linux-kernel - Re: [PATCH v3?] proc_sysctl: remove rcu_dereference() for accessing ->sysctl

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Message-id: <175159315670.565058.128329102948224076@noble.neil.brown.name>
Date: Fri, 04 Jul 2025 11:39:16 +1000
From: "NeilBrown" <neil@...wn.name>
To: "Al Viro" <viro@...iv.linux.org.uk>
Cc: "Kees Cook" <kees@...nel.org>, "Joel Granados" <joel.granados@...nel.org>,
 linux-fsdevel@...r.kernel.org, "LKML" <linux-kernel@...r.kernel.org>
Subject:
 Re: [PATCH v3?] proc_sysctl: remove rcu_dereference() for accessing ->sysctl

On Fri, 04 Jul 2025, Al Viro wrote:
> On Mon, Jun 16, 2025 at 12:49:51PM +1000, NeilBrown wrote:
> 
> > The reality is that ->sysctl does not need rcu protection.  There is no
> > concurrent update except that it can be set to NULL which is pointless.
> 
> I would rather *not* leave a dangling pointer there, and yes, it can
> end up being dangling.  kfree_rcu() from inside the ->evict_inode()
> may very well happen earlier than (also RCU-delayed) freeing of struct
> inode itself.

In that case could we move the proc_sys_evict_inode() call from
proc_evict_inode() to proc_free_inode(), and replace kfree_rcu() with
kfree()?
Or does the inode need to be deleted from ->sibling_inodes earlier than
free_inode?

> 
> What we can do is WRITE_ONCE() to set it to NULL on the evict_inode
> side and READ_ONCE() in the proc_sys_compare().

That is likely the simplest change.

Thanks,
NeilBrown

> 
> The reason why the latter is memory-safe is that ->d_compare() for
> non-in-lookup dentries is called either under rcu_read_lock() (in which
> case observing non-NULL means that kfree_rcu() couldn't have gotten to
> freeing the sucker) *or* under ->d_lock, in which case the inode can't
> reach ->evict_inode() until we are done.
> 
> So this predicate is very much relevant.  Have that fucker called with
> neither rcu_read_lock() nor ->d_lock, and you might very well end up
> with dereferencing an already freed ctl_table_header.
>