linux-kernel - Re: [Linux kernel bug] KASAN: slab-use-after-free Read in pressure

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Date: Sat, 25 May 2024 22:58:49 +0800
From: Chengming Zhou <zhouchengming@...edance.com>
To: Michal Koutný <mkoutny@...e.com>,
 Sam Sun <samsun1006219@...il.com>
Cc: linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
 hannes@...xchg.org, lizefan.x@...edance.com, tj@...nel.org,
 syzkaller-bugs@...glegroups.com, xrivendell7@...il.com
Subject: Re: [Linux kernel bug] KASAN: slab-use-after-free Read in
 pressure_write

On 2024/5/25 00:03, Michal Koutný wrote:
> On Fri, May 17, 2024 at 03:14:23PM GMT, Sam Sun <samsun1006219@...il.com> wrote:
>> ...
>> We analyzed the root cause of this problem. It happens when
>> concurrently accessing
>> "/sys/fs/cgroup/sys-fs-fuse-connections.mount/irq.pressure" and
>> "/sys/fs/cgroup/sys-fs-fuse-connections.mount/cgroup.pressure". If we
>> echo 0 to cgroup.pressure, kernel will invoke cgroup_pressure_write(),
>> and call kernfs_show(). It will set kn->flags to KERNFS_HIDDEN and
>> call kernfs_drain(), in which it frees kernfs_open_file *of. On the
>> other side, when accessing irq.pressure, kernel calls
>> pressure_write(), which will access of->priv. So that it triggers a
>> use-after-free.
> 
> Thanks for the nice breakdown.
> 
> What would you tell to something like below (not tested).

Thanks for the detailed report analysis and this fix patch.

I can still reproduce the UAF problem with this patch by running:

terminal 1: while true; do echo "some 150000 1000000" > cpu.pressure; done
terminal 2: while true; do echo 1 > cgroup.pressure; echo 0 > cgroup.pressure; done

Because we still access kernfs_open_file in pressure_write() before cgroup_mutex taken.

It seems like a problem with kernfs_drain()? I think it should make sure no active users
of kernfs_open_file when it returns, right? Will take a look again.

Thanks.

> 
> Regards,
> Michal
> 
> -- >8 --
> From f159b20051a921bcf990a4488ca6d49382b61a01 Mon Sep 17 00:00:00 2001
> From: =?UTF-8?q?Michal=20Koutn=C3=BD?= <mkoutny@...e.com>
> Date: Fri, 24 May 2024 16:50:24 +0200
> Subject: [PATCH] cgroup: Pin appropriate resources when creating PSI pressure
>  trigger
> MIME-Version: 1.0
> Content-Type: text/plain; charset=UTF-8
> Content-Transfer-Encoding: 8bit
> 
> Wrongly synchronized access to kernfs_open_file was detected by
> syzkaller when there is a race between trigger creation and disabling of
> pressure measurements for a cgroup (echo 0 >cgroup.pressure).
> 
> Use cgroup_mutex to synchronize whole duration of pressure_write() to
> prevent working with a free'd kernfs_open_file by excluding concurrent
> cgroup_pressure_write() (uses cgroup_mutex already).
> 
> Fixes: 0e94682b73bf ("psi: introduce psi monitor")
> Fixes: 34f26a15611a ("sched/psi: Per-cgroup PSI accounting disable/re-enable interface")
> Reported-by: Yue Sun <samsun1006219@...il.com>
> Reported-by: xingwei lee <xrivendell7@...il.com>
> Signed-off-by: Michal Koutný <mkoutny@...e.com>
> ---
>  kernel/cgroup/cgroup.c | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index e32b6972c478..e16ebd0c4977 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -3777,31 +3777,30 @@ static ssize_t pressure_write(struct kernfs_open_file *of, char *buf,
>  	struct psi_trigger *new;
>  	struct cgroup *cgrp;
>  	struct psi_group *psi;
> +	ssize_t ret = nbytes;
>  
>  	cgrp = cgroup_kn_lock_live(of->kn, false);
>  	if (!cgrp)
>  		return -ENODEV;
>  
> -	cgroup_get(cgrp);
> -	cgroup_kn_unlock(of->kn);
> -
>  	/* Allow only one trigger per file descriptor */
>  	if (ctx->psi.trigger) {
> -		cgroup_put(cgrp);
> -		return -EBUSY;
> +		ret = -EBUSY;
> +		goto out;
>  	}
>  
>  	psi = cgroup_psi(cgrp);
>  	new = psi_trigger_create(psi, buf, res, of->file, of);
>  	if (IS_ERR(new)) {
> -		cgroup_put(cgrp);
> -		return PTR_ERR(new);
> +		ret = PTR_ERR(new);
> +		goto out;
>  	}
>  
>  	smp_store_release(&ctx->psi.trigger, new);
> -	cgroup_put(cgrp);
>  
> -	return nbytes;
> +out:
> +	cgroup_kn_unlock(of->kn);
> +	return ret;
>  }
>  
>  static ssize_t cgroup_io_pressure_write(struct kernfs_open_file *of,