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

[Al Viro <viro@zeniv.linux.org.uk>]

On Fri, Jul 04, 2025 at 12:43:13AM +0100, Al Viro wrote:

> 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.

IOW, I would prefer to do this:

[PATCH] fix proc_sys_compare() handling of in-lookup dentries

There's one case where ->d_compare() can be called for an in-lookup
dentry; usually that's nothing special from ->d_compare() point of
view, but... proc_sys_compare() is weird.

The thing is, /proc/sys subdirectories can look differently for
different processes.  Up to and including having the same name
resolve to different dentries - all of them hashed.

The way it's done is ->d_compare() refusing to admit a match unless
this dentry is supposed to be visible to this caller.  The information
needed to discriminate between them is stored in inode; it is set
during proc_sys_lookup() and until it's done d_splice_alias() we really
can't tell who should that dentry be visible for.

Normally there's no negative dentries in /proc/sys; we can run into
a dying dentry in RCU dcache lookup, but those can be safely rejected.

However, ->d_compare() is also called for in-lookup dentries, before
they get positive - or hashed, for that matter.  In case of match
we will wait until dentry leaves in-lookup state and repeat ->d_compare()
afterwards.  In other words, the right behaviour is to treat the
name match as sufficient for in-lookup dentries; if dentry is not
for us, we'll see that when we recheck once proc_sys_lookup() is
done with it.
    
While we are at it, fix the misspelled READ_ONCE and WRITE_ONCE there.

Fixes: d9171b934526 ("parallel lookups machinery, part 4 (and last)")
Reported-by: NeilBrown <neilb@brown.name>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
diff --git a/fs/proc/inode.c b/fs/proc/inode.c
index a3eb3b740f76..3604b616311c 100644
--- a/fs/proc/inode.c
+++ b/fs/proc/inode.c
@@ -42,7 +42,7 @@ static void proc_evict_inode(struct inode *inode)
 
 	head = ei->sysctl;
 	if (head) {
-		RCU_INIT_POINTER(ei->sysctl, NULL);
+		WRITE_ONCE(ei->sysctl, NULL);
 		proc_sys_evict_inode(inode, head);
 	}
 }
diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index cc9d74a06ff0..08b78150cdde 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -918,17 +918,21 @@ static int proc_sys_compare(const struct dentry *dentry,
 	struct ctl_table_header *head;
 	struct inode *inode;
 
-	/* Although proc doesn't have negative dentries, rcu-walk means
-	 * that inode here can be NULL */
-	/* AV: can it, indeed? */
-	inode = d_inode_rcu(dentry);
-	if (!inode)
-		return 1;
 	if (name->len != len)
 		return 1;
 	if (memcmp(name->name, str, len))
 		return 1;
-	head = rcu_dereference(PROC_I(inode)->sysctl);
+
+	// false positive is fine here - we'll recheck anyway
+	if (d_in_lookup(dentry))
+		return 0;
+
+	inode = d_inode_rcu(dentry);
+	// we just might have run into dentry in the middle of __dentry_kill()
+	if (!inode)
+		return 1;
+
+	head = READ_ONCE(PROC_I(inode)->sysctl);
 	return !head || !sysctl_is_seen(head);
 }