This is a note to let you know that I've just added the patch titled fs: Protect reconfiguration of sb read-write from racing writes to the 6.4-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: fs-protect-reconfiguration-of-sb-read-write-from-rac.patch and it can be found in the queue-6.4 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. commit 8eb5af60150d9f07b12ba80f3a83ede0c3bc0707 Author: Jan Kara <jack@xxxxxxx> Date: Thu Jun 15 13:38:48 2023 +0200 fs: Protect reconfiguration of sb read-write from racing writes [ Upstream commit c541dce86c537714b6761a79a969c1623dfa222b ] The reconfigure / remount code takes a lot of effort to protect filesystem's reconfiguration code from racing writes on remounting read-only. However during remounting read-only filesystem to read-write mode userspace writes can start immediately once we clear SB_RDONLY flag. This is inconvenient for example for ext4 because we need to do some writes to the filesystem (such as preparation of quota files) before we can take userspace writes so we are clearing SB_RDONLY flag before we are fully ready to accept userpace writes and syzbot has found a way to exploit this [1]. Also as far as I'm reading the code the filesystem remount code was protected from racing writes in the legacy mount path by the mount's MNT_READONLY flag so this is relatively new problem. It is actually fairly easy to protect remount read-write from racing writes using sb->s_readonly_remount flag so let's just do that instead of having to workaround these races in the filesystem code. [1] https://lore.kernel.org/all/00000000000006a0df05f6667499@xxxxxxxxxx/T/ Signed-off-by: Jan Kara <jack@xxxxxxx> Message-Id: <20230615113848.8439-1-jack@xxxxxxx> Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> diff --git a/fs/super.c b/fs/super.c index 09668ddfbbd55..860d7a4b14c7c 100644 --- a/fs/super.c +++ b/fs/super.c @@ -903,6 +903,7 @@ int reconfigure_super(struct fs_context *fc) struct super_block *sb = fc->root->d_sb; int retval; bool remount_ro = false; + bool remount_rw = false; bool force = fc->sb_flags & SB_FORCE; if (fc->sb_flags_mask & ~MS_RMT_MASK) @@ -920,7 +921,7 @@ int reconfigure_super(struct fs_context *fc) bdev_read_only(sb->s_bdev)) return -EACCES; #endif - + remount_rw = !(fc->sb_flags & SB_RDONLY) && sb_rdonly(sb); remount_ro = (fc->sb_flags & SB_RDONLY) && !sb_rdonly(sb); } @@ -950,6 +951,14 @@ int reconfigure_super(struct fs_context *fc) if (retval) return retval; } + } else if (remount_rw) { + /* + * We set s_readonly_remount here to protect filesystem's + * reconfigure code from writes from userspace until + * reconfigure finishes. + */ + sb->s_readonly_remount = 1; + smp_wmb(); } if (fc->ops->reconfigure) {