Re: [PATCH net] kcm: Fix memory leak in error path of kcm_sendmsg()

[Paolo Abeni <pabeni@redhat.com>]

Hello,

On Sun, 2023-09-03 at 01:52 +0900, Shigeru Yoshida wrote:
> syzbot reported a memory leak like below [1]:
> 
> BUG: memory leak
> unreferenced object 0xffff88810b088c00 (size 240):
>   comm "syz-executor186", pid 5012, jiffies 4294943306 (age 13.680s)
>   hex dump (first 32 bytes):
>     00 89 08 0b 81 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:
>     [<ffffffff83e5d5ff>] __alloc_skb+0x1ef/0x230 net/core/skbuff.c:634
>     [<ffffffff84606e59>] alloc_skb include/linux/skbuff.h:1289 [inline]
>     [<ffffffff84606e59>] kcm_sendmsg+0x269/0x1050 net/kcm/kcmsock.c:815
>     [<ffffffff83e479c6>] sock_sendmsg_nosec net/socket.c:725 [inline]
>     [<ffffffff83e479c6>] sock_sendmsg+0x56/0xb0 net/socket.c:748
>     [<ffffffff83e47f55>] ____sys_sendmsg+0x365/0x470 net/socket.c:2494
>     [<ffffffff83e4c389>] ___sys_sendmsg+0xc9/0x130 net/socket.c:2548
>     [<ffffffff83e4c536>] __sys_sendmsg+0xa6/0x120 net/socket.c:2577
>     [<ffffffff84ad7bb8>] do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>     [<ffffffff84ad7bb8>] do_syscall_64+0x38/0xb0 arch/x86/entry/common.c:80
>     [<ffffffff84c0008b>] entry_SYSCALL_64_after_hwframe+0x63/0xcd
> 
> In kcm_sendmsg(), newly allocated socket buffers can be added to skb->next. If
> an error occurred and jumped to out_error label, those newly allocated socket
> buffers can be leaked. This patch fixes this issue by remembering the last
> allocated socket buffer in kcm_tx_msg(head)->last_skb.

I think the root cause should be clarified a little more. When the
'head' skb will be freed, all the frag_list skbs will be released, too.

AFAICS the issue is that in case of error after copying some bytes,
kcm_tx_msg(head)->last_skb is left unmodified and such reference is
used as the cursor to append newly allocated skbs to 'head'.

A later kcm_sendmsg will use an obsoleted 'last_skb' reference,
corrupting the 'head' frag_list and causing the leak.

The fix looks correct, but could you please send a v2 with an
updated/more extensive commit message?

> Link: https://syzkaller.appspot.com/bug?extid=6f98de741f7dbbfc4ccb [1]
> Reported-by: syzbot+6f98de741f7dbbfc4ccb@syzkaller.appspotmail.com
> Fixes: ab7ac4eb9832 ("kcm: Kernel Connection Multiplexor module")
> Signed-off-by: Shigeru Yoshida <syoshida@redhat.com>

Additionally please feed the patch to syzbot, so it can verify the fix,
and add the relevant tag.

Thanks,

Paolo