[PATCH v2] f2fs: disallow mmap write and data-modifying fallocate on atomic files

[Daeho Jeong <daeho43@gmail.com>]

From: Daeho Jeong <daehojeong@google.com>

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>
---
v2: serialize atomic flag check in f2fs_vm_page_mkwrite().
---
 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 ef4d218e694..7020b3e8940 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -187,6 +187,17 @@ 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);
 	if (unlikely(folio->mapping != inode->i_mapping ||
 			folio_pos(folio) > i_size_read(inode) ||
@@ -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)
-- 
2.56.0.385.gd3acb90ef8-goog