Messages in this thread | | | Date | Tue, 22 Nov 2022 11:11:14 +0000 | From | Mark Rutland <> | Subject | Re: [PATCH] arm64/mm: Intercept pfn changes in set_pte_at() |
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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.
We'd need to check that there aren't other cases like that.
Thanks, Mark.
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