FAILED: patch "[PATCH] f2fs: disallow mmap write and data-modifying fallocate on atomic files" failed to apply to 6.6-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x 06dfe4a7eb696d868b196c2a49873142f19bd862
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107053447-failed-6.6-06dfe4a7eb69@kernel.org' --subject-prefix 'PATCH 6.6.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 06dfe4a7eb696d868b196c2a49873142f19bd862 Mon Sep 17 00:00:00 2001
From: Daeho Jeong <daehojeong@google.com>
Date: Sat, 10 Oct 2026 02:41:33 +0000
Subject: [PATCH] f2fs: disallow mmap write and data-modifying fallocate on
 atomic files

Stores through a shared writable mapping and fallocate with
FALLOC_FL_PUNCH_HOLE, FALLOC_FL_ZERO_RANGE, FALLOC_FL_COLLAPSE_RANGE or
FALLOC_FL_INSERT_RANGE do not go through ->write_begin, so they bypass
the COW inode and modify the original inode directly. Reject them while
the file is in atomic write mode, as fallocate already does for pinned
and compressed files. A write fault gets SIGBUS and fallocate gets
-EOPNOTSUPP.

fallocate without these flags only preallocates blocks and stays
allowed.

f2fs_vm_page_mkwrite() cannot take inode_lock, so check the flag under
invalidate_lock, and hold invalidate_lock in
f2fs_ioc_start_atomic_write() from the flush until the flag is set.
Otherwise a fault could check the flag, then start_atomic_write could
flush and set the flag, and the fault would dirty the folio afterwards.

Cc: stable@kernel.org
Fixes: 3db1de0e582c ("f2fs: change the current atomic write way")
Signed-off-by: Daeho Jeong <daehojeong@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
---
 fs/f2fs/file.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index d4d859dbef880..58599ab4937de 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -188,6 +188,17 @@ static vm_fault_t f2fs_vm_page_mkwrite(struct vm_fault *vmf)
 	file_update_time(vmf->vma->vm_file);
 	filemap_invalidate_lock_shared(inode->i_mapping);
 
+	/*
+	 * mmap stores bypass the COW inode and would break atomicity. Check
+	 * under invalidate_lock: f2fs_ioc_start_atomic_write() holds it while
+	 * flushing dirty folios and setting FI_ATOMIC_FILE, so either it sees
+	 * our dirty folio and writes it back, or we see the flag.
+	 */
+	if (unlikely(f2fs_is_atomic_file(inode))) {
+		err = -EOPNOTSUPP;
+		goto out_sem;
+	}
+
 	folio_lock(folio);
 	/*
 	 * A concurrent buffered write can extend isize, so we store isize in a
@@ -2113,9 +2124,11 @@ static long f2fs_fallocate(struct file *file, int mode,
 
 	/*
 	 * Pinned file should not support partial truncation since the block
-	 * can be used by applications.
+	 * can be used by applications. Atomic files should not either, since
+	 * these modify the original inode directly and break atomicity.
 	 */
-	if ((f2fs_compressed_file(inode) || f2fs_is_pinned_file(inode)) &&
+	if ((f2fs_compressed_file(inode) || f2fs_is_pinned_file(inode) ||
+	     f2fs_is_atomic_file(inode)) &&
 		(mode & (FALLOC_FL_PUNCH_HOLE | FALLOC_FL_COLLAPSE_RANGE |
 			FALLOC_FL_ZERO_RANGE | FALLOC_FL_INSERT_RANGE))) {
 		ret = -EOPNOTSUPP;
@@ -2403,6 +2416,8 @@ static int f2fs_ioc_start_atomic_write(struct file *filp, bool truncate)
 
 	f2fs_down_write(&fi->i_gc_rwsem[WRITE]);
 	f2fs_down_write(&fi->i_gc_rwsem[READ]);
+	/* serialize with f2fs_vm_page_mkwrite(), see the comment there */
+	filemap_invalidate_lock(inode->i_mapping);
 
 	/*
 	 * Should wait end_io to count F2FS_WB_CP_DATA correctly by
@@ -2458,6 +2473,7 @@ static int f2fs_ioc_start_atomic_write(struct file *filp, bool truncate)
 	f2fs_i_size_write(fi->cow_inode, isize);
 
 out_unlock:
+	filemap_invalidate_unlock(inode->i_mapping);
 	f2fs_up_write(&fi->i_gc_rwsem[READ]);
 	f2fs_up_write(&fi->i_gc_rwsem[WRITE]);
 	if (ret)
-- 
2.53.0