netdev - Re: Possible kernel memory leak in bpf

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Date: Tue, 10 Oct 2023 21:39:28 -0700
From: Hsin-Wei Hung <hsinweih@....edu>
To: Hou Tao <houtao@...weicloud.com>
Cc: Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau <kafai@...com>, Song Liu <songliubraving@...com>, 
	Yonghong Song <yhs@...com>, John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>, 
	Network Development <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>, 
	Kumar Kartikeya Dwivedi <memxor@...il.com>
Subject: Re: Possible kernel memory leak in bpf_timer

On Sat, Oct 7, 2023 at 7:46 PM Hou Tao <houtao@...weicloud.com> wrote:
>
> Hi,
>
> On 9/27/2023 1:32 PM, Hsin-Wei Hung wrote:
> > Hi,
> >
> > We found a potential memory leak in bpf_timer in v5.15.26 using a
> > customized syzkaller for fuzzing bpf runtime. It can happen when
> > an arraymap is being released. An entry that has been checked by
> > bpf_timer_cancel_and_free() can again be initialized by bpf_timer_init().
> > Since both paths are almost identical between v5.15 and net-next,
> > I suspect this problem still exists. Below are kmemleak report and
> > some additional printks I inserted.
> >
> > [ 1364.081694] array_map_free_timers map:0xffffc900005a9000
> > [ 1364.081730] ____bpf_timer_init map:0xffffc900005a9000
> > timer:0xffff888001ab4080
> >
> > *no bpf_timer_cancel_and_free that will kfree struct bpf_hrtimer*
> > at 0xffff888001ab4080 is called
>
> I think the kmemleak happened as follows:
>
> bpf_timer_init()
>   lock timer->lock
>     read timer->timer as NULL
>     read map->usercnt != 0
>
>                 bpf_map_put_uref()
>                   // map->usercnt = 0
>                   atomic_dec_and_test(map->usercnt)
>                     array_map_free_timers()
>                     // just return and lead to mem leak
>                     find timer->timer is NULL
>
>     t = bpf_map_kmalloc_node()
>     timer->timer = t
>   unlock timer->lock
>
> Could you please try the attached patch to check whether the kmemleak
> problem has been fixed ?
>

Hi,

Sorry for the late reply to this thread.

KASAN is complaining about double-free/invalid-free in the kfree after
applying the patch. There are some cases that jump to "out" before the
bpf_hrtimer is allocated or when the bpf_hrtimer is already allocated.

I am still trying to have a standalone working POC. I think a key to
trigger this memory leak is to 1) have a large array map 2) a bpf
program init a timer in a small-index entry and then 3) release the
map.

-Amery


> >
> > [ 1383.907869] kmemleak: 1 new suspected memory leaks (see
> > /sys/kernel/debug/kmemleak)
> > BUG: memory leak
> > unreferenced object 0xffff888001ab4080 (size 96):
> >   comm "sshd", pid 279, jiffies 4295233126 (age 29.952s)
> >   hex dump (first 32 bytes):
> >     80 40 ab 01 80 88 ff ff 00 00 00 00 00 00 00 00  .@..............
> >     00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> >   backtrace:
> >     [<000000009d018da0>] bpf_map_kmalloc_node+0x89/0x1a0
> >     [<00000000ebcb33fc>] bpf_timer_init+0x177/0x320
> >     [<00000000fb7e90bf>] 0xffffffffc02a0358
> >     [<000000000c89ec4f>] __cgroup_bpf_run_filter_skb+0xcbf/0x1110
> >     [<00000000fd663fc0>] ip_finish_output+0x13d/0x1f0
> >     [<00000000acb3205c>] ip_output+0x19b/0x310
> >     [<000000006b584375>] __ip_queue_xmit+0x182e/0x1ed0
> >     [<00000000b921b07e>] __tcp_transmit_skb+0x2b65/0x37f0
> >     [<0000000026104b23>] tcp_write_xmit+0xf19/0x6290
> >     [<000000006dc71bc5>] __tcp_push_pending_frames+0xaf/0x390
> >     [<00000000251b364a>] tcp_push+0x452/0x6d0
> >     [<000000008522b7d3>] tcp_sendmsg_locked+0x2567/0x3030
> >     [<0000000038c644d2>] tcp_sendmsg+0x30/0x50
> >     [<000000009fe3413f>] inet_sendmsg+0xba/0x140
> >     [<0000000034d78039>] sock_sendmsg+0x13d/0x190
> >     [<00000000f55b8db6>] sock_write_iter+0x296/0x3d0
> >
> >
> > Thanks,
> > Hsin-Wei (Amery)
> >
> >
> > .
>