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

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Message-ID: <20250703234313.GM1880847@ZenIV>
Date: Fri, 4 Jul 2025 00:43:13 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: NeilBrown <neil@...wn.name>
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 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.

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().

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.