Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings by default

John Hubbard
2023-04-28T02:06:43Z

On 4/27/23 16:42, 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@quack2.suse.cz/
>
> Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
> ---
> include/linux/mm.h | 1 +
> mm/gup.c | 41 ++++++++++++++++++++++++++++++++++++++++-
> mm/mmap.c | 36 +++++++++++++++++++++++++++---------
> 3 files changed, 68 insertions(+), 10 deletions(-)
>

Looks good.

Reviewed-by: John Hubbard <jhubbard@nvidia.com>

thanks,
--
John Hubbard
NVIDIA

> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 37554b08bb28..f7da02fc89c6 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2433,6 +2433,7 @@ extern unsigned long move_page_tables(struct vm_area_struct *vma,
> #define MM_CP_UFFD_WP_ALL (MM_CP_UFFD_WP | \
> MM_CP_UFFD_WP_RESOLVE)
>
> +bool vma_needs_dirty_tracking(struct vm_area_struct *vma);
> int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot);
> static inline bool vma_wants_manual_pte_write_upgrade(struct vm_area_struct *vma)
> {
> diff --git a/mm/gup.c b/mm/gup.c
> index 1f72a717232b..d36a5db9feb1 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;
> +
> + /* 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);
> +}
> +
> static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
> {
> vm_flags_t vm_flags = vma->vm_flags;
> int write = (gup_flags & FOLL_WRITE);
> int foreign = (gup_flags & FOLL_REMOTE);
> + bool vma_anon = vma_is_anonymous(vma);
>
> if (vm_flags & (VM_IO | VM_PFNMAP))
> return -EFAULT;
>
> - if (gup_flags & FOLL_ANON && !vma_is_anonymous(vma))
> + if ((gup_flags & FOLL_ANON) && !vma_anon)
> return -EFAULT;
>
> if ((gup_flags & FOLL_LONGTERM) && vma_is_fsdax(vma))
> @@ -978,6 +1013,10 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
> return -EFAULT;
>
> if (write) {
> + if (!vma_anon &&
> + !writeable_file_mapping_allowed(vma, gup_flags))
> + return -EFAULT;
> +
> if (!(vm_flags & VM_WRITE)) {
> if (!(gup_flags & FOLL_FORCE))
> return -EFAULT;
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 536bbb8fa0ae..7b6344d1832a 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1475,6 +1475,31 @@ SYSCALL_DEFINE1(old_mmap, struct mmap_arg_struct __user *, arg)
> }
> #endif /* __ARCH_WANT_SYS_OLD_MMAP */
>
> +/* Do VMA operations imply write notify is required? */
> +static bool vm_ops_needs_writenotify(const struct vm_operations_struct *vm_ops)
> +{
> + return vm_ops && (vm_ops->page_mkwrite || vm_ops->pfn_mkwrite);
> +}
> +
> +/*
> + * Does this VMA require the underlying folios to have their dirty state
> + * tracked?
> + */
> +bool vma_needs_dirty_tracking(struct vm_area_struct *vma)
> +{
> + /* Does the filesystem need to be notified? */
> + if (vm_ops_needs_writenotify(vma->vm_ops))
> + return true;
> +
> + /* Specialty mapping? */
> + if (vma->vm_flags & VM_PFNMAP)
> + return false;
> +
> + /* Can the mapping track the dirty pages? */
> + return vma->vm_file && vma->vm_file->f_mapping &&
> + mapping_can_writeback(vma->vm_file->f_mapping);
> +}
> +
> /*
> * Some shared mappings will want the pages marked read-only
> * to track write events. If so, we'll downgrade vm_page_prot
> @@ -1484,14 +1509,13 @@ SYSCALL_DEFINE1(old_mmap, struct mmap_arg_struct __user *, arg)
> int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot)
> {
> vm_flags_t vm_flags = vma->vm_flags;
> - const struct vm_operations_struct *vm_ops = vma->vm_ops;
>
> /* If it was private or non-writable, the write bit is already clear */
> if ((vm_flags & (VM_WRITE|VM_SHARED)) != ((VM_WRITE|VM_SHARED)))
> return 0;
>
> /* The backer wishes to know when pages are first written to? */
> - if (vm_ops && (vm_ops->page_mkwrite || vm_ops->pfn_mkwrite))
> + if (vm_ops_needs_writenotify(vma->vm_ops))
> return 1;
>
> /* The open routine did something to the protections that pgprot_modify
> @@ -1511,13 +1535,7 @@ int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot)
> if (userfaultfd_wp(vma))
> return 1;
>
> - /* Specialty mapping? */
> - if (vm_flags & VM_PFNMAP)
> - return 0;
> -
> - /* Can the mapping track the dirty pages? */
> - return vma->vm_file && vma->vm_file->f_mapping &&
> - mapping_can_writeback(vma->vm_file->f_mapping);
> + return vma_needs_dirty_tracking(vma);
> }
>
> /*
> --
> 2.40.0