On Fri, Apr 28, 2023 at 12:42:32AM +0100, Lorenzo Stoakes wrote: > 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 results in both data being written to a folio without writenotify, and > the folio being dirtied unexpectedly (if the caller decides to do so). > > This issue was first reported by Jan Kara [1] in 2018, where the problem > resulted in file system crashes. > > This is only relevant when the mappings are file-backed and the underlying > file system requires folio dirty tracking. File systems which do not, such > as shmem or hugetlb, are not at risk and therefore can be written to > without issue. > > Unfortunately this limitation of GUP has been present for some time and > requires future rework of the GUP API in order to provide correct write > access to such mappings. > > However, for the time being we introduce this check to prevent the most > egregious case of this occurring, use of the FOLL_LONGTERM pin. > > These mappings are considerably more likely to be written to after > folios are cleaned and thus simply must not be permitted to do so. > > As part of this change we separate out vma_needs_dirty_tracking() as a > helper function to determine this which is distinct from > vma_wants_writenotify() which is specific to determining which PTE flags to > set. > > [1]:https://lore.kernel.org/linux-mm/20180103100430.GE4911@xxxxxxxxxxxxxx/ > > Suggested-by: Jason Gunthorpe <jgg@xxxxxxxxxx> > Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx> > --- > include/linux/mm.h | 1 + > mm/gup.c | 41 ++++++++++++++++++++++++++++++++++++++++- > mm/mmap.c | 36 +++++++++++++++++++++++++++--------- > 3 files changed, 68 insertions(+), 10 deletions(-) Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx> Jason
