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Fri, 10 Jul 2026 07:26:41 +0000 (UTC) Received: from sm-arm-grace07 (unknown [10.124.178.20]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange ECDHE (P-256) server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by az2nlsmom2.o.css.fujitsu.com (Postfix) with ESMTPS id 628291802EF7; Fri, 10 Jul 2026 07:26:36 +0000 (UTC) Date: Fri, 10 Jul 2026 16:26:32 +0900 From: Itaru Kitayama 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 , Sebastian Ene Subject: Re: [PATCH v2 0/6] KVM: arm64: ptdump: Shadow ptdump fixes Message-ID: References: <20260630121005.1130996-1-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=us-ascii Content-Disposition: inline In-Reply-To: On Fri, Jul 10, 2026 at 07:43:46AM +0100, Wei-Lin Chang wrote: > On Thu, Jul 09, 2026 at 04:34:52PM +0900, Itaru Kitayama wrote: > > On Fri, Jul 03, 2026 at 11:24:53AM +0100, Wei-Lin Chang wrote: > > > On Fri, Jul 03, 2026 at 08:02:56AM +0900, Itaru Kitayama wrote: > > > > On Thu, Jul 02, 2026 at 08:41:43AM +0100, Wei-Lin Chang wrote: > > > > > On Thu, Jul 02, 2026 at 03:55:48PM +0900, Itaru Kitayama wrote: > > > > > > Hi Wei-Lin, > > > > > > On Tue, Jun 30, 2026 at 01:09:59PM +0100, Wei-Lin Chang wrote: > > > > > > > Hi, > > > > > > > > > > > > > > This is v2 of fixing shadow ptdump debugfs files. Unfortunately I couldn't make > > > > > > > per mmu ptdump files work after all, mainly because there isn't a clean way to > > > > > > > locate the specific nested mmu for each ptdump file as the nested mmus could be > > > > > > > freed when the file gets opened. Therefore in this series a single file > > > > > > > "shadow_page_tables" is created that dumps all valid mmus' page table > > > > > > > information. > > > > > > > > > > > > > > An advantage of this is that this new ptdump file have a lifetime identical to > > > > > > > other ptdump files i.e. stage2_page_tables, ipa_range, etc., hence avoiding the > > > > > > > dentry UAF found last time [1]. > > > > > > > > > > > > > > With this all ptdump files are only removed when the last kvm reference gets > > > > > > > dropped and kvm_destroy_vm_debugfs() is called, in their open(), show() > > > > > > > functions the nested mmu array and mmu->pgt are checked with mmu_lock held to > > > > > > > prevent UAF. > > > > > > > > > > > > > > Patch 1-2: Undo previous shadow ptdump implementation. > > > > > > > Patch 3: Fix a mmu->pgt UAF that happens when ptdump files are read after > > > > > > > mmu->pgt is freed. > > > > > > > Patch 4-5: Preparation for the shadow page table dump file. > > > > > > > Patch 6: Implementation of the shadow page table dump file. > > > > > > > > > > > > > > The fixes are tested with CONFIG_PROVE_LOCKING, > > > > > > > CONFIG_DEBUG_ATOMIC_SLEEP, and CONFIG_KASAN. > > > > > > > > > > > > > > Thanks! > > > > > > > > > > > > Running your shadow stage 2 kselftest with bpftrace shows me that __kvm_pgtable_stage2_init() > > > > > > for shadow stage 2 translation tables are built with ia_bits = 52 and > > > > > > start_level = 0, but the debugfs entry for the active shadow stage 2 tables prints > > > > > > out that's 3 levels. Is this fully expected? > > > > > > > > > > Where is this level information you are seeing from? If it is > > > > > "stage2_level", that only reports the number of levels for the canonical > > > > > stage-2 (non nested). For nested mmus only the page tables are dumped in > > > > > nested/shadow_page_tables. > > > > > > > > Yes I know. The initial stage 2 translation table structure information is obtained by > > > > instrumenting the kernel using eBPF fexit to __kvm_pgtable_stage2_init(). > > > > > > > > Since you're correctly loopin over nested_mmus array, and the output is > > > > correctly shown using the kvm_pgtable information via the kvm_s2_mmu for > > > > them, I am confused at this moment. > > > > > > Two things here: > > > > > > Firstly ia_bits being 52 is not expected to me, as I didn't change the > > > VM's IPA size via KVM_VM_TYPE_ARM_IPA_SIZE in the selftest, to me I > > > would expect ia_bits to be 40 (the default). Start level being 0 looks > > > normal. > > > > Instrumenting the ptdump code you're proposing I see the shadow stage 2 > > translation tables are constructed with the max 52-bit IPA; irrespective > > of the IPA bits set in the init_s2_mmu() helper for the guest hypervisor > > setup in the selftest, I am guessing here to me fixed? 52-bit IPA size > > because it's awkward if nestd guest's IPA size is larger than the shadow > > stage 2's IPA size. > > Sorry, I got it wrong. It is correct that shadow stage-2 translation > tables are constructed with 52-bit IPA, irrespective of what L1 sets in > init_s2_mmu(). This is because L1 is free to decide what memory map it > exposes to its guest, so we only constrain it with the PARange exposed > to the guest. Please see the comments in init_nested_s2_mmu(). Thanks for the cofirmation, yes on QEMU the "max" CPU model that's what the PARange exposes, bpftrace was showing that too. > > > > > I confirmed using the __vm_create() helper with various guest modes > > supported that canonical stage 2's "ipa_range" changes according to the > > guest (L1) PA size so I wonder if it is a good idea to create something like > > shadow_ipa_range and shadow_stage2_levels files under nested directoy. > > I guess shadow_ipa_range and shadow_stage2_levels can be added if there > are explicit demand for it. Although right now they aren't too difficult > to infer from the PARange exposed to the guest. Surely, it's up to you. If you kindly take my Tested-by: tag, that'd be great the kernel I used for this series is the latest kvmarm/fixes, with your series applied I can see the mapping information extracted by walking the shadow stage 2 translation tables. Tested-by: Itaru Kitayama Thanks, Itaru. > > Thanks, > Wei-Lin Chang > > > > > Thanks, > > Itaru. > > > > > > > > Second thing is I still can't get what you are confused about, if it's > > > not about the first point. Note that the "3" in ptdump means it is a > > > level 3 mapping, not saying the page tables are using 3 levels. > > > > > > Happy to discuss more if you clarify. I'll also check the ia_bits when I > > > get to it. > > > > > > Thanks, > > > Wei-Lin Chang > > > > > > > > > > > Thanks, > > > > Itaru. > > > > > > > > > > > > > > Thanks, > > > > > Wei-Lin Chang > > > > > > > > > > > > > > > > > Thanks, > > > > > > Itaru. > > > > > > > > > > > > > > > > > > > > * Changes from v1 ([2]): > > > > > > > > > > > > > > - Move from per mmu ptdump files to one file that will dump all shadow page > > > > > > > tables. > > > > > > > > > > > > > > [1]: https://lore.kernel.org/kvmarm/ajty6I7ZqodP4ous@sm-arm-grace07/ > > > > > > > [2]: https://lore.kernel.org/kvmarm/20260623142443.648972-1-weilin.chang@arm.com/ > > > > > > > > > > > > > > Wei-Lin Chang (6): > > > > > > > KVM: arm64: ptdump: Remove shadow ptdump files > > > > > > > KVM: arm64: ptdump: Undo making the ptdump code mmu aware > > > > > > > KVM: arm64: ptdump: Fix UAF when mmu->pgt is freed > > > > > > > KVM: arm64: ptdump: Factor out initialization of > > > > > > > kvm_ptdump_guest_state > > > > > > > KVM: arm64: ptdump: Extract kvm_ptdump_guest_open() from canonical > > > > > > > ptdump path > > > > > > > KVM: arm64: ptdump: Introduce the shadow ptdump file > > > > > > > > > > > > > > arch/arm64/include/asm/kvm_host.h | 5 +- > > > > > > > arch/arm64/include/asm/kvm_mmu.h | 4 - > > > > > > > arch/arm64/kvm/nested.c | 18 +-- > > > > > > > arch/arm64/kvm/ptdump.c | 185 ++++++++++++++++++++---------- > > > > > > > 4 files changed, 135 insertions(+), 77 deletions(-) > > > > > > > > > > > > > > -- > > > > > > > 2.43.0 > > > > > > >