Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
From:   Lorenzo Stoakes <lstoakes@gmail.com>
To:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Muchun Song <muchun.song@linux.dev>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <brauner@kernel.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Lorenzo Stoakes <lstoakes@gmail.com>
Subject: [RFC PATCH 3/3] mm: perform the mapping_map_writable() check after call_mmap()
Date:   Mon,  3 Apr 2023 23:28:32 +0100
Message-Id: <c814a3694f09896e4ec85cbca74069ea6174ebb6.1680560277.git.lstoakes@gmail.com>
In-Reply-To: <cover.1680560277.git.lstoakes@gmail.com>
References: <cover.1680560277.git.lstoakes@gmail.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

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.

Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
---
 mm/mmap.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index c96dcce90772..a166e9f3c474 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2596,17 +2596,17 @@ 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 unmap_and_free_vma;
+		}
+
 		/*
 		 * Expansion is handled above, merging is handled below.
 		 * Drivers should not alter the address of the VMA.
-- 
2.40.0