On Tue, May 02, 2023 at 05:04:02PM +0200, David Hildenbrand wrote: > On 02.05.23 01:11, 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. > > > > This patch changes only the slow-path GUP functions, a following patch > > adapts the GUP-fast path along similar lines. > > > > [1]:https://lore.kernel.org/linux-mm/20180103100430.GE4911@xxxxxxxxxxxxxx/ > > > > Suggested-by: Jason Gunthorpe <jgg@xxxxxxxxxx> > > Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx> > > Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx> > > Reviewed-by: Mika Penttilä <mpenttil@xxxxxxxxxx> > > Reviewed-by: Jan Kara <jack@xxxxxxx> > > Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx> > > --- > > mm/gup.c | 41 ++++++++++++++++++++++++++++++++++++++++- > > 1 file changed, 40 insertions(+), 1 deletion(-) > > > > diff --git a/mm/gup.c b/mm/gup.c > > index ff689c88a357..0f09dec0906c 100644 > > --- a/mm/gup.c > > +++ b/mm/gup.c > > @@ -959,16 +959,51 @@ static int faultin_page(struct vm_area_struct *vma, > > return 0; > > } > > +/* > > + * 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. > > + * > > + * 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). > > + */ > > +static bool writeable_file_mapping_allowed(struct vm_area_struct *vma, > > + unsigned long gup_flags) > > +{ > > + /* If we aren't pinning then no problematic write can occur. */ > > + if (!(gup_flags & (FOLL_GET | FOLL_PIN))) > > + return true; > > I think we should really not look at FOLL_GET here. Just check for FOLL_PIN > (as said, even FOLL_LONGTERM would be sufficient, but I understand the > reasoning to keep it, although I would drop it :P ). It also better matches > your comment regarding pinning ... > > See the comment in is_valid_gup_args() regarding "LONGTERM can only be > specified when pinning". (well, there we also check that FOLL_PIN has to be > set ... ;) ) I think I will finally give in, in penance for the very silly mistake I made below... > > > + > > + /* We limit this check to the most egregious case - a long term pin. */ > > + if (!(gup_flags & FOLL_LONGTERM)) > > + return true; > > + > > + /* If the VMA requires dirty tracking then GUP will be problematic. */ > > + return vma_needs_dirty_tracking(vma); > > > ... should that be "!vma_needs_dirty_tracking(vma)" ? > > If the fs needs dirty tracking, it should be disallowed. > > Maybe that explains why it's still working for Matthew in his s390x test. > ... or I am too tired and messed up :) > No, no it was I who was too tired it seems! You're correct, this is wrong, will respin with fix :)) > -- > Thanks, > > David / dhildenb >
