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> 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. -- Best Regards, Yan, Zi
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