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From: Oliver Upton To: Will Deacon Cc: kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Marc Zyngier , Ricardo Koller Subject: Re: [RFC PATCH] KVM: arm64: Fix double-free following kvm_pgtable_stage2_free_unlinked() Message-ID: References: <20240212193052.27765-1-will@kernel.org> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20240212193052.27765-1-will@kernel.org> X-Migadu-Flow: FLOW_OUT X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20240212_121446_342074_2AD3E28A X-CRM114-Status: GOOD ( 21.30 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Mon, Feb 12, 2024 at 07:30:52PM +0000, Will Deacon wrote: > kvm_pgtable_stage2_free_unlinked() does the final put_page() on the > root page of the sub-tree before returning, so remove the additional > put_page() invocations in the callers. > > Cc: Marc Zyngier > Cc: Oliver Upton > Cc: Ricardo Koller > Signed-off-by: Will Deacon > --- > > Hi folks, > > Sending this as an RFC as I only spotted it from code inspection and I'm > surprised others aren't seeing fireworks if it's a genuine bug. I also > couldn't come up with a sensible Fixes tag, as all of: > > e7c05540c694b ("KVM: arm64: Add helper for creating unlinked stage2 subtrees") > 8f5a3eb7513fc ("KVM: arm64: Add kvm_pgtable_stage2_split()") > f6a27d6dc51b2 ("KVM: arm64: Drop last page ref in kvm_pgtable_stage2_free_removed()") > > are actually ok in isolation. Hrm. Please tell me I'm wrong? > > arch/arm64/kvm/hyp/pgtable.c | 2 -- > 1 file changed, 2 deletions(-) > > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c > index c651df904fe3..ab9d05fcf98b 100644 > --- a/arch/arm64/kvm/hyp/pgtable.c > +++ b/arch/arm64/kvm/hyp/pgtable.c > @@ -1419,7 +1419,6 @@ kvm_pte_t *kvm_pgtable_stage2_create_unlinked(struct kvm_pgtable *pgt, > level + 1); > if (ret) { > kvm_pgtable_stage2_free_unlinked(mm_ops, pgtable, level); > - mm_ops->put_page(pgtable); > return ERR_PTR(ret); > } AFAICT, this entire branch is effectively dead code, unless there's a KVM bug lurking behind the page table walk. The sub-tree isn't visible to other software or hardware walkers yet, so none of the PTE races could cause this to pop. So while this is very obviously a bug, it might be pure luck that folks haven't seen smoke here. Perhaps while fixing the bug we should take the opportunity to promote the condition to WARN_ON_ONCE(). > @@ -1502,7 +1501,6 @@ static int stage2_split_walker(const struct kvm_pgtable_visit_ctx *ctx, > > if (!stage2_try_break_pte(ctx, mmu)) { > kvm_pgtable_stage2_free_unlinked(mm_ops, childp, level); > - mm_ops->put_page(childp); > return -EAGAIN; > } This, on the other hand, seems possible. There exists a race where an old block PTE could have the AF set on it and the underlying cmpxchg() could fail. There shouldn't be a race with any software walkers, as we hold the MMU lock for write here. -- Thanks, Oliver _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel