by: Stefan Hajnoczi > > --- > > util/fdmon-io_uring.c | 26 +++++++++++++++++++------- > > 1 file changed, 19 insertions(+), 7 deletions(-) > >=20 > > diff --git a/util/fdmon-io_uring.c b/util/fdmon-io_uring.c > > index b0d68bdc44..2e40fff09a 100644 > > --- a/util/fdmon-io_uring.c > > +++ b/util/fdmon-io_uring.c > > @@ -52,9 +52,10 @@ enum { > > FDMON_IO_URING_ENTRIES =3D 128, /* sq/cq ring size */ > > =20 > > /* AioHandler::flags */ > > - FDMON_IO_URING_PENDING =3D (1 << 0), > > - FDMON_IO_URING_ADD =3D (1 << 1), > > - FDMON_IO_URING_REMOVE =3D (1 << 2), > > + FDMON_IO_URING_PENDING =3D (1 << 0), > > + FDMON_IO_URING_ADD =3D (1 << 1), > > + FDMON_IO_URING_REMOVE =3D (1 << 2), > > + FDMON_IO_URING_DELETE_AIO_HANDLER =3D (1 << 3), > > }; > > =20 > > static inline int poll_events_from_pfd(int pfd_events) > > @@ -218,6 +219,9 @@ static void fill_sq_ring(AioContext *ctx) > > if (flags & FDMON_IO_URING_REMOVE) { > > add_poll_remove_sqe(ctx, node); > > } > > + if (flags & FDMON_IO_URING_DELETE_AIO_HANDLER) { > > + QLIST_INSERT_HEAD_RCU(&ctx->deleted_aio_handlers, node, no= de_deleted); > > + } > > } > > } >=20 > Why is it safe to add new SQEs for the node and then add it to > ctx->deleted_aio_handlers without waiting for the CQEs first? I > expected this to be the first check in the loop iteration and to > contain a 'continue;' statement. >=20 > The POLL_REMOVE case is clear when looking at more context, it doesn't > pass the node. As for POLL_ADD, I suppose both flags are actually never > set together in practice because FDMON_IO_URING_DELETE_AIO_HANDLER is > only set when processing the CQE of POLL_ADD, so no new POLL_ADD for the > same node will be pending yet. And checking the callers, I see that > adding is only ever done with newly allocated nodes, so something like > removing and re-adding the same node doesn't happen either. >=20 > Could we then assert that FDMON_IO_URING_DELETE_AIO_HANDLER is never > combined with FDMON_IO_URING_ADD, but always with FDMON_IO_URING_REMOVE, > to make the assumptions more explicit? An update to my previous reply: REMOVE and DELETE_AIO_HANDLER are never set simultaneously: flags =3D qatomic_fetch_and(&node->flags, ~FDMON_IO_URING_REMOVE); ^^ REMOVE is cleared here ^^ if (flags & FDMON_IO_URING_REMOVE) { if (flags & FDMON_IO_URING_PENDING) { /* Still on ctx->submit_list, defer deletion until fill_sq_ring= () */ qatomic_or(&node->flags, FDMON_IO_URING_DELETE_AIO_HANDLER); ^^ DELETE_AIO_HANDLER is set here ^^ So the assertion should be: if (flags & FDMON_IO_URING_DELETE_AIO_HANDLER) { /* * process_cqe() sets this flag after ADD and REMOVE have been * cleared. They cannot be set again, so they must be clear. */ assert(!(flags & FDMON_IO_URING_ADD)); assert(!(flags & FDMON_IO_URING_REMOVE)); QLIST_INSERT_HEAD_RCU(&ctx->deleted_aio_handlers, node, node_deleted); } >=20 > > @@ -347,10 +356,13 @@ void fdmon_io_uring_destroy(AioContext *ctx) > > unsigned flags =3D qatomic_fetch_and(&node->flags, > > ~(FDMON_IO_URING_PENDING | > > FDMON_IO_URING_ADD | > > - FDMON_IO_URING_REMOVE)); > > + FDMON_IO_URING_REMOVE | > > + FDMON_IO_URING_DELETE_AIO_HANDLER)); > > =20 > > - if (flags & FDMON_IO_URING_REMOVE) { > > - QLIST_INSERT_HEAD_RCU(&ctx->deleted_aio_handlers, node= , node_deleted); > > + if ((flags & FDMON_IO_URING_REMOVE) || > > + (flags & FDMON_IO_URING_DELETE_AIO_HANDLER)) { >=20 > If my conclusion above is right, FDMON_IO_URING_REMOVE will be set in > both cases, so checking FDMON_IO_URING_DELETE_AIO_HANDLER is redundant. > Maybe assert this, too, when setting FDMON_IO_URING_DELETE_AIO_HANDLER. We need both here because either REMOVE or DELETE_AIO_HANDLER can be set, but not both. Stefan --pq8ywfv3rKnbJJ9N Content-Type: application/pgp-signature; name=signature.asc -----BEGIN PGP SIGNATURE----- iQEzBAEBCgAdFiEEhpWov9P5fNqsNXdanKSrs4Grc8gFAmh+p3IACgkQnKSrs4Gr c8gCqAf+MMq07x6zYtF+POVtDN11Ib7UmDZ6yevFkl/1DpMfdc26ECw/jQdGi2Yn +Opwqvk3GVK5fMISEJ/s8qSRmfoiww/3aDcgHygZScCN8UQwYVo14Hyx3V9kgtAu ZRsMG8vSisoLrCn8OxOmV7eenQmUA7Hzhis8fZj4hkUGIIdx5zx9JiiNLAQEOOgz mb79c7tMWVYpVQrkfoe/RGcXw/nNicupLY6LgNq+0XObFiwtuLAxMCyXeKXE6Z/n eE38TTY6YyzE2u01gaLRE6Pu9I4PcXMI+aTlPgon42j/CDSzLYX74zM2bM6g9jdf N/aHR7A4HMWiPyVB0mkYcyZvA/kzbg== =5R/k -----END PGP SIGNATURE----- --pq8ywfv3rKnbJJ9N--