Re: [PATCH] proc: Use new_inode not new_inode_pseudo

[Jan Kara]

On Fri 12-06-20 14:15:51, Eric W. Biederman wrote:
> 
> Recently syzbot reported that unmounting proc when there is an ongoing
> inotify watch on the root directory of proc could result in a use
> after free when the watch is removed after the unmount of proc
> when the watcher exits.
> 
> Commit 69879c01a0c3 ("proc: Remove the now unnecessary internal mount
> of proc") made it easier to unmount proc and allowed syzbot to see the
> problem, but looking at the code it has been around for a long time.
> 
> Looking at the code the fsnotify watch should have been removed by
> fsnotify_sb_delete in generic_shutdown_super.  Unfortunately the inode
> was allocated with new_inode_pseudo instead of new_inode so the inode
> was not on the sb->s_inodes list.  Which prevented
> fsnotify_unmount_inodes from finding the inode and removing the watch
> as well as made it so the "VFS: Busy inodes after unmount" warning
> could not find the inodes to warn about them.
> 
> Make all of the inodes in proc visible to generic_shutdown_super,
> and fsnotify_sb_delete by using new_inode instead of new_inode_pseudo.
> The only functional difference is that new_inode places the inodes
> on the sb->s_inodes list.
> 
> I wrote a small test program and I can verify that without changes it
> can trigger this issue, and by replacing new_inode_pseudo with
> new_inode the issues goes away.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Link: https://lkml.kernel.org/r/000000000000d788c905a7dfa3f4@xxxxxxxxxx
> Reported-by: syzbot+7d2debdcdb3cb93c1e5e@xxxxxxxxxxxxxxxxxxxxxxxxx
> Fixes: 0097875bd415 ("proc: Implement /proc/thread-self to point at the directory of the current thread")
> Fixes: 021ada7dff22 ("procfs: switch /proc/self away from proc_dir_entry")
> Fixes: 51f0885e5415 ("vfs,proc: guarantee unique inodes in /proc")
> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>

Thanks for analysing this! I agree with the analysis and the patch looks
good to me. You can add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza

> ---
>  fs/proc/inode.c       | 2 +-
>  fs/proc/self.c        | 2 +-
>  fs/proc/thread_self.c | 2 +-
>  3 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/proc/inode.c b/fs/proc/inode.c
> index f40c2532c057..28d6105e908e 100644
> --- a/fs/proc/inode.c
> +++ b/fs/proc/inode.c
> @@ -617,7 +617,7 @@ const struct inode_operations proc_link_inode_operations = {
>  
>  struct inode *proc_get_inode(struct super_block *sb, struct proc_dir_entry *de)
>  {
> -	struct inode *inode = new_inode_pseudo(sb);
> +	struct inode *inode = new_inode(sb);
>  
>  	if (inode) {
>  		inode->i_ino = de->low_ino;
> diff --git a/fs/proc/self.c b/fs/proc/self.c
> index ca5158fa561c..72cd69bcaf4a 100644
> --- a/fs/proc/self.c
> +++ b/fs/proc/self.c
> @@ -43,7 +43,7 @@ int proc_setup_self(struct super_block *s)
>  	inode_lock(root_inode);
>  	self = d_alloc_name(s->s_root, "self");
>  	if (self) {
> -		struct inode *inode = new_inode_pseudo(s);
> +		struct inode *inode = new_inode(s);
>  		if (inode) {
>  			inode->i_ino = self_inum;
>  			inode->i_mtime = inode->i_atime = inode->i_ctime = current_time(inode);
> diff --git a/fs/proc/thread_self.c b/fs/proc/thread_self.c
> index ac284f409568..a553273fbd41 100644
> --- a/fs/proc/thread_self.c
> +++ b/fs/proc/thread_self.c
> @@ -43,7 +43,7 @@ int proc_setup_thread_self(struct super_block *s)
>  	inode_lock(root_inode);
>  	thread_self = d_alloc_name(s->s_root, "thread-self");
>  	if (thread_self) {
> -		struct inode *inode = new_inode_pseudo(s);
> +		struct inode *inode = new_inode(s);
>  		if (inode) {
>  			inode->i_ino = thread_self_inum;
>  			inode->i_mtime = inode->i_atime = inode->i_ctime = current_time(inode);
> -- 
> 2.20.1
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR