[PATCH 6.6.y] mm: avoid unsafe VMA hook invocation when error arises on mmap hook

[Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>]

After an attempted mmap() fails, we are no longer in a situation where we
can safely interact with VMA hooks. This is currently not enforced, meaning
that we need complicated handling to ensure we do not incorrectly call
these hooks.

We can avoid the whole issue by treating the VMA as suspect the moment that
the file->f_ops->mmap() function reports an error by replacing whatever VMA
operations were installed with a dummy empty set of VMA operations.

We do so through a new helper function internal to mm - mmap_file() - which
is both more logically named than the existing call_mmap() function and
correctly isolates handling of the vm_op reassignment to mm.

All the existing invocations of call_mmap() outside of mm are ultimately
nested within the call_mmap() from mm, which we now replace.

It is therefore safe to leave call_mmap() in place as a convenience
function (and to avoid churn). The invokers are:

     ovl_file_operations -> mmap -> ovl_mmap() -> backing_file_mmap()
    coda_file_operations -> mmap -> coda_file_mmap()
     shm_file_operations -> shm_mmap()
shm_file_operations_huge -> shm_mmap()
            dma_buf_fops -> dma_buf_mmap_internal -> i915_dmabuf_ops
	                    -> i915_gem_dmabuf_mmap()

None of these callers interact with vm_ops or mappings in a problematic way
on error, quickly exiting out.

Reported-by: Jann Horn <jannh@xxxxxxxxxx>
Fixes: deb0f6562884 ("mm/mmap: undo ->mmap() when arch_validate_flags() fails")
Cc: stable <stable@xxxxxxxxxx>
Reviewed-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
Reviewed-by: Jann Horn <jannh@xxxxxxxxxx>
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
---
 mm/internal.h | 27 +++++++++++++++++++++++++++
 mm/mmap.c     |  4 ++--
 mm/nommu.c    |  4 ++--
 3 files changed, 31 insertions(+), 4 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index ef8d787a510c..d52d6b57dafb 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -83,6 +83,33 @@ static inline void *folio_raw_mapping(struct folio *folio)
 	return (void *)(mapping & ~PAGE_MAPPING_FLAGS);
 }
 
+/*
+ * This is a file-backed mapping, and is about to be memory mapped - invoke its
+ * mmap hook and safely handle error conditions. On error, VMA hooks will be
+ * mutated.
+ *
+ * @file: File which backs the mapping.
+ * @vma:  VMA which we are mapping.
+ *
+ * Returns: 0 if success, error otherwise.
+ */
+static inline int mmap_file(struct file *file, struct vm_area_struct *vma)
+{
+	int err = call_mmap(file, vma);
+
+	if (likely(!err))
+		return 0;
+
+	/*
+	 * OK, we tried to call the file hook for mmap(), but an error
+	 * arose. The mapping is in an inconsistent state and we most not invoke
+	 * any further hooks on it.
+	 */
+	vma->vm_ops = &vma_dummy_vm_ops;
+
+	return err;
+}
+
 void __acct_reclaim_writeback(pg_data_t *pgdat, struct folio *folio,
 						int nr_throttled);
 static inline void acct_reclaim_writeback(struct folio *folio)
diff --git a/mm/mmap.c b/mm/mmap.c
index 6530e9cac458..8a055bae6bdb 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2779,7 +2779,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 		}
 
 		vma->vm_file = get_file(file);
-		error = call_mmap(file, vma);
+		error = mmap_file(file, vma);
 		if (error)
 			goto unmap_and_free_vma;
 
@@ -2793,7 +2793,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 
 		vma_iter_config(&vmi, addr, end);
 		/*
-		 * If vm_flags changed after call_mmap(), we should try merge
+		 * If vm_flags changed after mmap_file(), we should try merge
 		 * vma again as we may succeed this time.
 		 */
 		if (unlikely(vm_flags != vma->vm_flags && prev)) {
diff --git a/mm/nommu.c b/mm/nommu.c
index 7f9e9e5a0e12..e976c62264c9 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -896,7 +896,7 @@ static int do_mmap_shared_file(struct vm_area_struct *vma)
 {
 	int ret;
 
-	ret = call_mmap(vma->vm_file, vma);
+	ret = mmap_file(vma->vm_file, vma);
 	if (ret == 0) {
 		vma->vm_region->vm_top = vma->vm_region->vm_end;
 		return 0;
@@ -929,7 +929,7 @@ static int do_mmap_private(struct vm_area_struct *vma,
 	 * happy.
 	 */
 	if (capabilities & NOMMU_MAP_DIRECT) {
-		ret = call_mmap(vma->vm_file, vma);
+		ret = mmap_file(vma->vm_file, vma);
 		/* shouldn't return success if we're not sharing */
 		if (WARN_ON_ONCE(!is_nommu_shared_mapping(vma->vm_flags)))
 			ret = -ENOSYS;
-- 
2.47.0