More information on the general COW issues can be found at [2]. This series is based on latest linus/master and [1]: [PATCH v3 00/16] mm: COW fixes part 2: reliable GUP pins of anonymous pages v2 is located at: https://github.com/davidhildenbrand/linux/tree/cow_fixes_part_3_v2 This series fixes memory corruptions when a GUP R/W reference (FOLL_WRITE | FOLL_GET) was taken on an anonymous page and COW logic fails to detect exclusivity of the page to then replacing the anonymous page by a copy in the page table: The GUP reference lost synchronicity with the pages mapped into the page tables. This series focuses on x86, arm64, s390x and ppc64/book3s -- other architectures are fairly easy to support by implementing __HAVE_ARCH_PTE_SWP_EXCLUSIVE. This primarily fixes the O_DIRECT memory corruptions that can happen on concurrent swapout, whereby we lose DMA reads to a page (modifying the user page by writing to it). O_DIRECT currently uses FOLL_GET for short-term (!FOLL_LONGTERM) DMA from/to a user page. In the long run, we want to convert it to properly use FOLL_PIN, and John is working on it, but that might take a while and might not be easy to backport. In the meantime, let's restore what used to work before we started modifying our COW logic: make R/W FOLL_GET references reliable as long as there is no fork() after GUP involved. This is just the natural follow-up of part 2, that will also further reduce "wrong COW" on the swapin path, for example, when we cannot remove a page from the swapcache due to concurrent writeback, or if we have two threads faulting on the same swapped-out page. Fixing O_DIRECT is just a nice side-product This issue, including other related COW issues, has been summarized in [3] under 2): " 2. Intra Process Memory Corruptions due to Wrong COW (FOLL_GET) It was discovered that we can create a memory corruption by reading a file via O_DIRECT to a part (e.g., first 512 bytes) of a page, concurrently writing to an unrelated part (e.g., last byte) of the same page, and concurrently write-protecting the page via clear_refs SOFTDIRTY tracking [6]. For the reproducer, the issue is that O_DIRECT grabs a reference of the target page (via FOLL_GET) and clear_refs write-protects the relevant page table entry. On successive write access to the page from the process itself, we wrongly COW the page when resolving the write fault, resulting in a loss of synchronicity and consequently a memory corruption. While some people might think that using clear_refs in this combination is a corner cases, it turns out to be a more generic problem unfortunately. For example, it was just recently discovered that we can similarly create a memory corruption without clear_refs, simply by concurrently swapping out the buffer pages [7]. Note that we nowadays even use the swap infrastructure in Linux without an actual swap disk/partition: the prime example is zram which is enabled as default under Fedora [10]. The root issue is that a write-fault on a page that has additional references results in a COW and thereby a loss of synchronicity and consequently a memory corruption if two parties believe they are referencing the same page. " We don't particularly care about R/O FOLL_GET references: they were never reliable and O_DIRECT doesn't expect to observe modifications from a page after DMA was started. Note that: * this only fixes the issue on x86, arm64, s390x and ppc64/book3s ("enterprise architectures"). Other architectures have to implement __HAVE_ARCH_PTE_SWP_EXCLUSIVE to achieve the same. * this does *not * consider any kind of fork() after taking the reference: fork() after GUP never worked reliably with FOLL_GET. * Not losing PG_anon_exclusive during swapout was the last remaining piece. KSM already makes sure that there are no other references on a page before considering it for sharing. Page migration maintains PG_anon_exclusive and simply fails when there are additional references (freezing the refcount fails). Only swapout code dropped the PG_anon_exclusive flag because it requires more work to remember + restore it. With this series in place, most COW issues of [3] are fixed on said architectures. Other architectures can implement __HAVE_ARCH_PTE_SWP_EXCLUSIVE fairly easily. [1] https://lkml.kernel.org/r/20220329160440.193848-1-david@xxxxxxxxxx [2] https://lkml.kernel.org/r/20211217113049.23850-1-david@xxxxxxxxxx [3] https://lore.kernel.org/r/3ae33b08-d9ef-f846-56fb-645e3b9b4c66@xxxxxxxxxx v2 -> v3: * Rebased and retested * "arm64/pgtable: support __HAVE_ARCH_PTE_SWP_EXCLUSIVE" -> Add RB and a comment to the patch description * "s390/pgtable: cleanup description of swp pte layout" -> Added * "s390/pgtable: support __HAVE_ARCH_PTE_SWP_EXCLUSIVE" -> Use new set_pte_bit()/clear_pte_bit() -> Fixups comments/patch description David Hildenbrand (8): mm/swap: remember PG_anon_exclusive via a swp pte bit mm/debug_vm_pgtable: add tests for __HAVE_ARCH_PTE_SWP_EXCLUSIVE x86/pgtable: support __HAVE_ARCH_PTE_SWP_EXCLUSIVE arm64/pgtable: support __HAVE_ARCH_PTE_SWP_EXCLUSIVE s390/pgtable: cleanup description of swp pte layout s390/pgtable: support __HAVE_ARCH_PTE_SWP_EXCLUSIVE powerpc/pgtable: remove _PAGE_BIT_SWAP_TYPE for book3s powerpc/pgtable: support __HAVE_ARCH_PTE_SWP_EXCLUSIVE for book3s arch/arm64/include/asm/pgtable-prot.h | 1 + arch/arm64/include/asm/pgtable.h | 23 ++++++-- arch/powerpc/include/asm/book3s/64/pgtable.h | 31 ++++++++--- arch/s390/include/asm/pgtable.h | 36 +++++++++---- arch/x86/include/asm/pgtable.h | 16 ++++++ arch/x86/include/asm/pgtable_64.h | 4 +- arch/x86/include/asm/pgtable_types.h | 5 ++ include/linux/pgtable.h | 29 +++++++++++ include/linux/swapops.h | 2 + mm/debug_vm_pgtable.c | 15 ++++++ mm/memory.c | 55 ++++++++++++++++++-- mm/rmap.c | 19 ++++--- mm/swapfile.c | 13 ++++- 13 files changed, 216 insertions(+), 33 deletions(-) -- 2.35.1
