Re: [PATCH v4 3/3] mm: perform the mapping_map_writable() check after call_mmap()

[Lorenzo Stoakes <lstoakes@gmail.com>]

On Thu, Oct 12, 2023 at 06:04:30PM +0100, Lorenzo Stoakes wrote:
> 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.
>
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=217238
> Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
> ---
>  mm/mmap.c | 23 ++++++++++++++---------
>  1 file changed, 14 insertions(+), 9 deletions(-)
>
[snip]

Andrew, could you apply the following -fix patch to this? As a bug was
detected in the implementation [0] - I was being over-zealous in setting
the writable_file_mapping flag and had falsely assumed vma->vm_file == file
in all instances of the cleanup. The fix is to only set it in one place.

[0]: https://lore.kernel.org/all/CA+G9fYtL7wK-dE-Tnz4t-GWmQb50EPYa=TWGjpgYU2Z=oeAO_w@mail.gmail.com/

----8<----
From 7feea6faada5b10a872c24755cc630220cba619a Mon Sep 17 00:00:00 2001
From: Lorenzo Stoakes <lstoakes@gmail.com>
Date: Mon, 16 Oct 2023 17:17:13 +0100
Subject: [PATCH] mm: perform the mapping_map_writable() check after
 call_mmap()

Do not set writable_file_mapping in an instance where it is not appropriate
to do so.

Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
---
 mm/mmap.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 7f45a08e7973..8b57e42fd980 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2923,10 +2923,8 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 	mm->map_count++;
 	if (vma->vm_file) {
 		i_mmap_lock_write(vma->vm_file->f_mapping);
-		if (vma_is_shared_maywrite(vma)) {
+		if (vma_is_shared_maywrite(vma))
 			mapping_allow_writable(vma->vm_file->f_mapping);
-			writable_file_mapping = true;
-		}
 
 		flush_dcache_mmap_lock(vma->vm_file->f_mapping);
 		vma_interval_tree_insert(vma, &vma->vm_file->f_mapping->i_mmap);
-- 
2.42.0