[PATCH v3 3/3] mm: enforce the mapping_map_writable() check after call_mmap()

[Lorenzo Stoakes <lstoakes@gmail.com>]

In order for an F_SEAL_WRITE sealed memfd mapping to have an opportunity to
clear VM_MAYWRITE in seal_check_write() we must be able to invoke either
the shmem_mmap() or hugetlbfs_file_mmap() f_ops->mmap() handler to do so.

We would otherwise fail the mapping_map_writable() check before we had
the opportunity to clear VM_MAYWRITE.

However, the existing logic in mmap_region() performs this check BEFORE
calling call_mmap() (which invokes file->f_ops->mmap()). We must enforce
this check AFTER the function call.

In order to avoid any risk of breaking call_mmap() handlers which assume
this will have been done first, we continue to mark the file writable
first, simply deferring enforcement of it failing until afterwards.

This enables mmap(..., PROT_READ, MAP_SHARED, fd, 0) mappings for memfd's
sealed via F_SEAL_WRITE to succeed, whereas previously they were not
permitted.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=217238
Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
---
 mm/mmap.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 6f6856b3267a..9fbee92aaaee 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2767,17 +2767,25 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 	vma->vm_pgoff = pgoff;
 
 	if (file) {
-		if (is_shared_maywrite(vm_flags)) {
-			error = mapping_map_writable(file->f_mapping);
-			if (error)
-				goto free_vma;
-		}
+		int writable_error = 0;
+
+		if (vma_is_shared_maywrite(vma))
+			writable_error = mapping_map_writable(file->f_mapping);
 
 		vma->vm_file = get_file(file);
 		error = call_mmap(file, vma);
 		if (error)
 			goto unmap_and_free_vma;
 
+		/*
+		 * call_mmap() may have changed VMA flags, so retry this check
+		 * if it failed before.
+		 */
+		if (writable_error && vma_is_shared_maywrite(vma)) {
+			error = writable_error;
+			goto close_and_free_vma;
+		}
+
 		/*
 		 * Expansion is handled above, merging is handled below.
 		 * Drivers should not alter the address of the VMA.
-- 
2.42.0