linux-kernel - Re: memory leak in xskq

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Date:   Wed, 16 Dec 2020 13:11:35 -0500
From:   Peilin Ye <yepeilin.cs@...il.com>
To:     syzbot <syzbot+cfa88ddd0655afa88763@...kaller.appspotmail.com>
Cc:     ast@...nel.org, bjorn.topel@...el.com, bpf@...r.kernel.org,
        daniel@...earbox.net, davem@...emloft.net, hawk@...nel.org,
        john.fastabend@...il.com, jonathan.lemon@...il.com,
        kuba@...nel.org, linux-kernel@...r.kernel.org,
        magnus.karlsson@...el.com, netdev@...r.kernel.org,
        syzkaller-bugs@...glegroups.com
Subject: Re: memory leak in xskq_create

Hi all,

On Sun, Dec 13, 2020 at 06:53:10AM -0800, syzbot wrote:
> BUG: memory leak
> unreferenced object 0xffff88810f897940 (size 64):
>   comm "syz-executor991", pid 8502, jiffies 4294942194 (age 14.080s)
>   hex dump (first 32 bytes):
>     7f 00 00 00 80 00 00 00 00 00 00 00 00 00 00 00  ................
>     00 a0 37 0c 81 88 ff ff 00 00 00 00 00 00 00 00  ..7.............
>   backtrace:
>     [<00000000639d0dd1>] xskq_create+0x23/0xd0 include/linux/slab.h:552
>     [<00000000b680b035>] xsk_init_queue net/xdp/xsk.c:508 [inline]
>     [<00000000b680b035>] xsk_setsockopt+0x1c4/0x590 net/xdp/xsk.c:875
>     [<000000002b302260>] __sys_setsockopt+0x1b0/0x360 net/socket.c:2132
>     [<00000000ae03723e>] __do_sys_setsockopt net/socket.c:2143 [inline]
>     [<00000000ae03723e>] __se_sys_setsockopt net/socket.c:2140 [inline]
>     [<00000000ae03723e>] __x64_sys_setsockopt+0x22/0x30 net/socket.c:2140
>     [<0000000005c2b4a0>] do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
>     [<0000000003db140f>] entry_SYSCALL_64_after_hwframe+0x44/0xa9

I have tested the following diff locally against syzbot's reproducer,
and sent a patch to it [1] for testing.  I will send a real patch here
tomorrow if syzbot is happy about it.  Please see explanation below.

--- a/net/xdp/xsk_buff_pool.c
+++ b/net/xdp/xsk_buff_pool.c
@@ -37,6 +37,9 @@ void xp_destroy(struct xsk_buff_pool *pool)
        if (!pool)
                return;

+       xskq_destroy(pool->fq);
+       xskq_destroy(pool->cq);
+
        kvfree(pool->heads);
        kvfree(pool);
 }
@@ -234,16 +237,6 @@ static void xp_release_deferred(struct work_struct *work)
        xp_clear_dev(pool);
        rtnl_unlock();

-       if (pool->fq) {
-               xskq_destroy(pool->fq);
-               pool->fq = NULL;
-       }
-
-       if (pool->cq) {
-               xskq_destroy(pool->cq);
-               pool->cq = NULL;
-       }
-
        xdp_put_umem(pool->umem, false);
        xp_destroy(pool);
 }

When xsk_bind() allocates `xs->pool` but something else goes wrong:

		xs->pool = xp_create_and_assign_umem(xs, xs->umem);
		[...]
		if (err) {
			xp_destroy(xs->pool);

xp_destroy() doesn't free `pool->{f,c}q`, causing a memory leak.  Move
`xskq_destroy(pool->{f,c}q)` from xp_release_deferred() to xp_destroy().

Also, since xskq_destroy() already does null check, I think it's
unnecessary to do it again here?

Thanks,
Peilin Ye

[1] https://syzkaller.appspot.com/bug?id=fea808dfe3c6dfdd6ba9778becbffe0b14e91294