[PATCH v6 2/3] mm/gup: disallow FOLL_LONGTERM GUP-nonfast writing to file-backed mappings

[Lorenzo Stoakes <lstoakes@gmail.com>]

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@quack2.suse.cz/

Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Mika Penttilä <mpenttil@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
---
 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;
+
+	/* 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;
-- 
2.40.1