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

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Date: Fri, 24 May 2024 18:03:16 +0200
From: Michal Koutný <mkoutny@...e.com>
To: 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, 
	Chengming Zhou <zhouchengming@...edance.com>
Subject: Re: [Linux kernel bug] KASAN: slab-use-after-free Read in
 pressure_write

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).

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,
-- 
2.44.0


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