On Tue, May 02, 2023 at 06:42:42PM +0200, David Hildenbrand wrote: > On 02.05.23 18:34, 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 | 43 ++++++++++++++++++++++++++++++++++++++++++- > > 1 file changed, 42 insertions(+), 1 deletion(-) > > > > diff --git a/mm/gup.c b/mm/gup.c > > index ff689c88a357..6e209ca10967 100644 > > --- a/mm/gup.c > > +++ b/mm/gup.c > > @@ -959,16 +959,53 @@ 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. A long term > > + * pin is the most egregious case so this is the case we disallow. > > + */ > > + if (!(gup_flags & (FOLL_PIN | FOLL_LONGTERM))) > > + return true; > > If you really want to keep FOLL_PIN here ... this has to be > > if ((gup_flags & (FOLL_PIN | FOLL_LONGTERM)) != (FOLL_PIN | FOLL_LONGTERM)) > > or two separate checks. > > Otherwise you'd also proceed if only FOLL_PIN is set. > > Unless my tired eyes betrayed me. Your tired eyes are rapidly taking on the firey visage of the dark lord Sauron... but also, ugh god pints_owed_to_myself++. Sorry this was a me rushing it out of shame thing. Will fix on respin... apologies for spam everyone :) > > > -- > Thanks, > > David / dhildenb >
