linux-kernel - [PATCH v2 3/3] mm: perform the mapping_map_writable() check after call

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Message-Id: <6f3aea05c9cc46094b029cbd1138d163c1ae7f9d.1682890156.git.lstoakes@gmail.com>
Date:   Sun, 30 Apr 2023 23:26:07 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        linux-fsdevel@...r.kernel.org, Jan Kara <jack@...e.cz>,
        Hugh Dickins <hughd@...gle.com>,
        Lorenzo Stoakes <lstoakes@...il.com>
Subject: [PATCH v2 3/3] mm: perform the mapping_map_writable() check after call_mmap()

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.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=217238
Signed-off-by: Lorenzo Stoakes <lstoakes@...il.com>
---
 mm/mmap.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 646e34e95a37..1608d7f5a293 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2642,17 +2642,17 @@ 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;
-		}
-
 		vma->vm_file = get_file(file);
 		error = call_mmap(file, vma);
 		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;
+		}
+
 		/*
 		 * Expansion is handled above, merging is handled below.
 		 * Drivers should not alter the address of the VMA.
--
2.40.1