linux-kernel - Re: [PATCH] kernfs: Fix UAF in PSI polling when open file is released

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <ql5573r2nbex53fyygwczyjipmtalc22n3hxxzmqwi2sgadodt@a5pesn5gunf2>
Date: Fri, 15 Aug 2025 16:42:06 +0200
From: Michal Koutný <mkoutny@...e.com>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: Chen Ridong <chenridong@...weicloud.com>, tj@...nel.org, 
	hannes@...xchg.org, peterz@...radead.org, zhouchengming@...edance.com, 
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org, lujialin4@...wei.com, 
	chenridong@...wei.com
Subject: Re: [PATCH] kernfs: Fix UAF in PSI polling when open file is released

On Fri, Aug 15, 2025 at 08:11:39AM +0200, Greg KH <gregkh@...uxfoundation.org> wrote:
> > diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
> > index a6c692cac616..d5d01f0b9392 100644
> > --- a/fs/kernfs/file.c
> > +++ b/fs/kernfs/file.c
> > @@ -852,7 +852,7 @@ static __poll_t kernfs_fop_poll(struct file *filp, poll_table *wait)
> >  	struct kernfs_node *kn = kernfs_dentry_node(filp->f_path.dentry);
> >  	__poll_t ret;
> >  
> > -	if (!kernfs_get_active(kn))
> > +	if (of->released || !kernfs_get_active(kn))
> 
> I can see why the cgroup change is needed,

I don't see it that much. of->priv isn't checked in cgroup code anywhere
so it isn't helpful zeroing. As Ridong writes it may trade UaF for NULL
pointer deref :-/ (Additionally, same zeroing would be needed in error
path in cgroup_file_open().)

I _think_ the place to cleanup would be in
@@ -3978,6 +3978,8 @@ static ssize_t cgroup_pressure_write(struct kernfs_open_file *of,
                psi->enabled = enable;
                if (enable)
                        psi_cgroup_restart(psi);
+               else
+                       psi_trigger_destroy(???);
        }

        cgroup_kn_unlock(of->kn);

The issue is that cgroup_pressure_write doesn't know all possible
triggers to be cancelled. (The fix with of->released would only
sanitize effect but not the cause IMO.)

HTH,
Michal

Download attachment "signature.asc" of type "application/pgp-signature" (229 bytes)