FAILED: patch "[PATCH] nilfs2: fix use-after-free bug of ns_writer on remount" failed to apply to 4.14-stable tree

The patch below does not apply to the 4.14-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 <[email protected]>.

Possible dependencies:

8cccf05fe857 ("nilfs2: fix use-after-free bug of ns_writer on remount")
1751e8a6cb93 ("Rename superblock flags (MS_xyz -> SB_xyz)")
b04a23421bf6 ("Merge branch 'overlayfs-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/vfs")

thanks,

greg k-h

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

From 8cccf05fe857a18ee26e20d11a8455a73ffd4efd Mon Sep 17 00:00:00 2001
From: Ryusuke Konishi <[email protected]>
Date: Fri, 4 Nov 2022 23:29:59 +0900
Subject: [PATCH] nilfs2: fix use-after-free bug of ns_writer on remount

If a nilfs2 filesystem is downgraded to read-only due to metadata corruption on disk and is remounted read/write, or if emergency read-only remount is performed, detaching a log writer and synchronizing the filesystem can be done at the same time.

In these cases, use-after-free of the log writer (hereinafter nilfs->ns_writer) can happen as shown in the scenario below:

Task1 Task2
-------------------------------- ------------------------------
nilfs_construct_segment
nilfs_segctor_sync
init_wait
init_waitqueue_entry
add_wait_queue
schedule
nilfs_remount (R/W remount case)
nilfs_attach_log_writer
nilfs_detach_log_writer
nilfs_segctor_destroy
kfree
finish_wait
_raw_spin_lock_irqsave
__raw_spin_lock_irqsave
do_raw_spin_lock
debug_spin_lock_before <-- use-after-free

While Task1 is sleeping, nilfs->ns_writer is freed by Task2. After Task1 waked up, Task1 accesses nilfs->ns_writer which is already freed. This scenario diagram is based on the Shigeru Yoshida's post [1].

This patch fixes the issue by not detaching nilfs->ns_writer on remount so that this UAF race doesn't happen. Along with this change, this patch also inserts a few necessary read-only checks with superblock instance where only the ns_writer pointer was used to check if the filesystem is read-only.

index b4cebad21b48..3335ef352915 100644
@@ -317,7 +317,7 @@ void nilfs_relax_pressure_in_lock(struct super_block *sb)
 	struct the_nilfs *nilfs = sb->s_fs_info;
 	struct nilfs_sc_info *sci = nilfs->ns_writer;
 
-	if (!sci || !sci->sc_flush_request)
+	if (sb_rdonly(sb) || unlikely(!sci) || !sci->sc_flush_request)
 		return;
 
 	set_bit(NILFS_SC_PRIOR_FLUSH, &sci->sc_flags);

@@ -2242,7 +2242,7 @@ int nilfs_construct_segment(struct super_block *sb)
 	struct nilfs_sc_info *sci = nilfs->ns_writer;
 	struct nilfs_transaction_info *ti;
 
-	if (!sci)
+	if (sb_rdonly(sb) || unlikely(!sci))
 		return -EROFS;
 
 	/* A call inside transactions causes a deadlock. */

@@ -2280,7 +2280,7 @@ int nilfs_construct_dsync_segment(struct super_block *sb, struct inode *inode,
 	struct nilfs_transaction_info ti;
 	int err = 0;
 
-	if (!sci)
+	if (sb_rdonly(sb) || unlikely(!sci))
 		return -EROFS;
 
 	nilfs_transaction_lock(sb, &ti, 0);

@@ -2776,11 +2776,12 @@ int nilfs_attach_log_writer(struct super_block *sb, struct nilfs_root *root)
 
 	if (nilfs->ns_writer) {
 		/*
-		 * This happens if the filesystem was remounted
-		 * read/write after nilfs_error degenerated it into a
-		 * read-only mount.
+		 * This happens if the filesystem is made read-only by
+		 * __nilfs_error or nilfs_remount and then remounted
+		 * read/write.  In these cases, reuse the existing
+		 * writer.
 		 */
-		nilfs_detach_log_writer(sb);
+		return 0;
 	}
 
 	nilfs->ns_writer = nilfs_segctor_new(sb, root);

index ba108f915391..6edb6e0dd61f 100644
@@ -1133,8 +1133,6 @@ static int nilfs_remount(struct super_block *sb, int *flags, char *data)
 	if ((bool)(*flags & SB_RDONLY) == sb_rdonly(sb))
 		goto out;
 	if (*flags & SB_RDONLY) {
-		/* Shutting down log writer */
-		nilfs_detach_log_writer(sb);
 		sb->s_flags |= SB_RDONLY;
 
 		/*