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

[Lorenzo Stoakes]

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(-)
>
> 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

Apologies, for some reason I forgot to include the revision list in that
patch, enclosed below:-

v5:
- Rebased on latest mm-unstable as of 25th April 2023.
- Some small refactorings suggested by John.
- Added an extended description of the problem in the comment around
  writeable_file_mapping_allowed() for clarity.
- Updated commit message as suggested by Mika and John.

v4:
- Split out vma_needs_dirty_tracking() from vma_wants_writenotify() to
  reduce duplication and update to use this in the GUP check. Note that
  both separately check vm_ops_needs_writenotify() as the latter needs to
  test this before the vm_pgprot_modify() test, resulting in
  vma_wants_writenotify() checking this twice, however it is such a small
  check this should not be egregious.
https://lore.kernel.org/all/3b92d56f55671a0389252379237703df6e86ea48.1682464032.git.lstoakes@xxxxxxxxx/

v3:
- Rebased on latest mm-unstable as of 24th April 2023.
- Explicitly check whether file system requires folio dirtying. Note that
  vma_wants_writenotify() could not be used directly as it is very much focused
  on determining if the PTE r/w should be set (e.g. assuming private mapping
  does not require it as already set, soft dirty considerations).
- Tested code against shmem and hugetlb mappings - confirmed that these are not
  disallowed by the check.
- Eliminate FOLL_ALLOW_BROKEN_FILE_MAPPING flag and instead perform check only
  for FOLL_LONGTERM pins.
- As a result, limit check to internal GUP code.
 https://lore.kernel.org/all/23c19e27ef0745f6d3125976e047ee0da62569d4.1682406295.git.lstoakes@xxxxxxxxx/

v2:
- Add accidentally excluded ptrace_access_vm() use of
  FOLL_ALLOW_BROKEN_FILE_MAPPING.
- Tweak commit message.
https://lore.kernel.org/all/c8ee7e02d3d4f50bb3e40855c53bda39eec85b7d.1682321768.git.lstoakes@xxxxxxxxx/

v1:
https://lore.kernel.org/all/f86dc089b460c80805e321747b0898fd1efe93d7.1682168199.git.lstoakes@xxxxxxxxx/