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Thu, 08 Jul 2021 05:47:06 +0000 Subject: Re: [PATCH 4/8] xen/blkfront: don't trust the backend response data blindly To: Jan Beulich Cc: Konrad Rzeszutek Wilk , =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= , Boris Ostrovsky , Stefano Stabellini , Jens Axboe , xen-devel@lists.xenproject.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org References: <20210513100302.22027-1-jgross@suse.com> <20210513100302.22027-5-jgross@suse.com> <315ad8b9-8a98-8d3e-f66c-ab32af2731a8@suse.com> <6095c4b9-a9bb-8a38-fb6c-a5483105b802@suse.com> <030ef85e-b5af-f46e-c8dc-88b8d195c4e1@suse.com> <477f01cd-8793-705c-10f9-cf0c0cd6ed84@suse.com> From: Juergen Gross Message-ID: Date: Thu, 8 Jul 2021 07:47:06 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <477f01cd-8793-705c-10f9-cf0c0cd6ed84@suse.com> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="65Cl9OH3VxhoDnxdHBagUmL999Yvk5KRj" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --65Cl9OH3VxhoDnxdHBagUmL999Yvk5KRj Content-Type: multipart/mixed; boundary="TaRSfQAduiJvbDCSd5wNF88LIRKqIxccY"; protected-headers="v1" From: Juergen Gross To: Jan Beulich Cc: Konrad Rzeszutek Wilk , =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= , Boris Ostrovsky , Stefano Stabellini , Jens Axboe , xen-devel@lists.xenproject.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org Message-ID: Subject: Re: [PATCH 4/8] xen/blkfront: don't trust the backend response data blindly References: <20210513100302.22027-1-jgross@suse.com> <20210513100302.22027-5-jgross@suse.com> <315ad8b9-8a98-8d3e-f66c-ab32af2731a8@suse.com> <6095c4b9-a9bb-8a38-fb6c-a5483105b802@suse.com> <030ef85e-b5af-f46e-c8dc-88b8d195c4e1@suse.com> <477f01cd-8793-705c-10f9-cf0c0cd6ed84@suse.com> In-Reply-To: <477f01cd-8793-705c-10f9-cf0c0cd6ed84@suse.com> --TaRSfQAduiJvbDCSd5wNF88LIRKqIxccY Content-Type: multipart/mixed; boundary="------------1299B9F7BEB5CBBFFB8C69D9" Content-Language: en-US This is a multi-part message in MIME format. --------------1299B9F7BEB5CBBFFB8C69D9 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: quoted-printable On 17.05.21 17:33, Jan Beulich wrote: > On 17.05.2021 17:22, Juergen Gross wrote: >> On 17.05.21 17:12, Jan Beulich wrote: >>> On 17.05.2021 16:23, Juergen Gross wrote: >>>> On 17.05.21 16:11, Jan Beulich wrote: >>>>> On 13.05.2021 12:02, Juergen Gross wrote: >>>>>> @@ -1574,10 +1580,16 @@ static irqreturn_t blkif_interrupt(int irq= , void *dev_id) >>>>>> spin_lock_irqsave(&rinfo->ring_lock, flags); >>>>>> again: >>>>>> rp =3D rinfo->ring.sring->rsp_prod; >>>>>> + if (RING_RESPONSE_PROD_OVERFLOW(&rinfo->ring, rp)) { >>>>>> + pr_alert("%s: illegal number of responses %u\n", >>>>>> + info->gd->disk_name, rp - rinfo->ring.rsp_cons); >>>>>> + goto err; >>>>>> + } >>>>>> rmb(); /* Ensure we see queued responses up to 'rp'. */ >>>>> >>>>> I think you want to insert after the barrier. >>>> >>>> Why? The relevant variable which is checked is "rp". The result of t= he >>>> check is in no way depending on the responses themselves. And any ch= ange >>>> of rsp_cons is protected by ring_lock, so there is no possibility of= >>>> reading an old value here. >>> >>> But this is a standard double read situation: You might check a value= >>> and then (via a separate read) use a different one past the barrier. >> >> Yes and no. >> >> rsp_cons should never be written by the other side, and additionally >> it would be read multiple times anyway. >=20 > But I'm talking about rsp_prod, as that's what rp gets loaded from. Oh, now I get your problem. But shouldn't that better be solved by using READ_ONCE() for reading rp instead? 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