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[35.233.69.230]) by smtp.gmail.com with ESMTPSA id r10sm7196657wmh.45.2021.03.03.03.09.01 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 03 Mar 2021 03:09:01 -0800 (PST) Date: Wed, 3 Mar 2021 11:08:56 +0000 From: Quentin Perret To: Marc Zyngier Cc: Jia He , kvmarm@lists.cs.columbia.edu, James Morse , Julien Thierry , Suzuki K Poulose , Catalin Marinas , Will Deacon , Gavin Shan , Yanan Wang , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] KVM: arm64: Fix unaligned addr case in mmu walking Message-ID: References: <20210303024225.2591-1-justin.he@arm.com> <87sg5czhny.wl-maz@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87sg5czhny.wl-maz@kernel.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wednesday 03 Mar 2021 at 09:54:25 (+0000), Marc Zyngier wrote: > Hi Jia, > > On Wed, 03 Mar 2021 02:42:25 +0000, > Jia He wrote: > > > > If the start addr is not aligned with the granule size of that level. > > loop step size should be adjusted to boundary instead of simple > > kvm_granual_size(level) increment. Otherwise, some mmu entries might miss > > the chance to be walked through. > > E.g. Assume the unmap range [data->addr, data->end] is > > [0xff00ab2000,0xff00cb2000] in level 2 walking and NOT block mapping. > > When does this occur? Upgrade from page mappings to block? Swap out? > > > And the 1st part of that pmd entry is [0xff00ab2000,0xff00c00000]. The > > pmd value is 0x83fbd2c1002 (not valid entry). In this case, data->addr > > should be adjusted to 0xff00c00000 instead of 0xff00cb2000. > > Let me see if I understand this. Assuming 4k pages, the region > described above spans *two* 2M entries: > > (a) ff00ab2000-ff00c00000, part of ff00a00000-ff00c00000 > (b) ff00c00000-ff00db2000, part of ff00c00000-ff00e00000 > > (a) has no valid mapping, but (b) does. Because we fail to correctly > align on a block boundary when skipping (a), we also skip (b), which > is then left mapped. > > Did I get it right? If so, yes, this is... annoying. > > Understanding the circumstances this triggers in would be most > interesting. This current code seems to assume that we get ranges > aligned to mapping boundaries, but I seem to remember that the old > code did use the stage2_*_addr_end() helpers to deal with this case. > > Will: I don't think things have changed in that respect, right? Indeed we should still use stage2_*_addr_end(), especially in the unmap path that is mentioned here, so it would be helpful to have a little bit more context. > > Without this fix, userspace "segment fault" error can be easily > > triggered by running simple gVisor runsc cases on an Ampere Altra > > server: > > docker run --runtime=runsc -it --rm ubuntu /bin/bash > > > > In container: > > for i in `seq 1 100`;do ls;done > > The workload on its own isn't that interesting. What I'd like to > understand is what happens on the host during that time. > > > > > Reported-by: Howard Zhang > > Signed-off-by: Jia He > > --- > > arch/arm64/kvm/hyp/pgtable.c | 1 + > > 1 file changed, 1 insertion(+) > > > > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c > > index bdf8e55ed308..4d99d07c610c 100644 > > --- a/arch/arm64/kvm/hyp/pgtable.c > > +++ b/arch/arm64/kvm/hyp/pgtable.c > > @@ -225,6 +225,7 @@ static inline int __kvm_pgtable_visit(struct kvm_pgtable_walk_data *data, > > goto out; > > > > if (!table) { > > + data->addr = ALIGN_DOWN(data->addr, kvm_granule_size(level)); > > data->addr += kvm_granule_size(level); > > goto out; > > } > > It otherwise looks good to me. Quentin, Will: unless you object to > this, I plan to take it in the next round of fixes with Though I'm still unsure how we hit that today, the change makes sense on its own I think, so no objection from me. Thanks, Quentin