Messages in this thread | | | Date | Wed, 23 Nov 2022 09:57:37 +0530 | Subject | Re: [PATCH] arm64/mm: Intercept pfn changes in set_pte_at() | From | Anshuman Khandual <> |
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On 11/22/22 16:41, Mark Rutland wrote: > On Tue, Nov 22, 2022 at 09:57:49AM +0000, Will Deacon wrote: >> On Tue, Nov 22, 2022 at 01:43:17PM +0530, Anshuman Khandual wrote: >>> >>> >>> On 11/18/22 19:43, Will Deacon wrote: >>>> On Wed, Nov 16, 2022 at 08:40:01AM +0530, Anshuman Khandual wrote: >>>>> Changing pfn on a user page table mapped entry, without first going through >>>>> break-before-make (BBM) procedure is unsafe. This just updates set_pte_at() >>>>> to intercept such changes, via an updated pgattr_change_is_safe(). This new >>>>> check happens via __check_racy_pte_update(), which has now been renamed as >>>>> __check_safe_pte_update(). >>>>> >>>>> Cc: Catalin Marinas <catalin.marinas@arm.com> >>>>> Cc: Will Deacon <will@kernel.org> >>>>> Cc: Mark Rutland <mark.rutland@arm.com> >>>>> Cc: Andrew Morton <akpm@linux-foundation.org> >>>>> Cc: linux-arm-kernel@lists.infradead.org >>>>> Cc: linux-kernel@vger.kernel.org >>>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com> >>>>> --- >>>>> This applies on v6.1-rc4 >>>>> >>>>> arch/arm64/include/asm/pgtable.h | 8 ++++++-- >>>>> arch/arm64/mm/mmu.c | 8 +++++++- >>>>> 2 files changed, 13 insertions(+), 3 deletions(-) >>>> >>>> I remember Mark saying that BBM is sometimes violated by the core code in >>>> cases where the pte isn't actually part of a live pgtable (e.g. if it's on >>>> the stack or part of a newly allocated table). Won't that cause false >>>> positives here? >>> >>> Could you please elaborate ? If the pte is not on a live page table, then >>> pte_valid() will return negative on such entries. So any update there will >>> be safe. I am wondering, how this change will cause false positives which >>> would not have been possible earlier. >> >> I don't think pte_valid() will always return false for these entries. >> Consider, for example, ptes which are valid but which live in a table that >> is not reachable by the MMU. I think this is what Mark had in mind, but it >> would be helpful if he could chime in with the specific example he ran into. > > Yup -- that was the case I had in mind. IIRC I hit that in the past when trying > to do something similar, but I can't recall exactly where that was. I suspect > that was probably to do with page migration or huge page splitting/merging. > > Looking around, at least __split_huge_zero_page_pmd() and > __split_huge_pmd_locked() do something like that, creating a temporary pmd > entry on the stack, populating a table of non-live but valid ptes, then > plumbing it into the real pmd.
In both cases i.e __split_huge_zero_page_pmd() and __split_huge_pmd_locked(), the entry is first asserted to be empty via pte_none(), before writing a new value in there. set_pte_at() would still consider such updates safe because pfn_valid(old) will return negative on such entries.
VM_BUG_ON(!pte_none(*pte)); set_pte_at(mm, haddr, pte, entry);
But if these entries still get updated yet again (while still being inactive) with new pte values, then set_pte_at() would complain for the pfn update on the entry, while being "valid". But is this a viable scenario ?
> > We'd need to check that there aren't other cases like that. > Sure, might be some what tricky but anything in particular to be looked into ? I guess if this change gets into a CI system which runs all memory stress tests for long enough with CONFIG_DEBUG_VM enabled, we might get some more clue if there are other similar scenarios possible.
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