[PATCH 5.4.y 3/3] mm: perform the mapping_map_writable() check after call_mmap()

["Isaac J. Manjarres" <isaacmanjarres@google.com>]

From: Lorenzo Stoakes <lstoakes@gmail.com>

[ Upstream commit 158978945f3173b8c1a88f8c5684a629736a57ac ]

In order for a F_SEAL_WRITE sealed memfd mapping to have an opportunity to
clear VM_MAYWRITE, we must be able to invoke the appropriate
vm_ops->mmap() handler to do so.  We would otherwise fail the
mapping_map_writable() check before we had the opportunity to avoid it.

This patch moves this check after the call_mmap() invocation.  Only memfd
actively denies write access causing a potential failure here (in
memfd_add_seals()), so there should be no impact on non-memfd cases.

This patch makes the userland-visible change that MAP_SHARED, PROT_READ
mappings of an F_SEAL_WRITE sealed memfd mapping will now succeed.

There is a delicate situation with cleanup paths assuming that a writable
mapping must have occurred in circumstances where it may now not have.  In
order to ensure we do not accidentally mark a writable file unwritable by
mistake, we explicitly track whether we have a writable mapping and unmap
only if we do.

[lstoakes@gmail.com: do not set writable_file_mapping in inappropriate case]
  Link: https://lkml.kernel.org/r/c9eb4cc6-7db4-4c2b-838d-43a0b319a4f0@lucifer.local
Link: https://bugzilla.kernel.org/show_bug.cgi?id=217238
Link: https://lkml.kernel.org/r/55e413d20678a1bb4c7cce889062bbb07b0df892.1697116581.git.lstoakes@gmail.com
Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: stable@vger.kernel.org
[isaacmanjarres: added error handling to cleanup the work done by the
mmap() callback and removed unused label.]
Signed-off-by: Isaac J. Manjarres <isaacmanjarres@google.com>
---
 mm/mmap.c | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index cb712ae731cd..e591a82a26a8 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1718,6 +1718,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 {
 	struct mm_struct *mm = current->mm;
 	struct vm_area_struct *vma, *prev;
+	bool writable_file_mapping = false;
 	int error;
 	struct rb_node **rb_link, *rb_parent;
 	unsigned long charged = 0;
@@ -1785,11 +1786,6 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 			if (error)
 				goto free_vma;
 		}
-		if (is_shared_maywrite(vm_flags)) {
-			error = mapping_map_writable(file->f_mapping);
-			if (error)
-				goto allow_write_and_free_vma;
-		}
 
 		/* ->mmap() can change vma->vm_file, but must guarantee that
 		 * vma_link() below can deny write-access if VM_DENYWRITE is set
@@ -1801,6 +1797,14 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 		if (error)
 			goto unmap_and_free_vma;
 
+		if (vma_is_shared_maywrite(vma)) {
+			error = mapping_map_writable(file->f_mapping);
+			if (error)
+				goto close_and_free_vma;
+
+			writable_file_mapping = true;
+		}
+
 		/* Can addr have changed??
 		 *
 		 * Answer: Yes, several device drivers can do it in their
@@ -1823,7 +1827,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 	vma_link(mm, vma, prev, rb_link, rb_parent);
 	/* Once vma denies write, undo our temporary denial count */
 	if (file) {
-		if (is_shared_maywrite(vm_flags))
+		if (writable_file_mapping)
 			mapping_unmap_writable(file->f_mapping);
 		if (vm_flags & VM_DENYWRITE)
 			allow_write_access(file);
@@ -1858,15 +1862,17 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 
 	return addr;
 
+close_and_free_vma:
+	if (vma->vm_ops && vma->vm_ops->close)
+		vma->vm_ops->close(vma);
 unmap_and_free_vma:
 	vma->vm_file = NULL;
 	fput(file);
 
 	/* Undo any partial mapping done by a device driver. */
 	unmap_region(mm, vma, prev, vma->vm_start, vma->vm_end);
-	if (is_shared_maywrite(vm_flags))
+	if (writable_file_mapping)
 		mapping_unmap_writable(file->f_mapping);
-allow_write_and_free_vma:
 	if (vm_flags & VM_DENYWRITE)
 		allow_write_access(file);
 free_vma:
-- 
2.50.1.552.g942d659e1b-goog