[PATCH net] rtnetlink: invoke 'cb->done' destructor before 'cb->args' reset

[Alexey Kodanev <alexey.kodanev@oracle.com>]

cb->args[2] can store the pointer to the struct fib6_walker,
allocated in inet6_dump_fib(). On the next loop iteration in
rtnl_dump_all(), 'memset(&cb, 0, sizeof(cb->args))' can reset
that pointer, leaking the memory [1].

Fix it by calling cb->done, if it is set, before filling 'cb->args'
with zeros.

Looks like the recent changes in rtnl_dump_all() contributed to
the appearance of this kmemleak [1], commit c63586dc9b3e ("net:
rtnl_dump_all needs to propagate error from dumpit function")
breaks the loop only on an error now.

[1]:
unreferenced object 0xffff88001322a200 (size 96):
  comm "sshd", pid 1484, jiffies 4296032768 (age 1432.542s)
  hex dump (first 32 bytes):
    00 01 00 00 00 00 ad de 00 02 00 00 00 00 ad de  ................
    18 09 41 36 00 88 ff ff 18 09 41 36 00 88 ff ff  ..A6......A6....
  backtrace:
    [<0000000095846b39>] kmem_cache_alloc_trace+0x151/0x220
    [<000000007d12709f>] inet6_dump_fib+0x68d/0x940
    [<000000002775a316>] rtnl_dump_all+0x1d9/0x2d0
    [<00000000d7cd302b>] netlink_dump+0x945/0x11a0
    [<000000002f43485f>] __netlink_dump_start+0x55d/0x800
    [<00000000f76bbeec>] rtnetlink_rcv_msg+0x4fa/0xa00
    [<000000009b5761f3>] netlink_rcv_skb+0x29c/0x420
    [<0000000087a1dae1>] rtnetlink_rcv+0x15/0x20
    [<00000000691b703b>] netlink_unicast+0x4e3/0x6c0
    [<00000000b5be0204>] netlink_sendmsg+0x7f2/0xba0
    [<0000000096d2aa60>] sock_sendmsg+0xba/0xf0
    [<000000008c1b786f>] __sys_sendto+0x1e4/0x330
    [<0000000019587b3f>] __x64_sys_sendto+0xe1/0x1a0
    [<00000000071f4d56>] do_syscall_64+0x9f/0x300
    [<000000002737577f>] entry_SYSCALL_64_after_hwframe+0x44/0xa9
    [<0000000057587684>] 0xffffffffffffffff

Fixes: 1b43af5480c3 ("[IPV6]: Increase number of possible routing tables to 2^32")
Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
---
 net/core/rtnetlink.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index f679c7a..314c683 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -3362,6 +3362,8 @@ static int rtnl_dump_all(struct sk_buff *skb, struct netlink_callback *cb)
 			continue;
 
 		if (idx > s_idx) {
+			if (cb->done)
+				cb->done(cb);
 			memset(&cb->args[0], 0, sizeof(cb->args));
 			cb->prev_seq = 0;
 			cb->seq = 0;
-- 
1.8.3.1