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Thu, 08 Jul 2021 06:41:00 +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> <5a9dcc69-385a-eda5-6974-cb962ae62601@suse.com> From: Juergen Gross Message-ID: <7f606c27-173a-542d-406d-196dae784edd@suse.com> Date: Thu, 8 Jul 2021 08:40:59 +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: <5a9dcc69-385a-eda5-6974-cb962ae62601@suse.com> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="Ja2XZNSE8nO4IrLKl6sE4EnuPN7605kJy" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --Ja2XZNSE8nO4IrLKl6sE4EnuPN7605kJy Content-Type: multipart/mixed; boundary="LM4uTaxxPZvHMHYeVdJJN6jdt4QkBjnKN"; 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: <7f606c27-173a-542d-406d-196dae784edd@suse.com> 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> <5a9dcc69-385a-eda5-6974-cb962ae62601@suse.com> In-Reply-To: <5a9dcc69-385a-eda5-6974-cb962ae62601@suse.com> --LM4uTaxxPZvHMHYeVdJJN6jdt4QkBjnKN Content-Type: multipart/mixed; boundary="------------2D62ABF81D642597E69B3D65" Content-Language: en-US This is a multi-part message in MIME format. --------------2D62ABF81D642597E69B3D65 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: quoted-printable On 08.07.21 08:37, Jan Beulich wrote: > On 08.07.2021 07:47, Juergen Gross wrote: >> 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 i= rq, 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= the >>>>>> check is in no way depending on the responses themselves. And any = change >>>>>> 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 val= ue >>>>> and then (via a separate read) use a different one past the barrier= =2E >>>> >>>> Yes and no. >>>> >>>> rsp_cons should never be written by the other side, and additionally= >>>> it would be read multiple times anyway. >>> >>> 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 r= p >> instead? >=20 > Not sure - the rmb() is needed anyway aiui, and hence you could as well= > move your code addition. Sure. My question was rather: does the rmb() really eliminate the possibility of a double read introduced by the compiler? If yes, moving the code is the correct solution. 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