linux-hardening - Re: [PATCH 03/19] fs: release anon dev_t in deactivate_locked

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Date: Tue, 26 Sep 2023 22:25:15 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Christoph Hellwig <hch@....de>
Cc: Christian Brauner <brauner@...nel.org>,
	Heiko Carstens <hca@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>,
	Alexander Gordeev <agordeev@...ux.ibm.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Reinette Chatre <reinette.chatre@...el.com>,
	Miquel Raynal <miquel.raynal@...tlin.com>,
	Richard Weinberger <richard@....at>,
	Vignesh Raghavendra <vigneshr@...com>,
	Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>,
	Tejun Heo <tj@...nel.org>,
	Trond Myklebust <trond.myklebust@...merspace.com>,
	Anna Schumaker <anna@...nel.org>, Kees Cook <keescook@...omium.org>,
	Damien Le Moal <dlemoal@...nel.org>,
	Naohiro Aota <naohiro.aota@....com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-s390@...r.kernel.org, linux-rdma@...r.kernel.org,
	linux-nfs@...r.kernel.org, linux-hardening@...r.kernel.org,
	cgroups@...r.kernel.org
Subject: Re: [PATCH 03/19] fs: release anon dev_t in deactivate_locked_super

On Tue, Sep 26, 2023 at 11:38:34AM +0200, Christoph Hellwig wrote:

> How?
> 
> Old sequence before his patch:
> 
> 	deactivate_locked_super()
> 	  -> kill_anon_super()
> 	    -> generic_shutdown_super()
> 	    -> kill_super_notify()
> 	    -> free_anon_bdev()
> 	  -> kill_super_notify()
> 
> New sequence with this patch:
> 
> 	deactivate_locked_super()
> 	  -> generic_shutdown_super()
> 	    -> kill_super_notify()
> 	    -> free_anon_bdev()
> 

Before your patch: foo_kill_super() calls kill_anon_super(),
which calls kill_super_notify(), which removes the sucker from
the list, then frees ->s_fs_info.  After your patch:
removal from the lists happens via the call of kill_super_notify()
*after* both of your methods had been called, while freeing
->s_fs_info happens from the method call.  IOW, you've restored
the situation prior to "super: ensure valid info".  The whole
point of that commit had been to make sure that we have nothing
in the lists with ->s_fs_info pointing to a freed object.

It's not about free_anon_bdev(); that part is fine - it's the
"we can drop the weird second call site of kill_super_notify()"
thing that is broken.

Al, still slogging through the rcu pathwalk races in the methods...
The latest catch: nfs_set_verifier() can get called on a dentry
that had just been seen to have positive parent, but is not
pinned down.
	grab ->d_lock; OK, we know that dentry won't get freed under us
	fetch ->d_parent->d_inode
	pass that to nfs_verify_change_attribute()
... which assumes that inode it's been given is not NULL.  Normally it
would've been - ->d_lock stabilizes ->d_parent, and negative dentries
obviously have no children.  Except that we might've been just hit
by dentry_kill() due to eviction on memory pressure, got ->d_lock
right after that and proceeded to play with ->d_parent, just as
that parent is going through dentry_kill() from the same eviction on
memory pressure...  If it gets to dentry_unlink_inode() before we get to
fetching ->d_parent->d_inode, nfs_verify_change_attribute(NULL, whatever)
is going to oops...