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. -- Thanks, David / dhildenb
