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Tue, 2 Jun 2020 09:39:29 +0000 (UTC) MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Date: Tue, 02 Jun 2020 15:09:29 +0530 From: guptap@codeaurora.org To: Robin Murphy Cc: Andrew Morton , mhocko@suse.com, joro@8bytes.org, linux-mm@kvack.org, iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org, owner-linux-mm@kvack.org, stable@vger.kernel.org Subject: Re: [PATCH] iommu/dma: limit iova free size to unmmaped iova In-Reply-To: <4ba082d3bb965524157704ea1ffb1ff4@codeaurora.org> References: <20200521113004.12438-1-guptap@codeaurora.org> <7aaa8dcc-6a47-f256-431d-2a1b034b4076@arm.com> <90662ef3123dbf2e93f9718ee5cc14a7@codeaurora.org> <2d873ab9-ebb9-3c2d-f129-55a036ab47d0@arm.com> <4ba082d3bb965524157704ea1ffb1ff4@codeaurora.org> Message-ID: <2bfb4ce3a2dfeb2d585f0308a9002feb@codeaurora.org> X-Sender: guptap@codeaurora.org User-Agent: Roundcube Webmail/1.3.9 X-Rspamd-Queue-Id: 9B770989E X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam03 Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 2020-05-26 12:49, guptap@codeaurora.org wrote: > On 2020-05-22 14:54, Robin Murphy wrote: >> On 2020-05-22 07:25, guptap@codeaurora.org wrote: >>> On 2020-05-22 01:46, Robin Murphy wrote: >>>> On 2020-05-21 12:30, Prakash Gupta wrote: >>> I agree, we shouldn't be freeing the partial iova. Instead just=20 >>> making >>> sure if unmap was successful should be sufficient before freeing=20 >>> iova. So change >>> can instead be something like this: >>>=20 >>> -=C2=A0=C2=A0=C2=A0 iommu_dma_free_iova(cookie, dma_addr, size); >>> +=C2=A0=C2=A0=C2=A0 if (unmapped) >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 iommu_dma_free_iova(cooki= e, dma_addr, size); >>>=20 >>>> TBH my gut feeling here is that you're really just trying to treat a >>>> symptom of another bug elsewhere, namely some driver calling >>>> dma_unmap_* or dma_free_* with the wrong address or size in the=20 >>>> first >>>> place. >>>>=20 >>> This condition would arise only if driver calling dma_unmap/free_*=20 >>> with 0 >>> iova_pfn. This will be flagged with a warning during unmap but will=20 >>> trigger >>> panic later on while doing unrelated dma_map/unmap_*. If unmapped has= =20 >>> already >>> failed for invalid iova, there is no reason we should consider this=20 >>> as valid >>> iova and free. This part should be fixed. >>=20 >> I disagree. In general, if drivers call the DMA API incorrectly it is >> liable to lead to data loss, memory corruption, and various other >> unpleasant misbehaviour - it is not the DMA layer's job to attempt to >> paper over driver bugs. >>=20 >> There *is* an argument for downgrading the BUG_ON() in >> iova_magazine_free_pfns() to a WARN_ON(), since frankly it isn't a >> sufficiently serious condition to justify killing the whole machine >> immediately, but NAK to bodging the iommu-dma mid-layer to "fix" that. >> A serious bug already happened elsewhere, so trying to hide the >> fallout really doesn't help anyone. >>=20 > Sorry for delayed response, it was a long weekend. > I agree that invalid DMA API call can result in unexpected issues and=20 > client > should fix it, but then the present behavior makes it difficult to=20 > catch cases > when driver is making wrong DMA API calls. When invalid iova pfn is=20 > passed it > doesn't fail then and there, though DMA layer is aware of iova being=20 > invalid. It > fails much after that in the context of an valid map/unmap, with=20 > BUG_ON(). >=20 > Downgrading BUG_ON() to WARN_ON() in iova_magazine_free_pfns() will not= =20 > help > much as invalid iova will cause NULL pointer dereference. >=20 > I see no reason why DMA layer wants to free an iova for which unmapped=20 > failed. > IMHO queuing an invalid iova (which already failed unmap) to rcache=20 > which > eventually going to crash the system looks like iommu-dma layer issue. >=20 > Thanks, > Prakash ping?