Re: [PATCH net] gtp: fix use-after-free and null-ptr-deref in gtp_newlink()

Hi,

29.02.2024 02:51, Pablo Neira Ayuso wrote:

On Wed, Feb 28, 2024 at 02:47:03PM +0300, Alexander Ofitserov wrote:

...

The gtp_link_ops operations structure for the subsystem must be registered after registering the gtp_net_ops pernet operations structure.

A fix for this was already applied, see:

commit 136cfaca22567a03bbb3bf53a43d8cb5748b80ec Author: Vasiliy Kovalev kovalev@altlinux.org Date: Wed Feb 14 19:27:33 2024 +0300

 gtp: fix use-after-free and null-ptr-deref in gtp_genl_dump_pdp()

...

diff --git a/drivers/net/gtp.c b/drivers/net/gtp.c index 2129ae42c7030..0ddec4cc84093 100644 --- a/drivers/net/gtp.c +++ b/drivers/net/gtp.c @@ -1903,26 +1903,26 @@ static int __init gtp_init(void) get_random_bytes(&gtp_h_initval, sizeof(gtp_h_initval));

• err = rtnl_link_register(&gtp_link_ops);

• err = register_pernet_subsys(&gtp_net_ops); if (err < 0) goto error_out;

BTW, I like that this calls register_pernet_subsys() before rtnl_link_register(), where a rtnetlink request could come before pernet is set up.

...

• err = register_pernet_subsys(&gtp_net_ops);

• err = rtnl_link_register(&gtp_link_ops); if (err < 0)

• goto unreg_rtnl_link;
  

• goto unreg_pernet_subsys;
  

err = genl_register_family(&gtp_genl_family); if (err < 0)

• goto unreg_pernet_subsys;
  

• goto unreg_rtnl_link;
  

pr_info("GTP module loaded (pdp ctx size %zd bytes)\n", sizeof(struct pdp_ctx)); return 0; -unreg_pernet_subsys:

• unregister_pernet_subsys(&gtp_net_ops); unreg_rtnl_link: rtnl_link_unregister(&gtp_link_ops);

+unreg_pernet_subsys:

• unregister_pernet_subsys(&gtp_net_ops); error_out: pr_err("error loading GTP module loaded\n"); return err;

-- 2.42.1

This patch fixes another problem, but a similar one, since the sequence is incorrect when registering subsystems.

Initially, the registration sequence in the gtp module was as follows:

1) rtnl_link_register();

2) genl_register_family();

3) register_pernet_subsys();

During debugging of the module, when starting the syzkaller reproducer, it turned out that after genl_register_family() (2),

without waiting for register_pernet_subsys()(3), the .dumpit event is triggered, in which the data of the unregistered pernet subsystem is accessed.

That is, the bug was fixed by the commit

136cfaca2256 ("gtp: fix use-after-free and null-ptr-deref in gtp_genl_dump_pdp()") [1]

and the registration sequence became as follows:

1) rtnl_link_register();

2) register_pernet_subsys();

3) genl_register_family();

However, syzkaller has discovered another problem:

after registering rtnl_link_register, the .newlink event is triggered, in which the data of the unregistered pernet subsystem is accessed.

This problem is reproducible on current stable kernels and the latest upstream kernel 6.8-rc6, in which the patch 136cfaca2256 [1] is applied.

Therefore, the correct sequence should be as follows:

1) register_pernet_subsys();

2) rtnl_link_register();

3) genl_register_family();

The proposed patch is developed on top of the commit changes [1], does not conflict with it and fixes the described bug.

[1] https://lore.kernel.org/lkml/20240220160434.29bcaf43@kernel.org/T/#mb1f72c2a...