On 02/05/2024 02:27, Zi Yan wrote: > On 1 May 2024, at 10:33, Ryan Roberts wrote: > >> __split_huge_pmd_locked() can be called for a present THP, devmap or >> (non-present) migration entry. It calls pmdp_invalidate() >> unconditionally on the pmdp and only determines if it is present or not >> based on the returned old pmd. This is a problem for the migration entry >> case because pmd_mkinvalid(), called by pmdp_invalidate() must only be >> called for a present pmd. >> >> On arm64 at least, pmd_mkinvalid() will mark the pmd such that any >> future call to pmd_present() will return true. And therefore any >> lockless pgtable walker could see the migration entry pmd in this state >> and start interpretting the fields as if it were present, leading to >> BadThings (TM). GUP-fast appears to be one such lockless pgtable walker. >> >> x86 does not suffer the above problem, but instead pmd_mkinvalid() will >> corrupt the offset field of the swap entry within the swap pte. See link >> below for discussion of that problem. >> >> Fix all of this by only calling pmdp_invalidate() for a present pmd. And >> for good measure let's add a warning to all implementations of >> pmdp_invalidate[_ad](). I've manually reviewed all other >> pmdp_invalidate[_ad]() call sites and believe all others to be >> conformant. >> >> This is a theoretical bug found during code review. I don't have any >> test case to trigger it in practice. >> >> Cc: stable@xxxxxxxxxxxxxxx >> Link: https://lore.kernel.org/all/0dd7827a-6334-439a-8fd0-43c98e6af22b@xxxxxxx/ >> Fixes: 84c3fc4e9c56 ("mm: thp: check pmd migration entry in common path") >> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx> >> --- >> >> Right v3; this goes back to the original approach in v1 to fix core-mm rather >> than push the fix into arm64, since we discovered that x86 can't handle >> pmd_mkinvalid() being called for non-present pmds either. >> >> I'm pulling in more arch maintainers because this version adds some warnings in >> arch code to help spot incorrect usage. >> >> Although Catalin had already accepted v2 (fixing arm64) [2] into for-next/fixes, >> he's agreed to either remove or revert it. >> >> >> Changes since v1 [1] >> ==================== >> >> - Improve pmdp_mkinvalid() docs to make it clear it can only be called for >> present pmd (per JohnH, Zi Yan) >> - Added warnings to arch overrides of pmdp_invalidate[_ad]() (per Zi Yan) >> - Moved comment next to new location of pmpd_invalidate() (per Zi Yan) >> >> >> [1] https://lore.kernel.org/linux-mm/20240425170704.3379492-1-ryan.roberts@xxxxxxx/ >> [2] https://lore.kernel.org/all/20240430133138.732088-1-ryan.roberts@xxxxxxx/ >> >> Thanks, >> Ryan >> >> >> Documentation/mm/arch_pgtable_helpers.rst | 6 ++- >> arch/powerpc/mm/book3s64/pgtable.c | 1 + >> arch/s390/include/asm/pgtable.h | 4 +- >> arch/sparc/mm/tlb.c | 1 + >> arch/x86/mm/pgtable.c | 2 + >> mm/huge_memory.c | 49 ++++++++++++----------- >> mm/pgtable-generic.c | 2 + >> 7 files changed, 39 insertions(+), 26 deletions(-) > > The changes in Documentation/mm and mm/* look good to me. Thanks. > Reviewed-by: Zi Yan <ziy@xxxxxxxxxx> Thanks! > > I wonder if making Documentation/mm and mm/* changes in a separate patch > would be better, since you will not need acks from arch maintainers and > get the patch in quicker. Yeah maybe - I considered that, but then decided I'm literally just adding a debug warning to the arch code so it shouldn't be too controversial. Perhaps wait a few days for acks and if nothing turns up then I'll re-post with it split? > > > -- > Best Regards, > Yan, Zi
