Writing to file-backed mappings which require folio dirty tracking using GUP is a fundamentally broken operation, as kernel write access to GUP mappings do not adhere to the semantics expected by a file system. A GUP caller uses the direct mapping to access the folio, which does not cause write notify to trigger, nor does it enforce that the caller marks the folio dirty. The problem arises when, after an initial write to the folio, writeback results in the folio being cleaned and then the caller, via the GUP interface, writes to the folio again. As a result of the use of this secondary, direct, mapping to the folio no write notify will occur, and if the caller does mark the folio dirty, this will be done so unexpectedly. For example, consider the following scenario:- 1. A folio is written to via GUP which write-faults the memory, notifying the file system and dirtying the folio. 2. Later, writeback is triggered, resulting in the folio being cleaned and the PTE being marked read-only. 3. The GUP caller writes to the folio, as it is mapped read/write via the direct mapping. 4. The GUP caller, now done with the page, unpins it and sets it dirty (though it does not have to). This change updates both the PUP FOLL_LONGTERM slow and fast APIs. As pin_user_pages_fast_only() does not exist, we can rely on a slightly imperfect whitelisting in the PUP-fast case and fall back to the slow case should this fail. v8: - Fixed typo writeable -> writable. - Fixed bug in writable_file_mapping_allowed() - must check combination of FOLL_PIN AND FOLL_LONGTERM not either/or. - Updated vma_needs_dirty_tracking() to include write/shared to account for MAP_PRIVATE mappings. - Move to open-coding the checks in folio_pin_allowed() so we can READ_ONCE() the mapping and avoid unexpected compiler loads. Rename to account for fact we now check flags here. - Disallow mapping == NULL or mapping & PAGE_MAPPING_FLAGS other than anon. Defer to slow path. - Perform GUP-fast check _after_ the lowest page table level is confirmed to be stable. - Updated comments and commit message for final patch as per Jason's suggestions. v7: - Fixed very silly bug in writeable_file_mapping_allowed() inverting the logic. - Removed unnecessary RCU lock code and replaced with adaptation of Peter's idea. - Removed unnecessary open-coded folio_test_anon() in folio_longterm_write_pin_allowed() and restructured to generally permit NULL folio_mapping(). https://lore.kernel.org/all/cover.1683044162.git.lstoakes@xxxxxxxxx/ v6: - Rebased on latest mm-unstable as of 28th April 2023. - Add PUP-fast check with handling for rcu-locked TLB shootdown to synchronise correctly. - Split patch series into 3 to make it more digestible. https://lore.kernel.org/all/cover.1682981880.git.lstoakes@xxxxxxxxx/ v5: - Rebased on latest mm-unstable as of 25th April 2023. - Some small refactorings suggested by John. - Added an extended description of the problem in the comment around writeable_file_mapping_allowed() for clarity. - Updated commit message as suggested by Mika and John. https://lore.kernel.org/all/6b73e692c2929dc4613af711bdf92e2ec1956a66.1682638385.git.lstoakes@xxxxxxxxx/ v4: - Split out vma_needs_dirty_tracking() from vma_wants_writenotify() to reduce duplication and update to use this in the GUP check. Note that both separately check vm_ops_needs_writenotify() as the latter needs to test this before the vm_pgprot_modify() test, resulting in vma_wants_writenotify() checking this twice, however it is such a small check this should not be egregious. https://lore.kernel.org/all/3b92d56f55671a0389252379237703df6e86ea48.1682464032.git.lstoakes@xxxxxxxxx/ v3: - Rebased on latest mm-unstable as of 24th April 2023. - Explicitly check whether file system requires folio dirtying. Note that vma_wants_writenotify() could not be used directly as it is very much focused on determining if the PTE r/w should be set (e.g. assuming private mapping does not require it as already set, soft dirty considerations). - Tested code against shmem and hugetlb mappings - confirmed that these are not disallowed by the check. - Eliminate FOLL_ALLOW_BROKEN_FILE_MAPPING flag and instead perform check only for FOLL_LONGTERM pins. - As a result, limit check to internal GUP code. https://lore.kernel.org/all/23c19e27ef0745f6d3125976e047ee0da62569d4.1682406295.git.lstoakes@xxxxxxxxx/ v2: - Add accidentally excluded ptrace_access_vm() use of FOLL_ALLOW_BROKEN_FILE_MAPPING. - Tweak commit message. https://lore.kernel.org/all/c8ee7e02d3d4f50bb3e40855c53bda39eec85b7d.1682321768.git.lstoakes@xxxxxxxxx/ v1: https://lore.kernel.org/all/f86dc089b460c80805e321747b0898fd1efe93d7.1682168199.git.lstoakes@xxxxxxxxx/ Lorenzo Stoakes (3): mm/mmap: separate writenotify and dirty tracking logic mm/gup: disallow FOLL_LONGTERM GUP-nonfast writing to file-backed mappings mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings include/linux/mm.h | 1 + mm/gup.c | 146 ++++++++++++++++++++++++++++++++++++++++++++- mm/mmap.c | 53 ++++++++++++---- 3 files changed, 186 insertions(+), 14 deletions(-) -- 2.40.1
