Re: [PATCH] arm64/mm: Intercept pfn changes in set_pte_at()

[Will Deacon]

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@xxxxxxx>
> >> Cc: Will Deacon <will@xxxxxxxxxx>
> >> Cc: Mark Rutland <mark.rutland@xxxxxxx> 
> >> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> 
> >> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> >> Cc: linux-kernel@xxxxxxxxxxxxxxx
> >> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> >> ---
> >> 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