Messages in this thread | | | Date | Tue, 22 Nov 2022 09:57:49 +0000 | From | Will Deacon <> | Subject | Re: [PATCH] arm64/mm: Intercept pfn changes in set_pte_at() |
| |
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.
Will
| |