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[104.155.91.135]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-49fbd22dbf2sm50558415e9.10.2026.09.17.01.11.54 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 17 Sep 2026 01:11:54 -0700 (PDT) Date: Thu, 17 Sep 2026 09:11:50 +0100 From: Vincent Donnefort To: Wei-Lin Chang Cc: linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org, Marc Zyngier , Oliver Upton , Fuad Tabba , Joey Gouly , Steffen Eiden , Suzuki K Poulose , Zenghui Yu , Catalin Marinas , Will Deacon , Mark Rutland , Sebastian Ene , Itaru Kitayama , Sashiko AI , stable@vger.kernel.org Subject: Re: [PATCH v3 1/2] KVM: arm64: ptdump: Check the page tables aren't freed when accessing Message-ID: References: <20260916230337.4162485-1-weilin.chang@arm.com> <20260916230337.4162485-2-weilin.chang@arm.com> Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20260916230337.4162485-2-weilin.chang@arm.com> On Thu, Sep 17, 2026 at 12:03:36AM +0100, Wei-Lin Chang wrote: > An open debugfs file keeps the KVM structure alive, but does not prevent > mmu notifier release from freeing the stage-2 page tables when the VMM’s > address space is torn down. Therefore page tables belonging to the mmus > could have been freed when a thread opens or reads the ptdump files. > Take the mmu_lock and check mmu->pgt is still alive before accessing the > page tables. As Sashiko said, the read_lock is probably enough, including the existing one in kvm_ptdump_guest_show() With that change: Reviewed-by: Vincent Donnefort Tested-by: Vincent Donnefort > > Also for the ipa_range file and stage2_levels file, switch from keeping > the pointer to the page tables to keeping the pointer to the mmu. > > Fixes: 7c4f73548ed1 ("KVM: arm64: Register ptdump with debugfs on guest creation") > Reported-by: Sashiko AI > Closes: https://lore.kernel.org/kvmarm/20260623144054.B91D21F000E9@smtp.kernel.org/ > Signed-off-by: Wei-Lin Chang > Cc: stable@vger.kernel.org > --- > arch/arm64/kvm/ptdump.c | 43 +++++++++++++++++++++++++++++++---------- > 1 file changed, 33 insertions(+), 10 deletions(-) > > diff --git a/arch/arm64/kvm/ptdump.c b/arch/arm64/kvm/ptdump.c > index 69899797dbad..4ead856c181a 100644 > --- a/arch/arm64/kvm/ptdump.c > +++ b/arch/arm64/kvm/ptdump.c > @@ -115,15 +115,28 @@ static int kvm_ptdump_build_levels(struct ptdump_pg_level *level, u32 start_lvl) > > static struct kvm_ptdump_guest_state *kvm_ptdump_parser_create(struct kvm_s2_mmu *mmu) > { > + struct kvm *kvm = kvm_s2_mmu_to_kvm(mmu); > struct kvm_ptdump_guest_state *st; > - struct kvm_pgtable *pgtable = mmu->pgt; > + s8 start_level; > int ret; > > + /* > + * We only need the pgt start level to initialize the ptdump, get it > + * while holding the mmu_lock. It's fine if the pgt gets freed > + * afterwards, we'll check again when doing the actual dump. > + */ > + scoped_guard(write_lock, &kvm->mmu_lock) { > + if (mmu->pgt) > + start_level = mmu->pgt->start_level; > + else > + return ERR_PTR(-ENOENT); > + } > + > st = kzalloc_obj(struct kvm_ptdump_guest_state, GFP_KERNEL_ACCOUNT); > if (!st) > return ERR_PTR(-ENOMEM); > > - ret = kvm_ptdump_build_levels(&st->level[0], pgtable->start_level); > + ret = kvm_ptdump_build_levels(&st->level[0], start_level); > if (ret) { > kfree(st); > return ERR_PTR(ret); > @@ -149,6 +162,9 @@ static int kvm_ptdump_guest_show(struct seq_file *m, void *unused) > }; > > guard(write_lock)(&kvm->mmu_lock); > + if (!mmu->pgt) > + return 0; > + > st->parser_state = (struct ptdump_pg_state) { > .marker = &st->ipa_marker[0], > .end_address = BIT(mmu->pgt->ia_bits), > @@ -211,17 +227,27 @@ static const struct file_operations kvm_ptdump_guest_fops = { > > static int kvm_pgtable_range_show(struct seq_file *m, void *unused) > { > - struct kvm_pgtable *pgtable = m->private; > + struct kvm_s2_mmu *mmu = m->private; > + struct kvm *kvm = kvm_s2_mmu_to_kvm(mmu); > + > + guard(write_lock)(&kvm->mmu_lock); > + > + if (mmu->pgt) > + seq_printf(m, "%2u\n", mmu->pgt->ia_bits); > > - seq_printf(m, "%2u\n", pgtable->ia_bits); > return 0; > } > > static int kvm_pgtable_levels_show(struct seq_file *m, void *unused) > { > - struct kvm_pgtable *pgtable = m->private; > + struct kvm_s2_mmu *mmu = m->private; > + struct kvm *kvm = kvm_s2_mmu_to_kvm(mmu); > + > + guard(write_lock)(&kvm->mmu_lock); > + > + if (mmu->pgt) > + seq_printf(m, "%1d\n", KVM_PGTABLE_MAX_LEVELS - mmu->pgt->start_level); > > - seq_printf(m, "%1d\n", KVM_PGTABLE_MAX_LEVELS - pgtable->start_level); > return 0; > } > > @@ -230,15 +256,12 @@ static int kvm_pgtable_debugfs_open(struct inode *m, struct file *file, > { > struct kvm_s2_mmu *mmu = m->i_private; > struct kvm *kvm = kvm_s2_mmu_to_kvm(mmu); > - struct kvm_pgtable *pgtable; > int ret; > > if (!kvm_get_kvm_safe(kvm)) > return -ENOENT; > > - pgtable = mmu->pgt; > - > - ret = single_open(file, show, pgtable); > + ret = single_open(file, show, mmu); > if (ret < 0) > kvm_put_kvm(kvm); > return ret; > -- > 2.43.0 > -- Vincent