2018-07-06 13:38:51

by Andrey Ryabinin

Subject: [PATCH] nf_conntrack: Fix possible possible crash on module loading.

Loading the nf_conntrack module with doubled hashsize parameter, i.e.
modprobe nf_conntrack hashsize=12345 hashsize=12345
causes NULL-ptr deref.

If 'hashsize' specified twice, the nf_conntrack_set_hashsize() function
will be called also twice.
The first nf_conntrack_set_hashsize() call will set the
'nf_conntrack_htable_size' variable:

nf_conntrack_set_hashsize()
...
/* On boot, we can set this without any fancy locking. */
if (!nf_conntrack_htable_size)
return param_set_uint(val, kp);

But on the second invocation, the nf_conntrack_htable_size is already set,
so the nf_conntrack_set_hashsize() will take a different path and call
the nf_conntrack_hash_resize() function. Which will crash on the attempt
to dereference 'nf_conntrack_hash' pointer:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
RIP: 0010:nf_conntrack_hash_resize+0x255/0x490 [nf_conntrack]
Call Trace:
nf_conntrack_set_hashsize+0xcd/0x100 [nf_conntrack]
parse_args+0x1f9/0x5a0
load_module+0x1281/0x1a50
__se_sys_finit_module+0xbe/0xf0
do_syscall_64+0x7c/0x390
entry_SYSCALL_64_after_hwframe+0x49/0xbe

Fix this, by checking !nf_conntrack_hash instead of
!nf_conntrack_htable_size. nf_conntrack_hash will be initialized only
after the module loaded, so the second invocation of the
nf_conntrack_set_hashsize() won't crash, it will just reinitialize
nf_conntrack_htable_size again.

Signed-off-by: Andrey Ryabinin <[email protected]>
---
net/netfilter/nf_conntrack_core.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
index 6c2d22d63f48..9f52f0e1478b 100644
--- a/net/netfilter/nf_conntrack_core.c
+++ b/net/netfilter/nf_conntrack_core.c
@@ -2151,7 +2151,7 @@ int nf_conntrack_set_hashsize(const char *val, const struct kernel_param *kp)
return -EOPNOTSUPP;

/* On boot, we can set this without any fancy locking. */
- if (!nf_conntrack_htable_size)
+ if (!nf_conntrack_hash)
return param_set_uint(val, kp);

rc = kstrtouint(val, 0, &hashsize);
--
2.16.4

2018-07-09 16:04:42

by Pablo Neira Ayuso

Subject: Re: [PATCH] nf_conntrack: Fix possible possible crash on module loading.

On Fri, Jul 06, 2018 at 04:38:53PM +0300, Andrey Ryabinin wrote:
> Loading the nf_conntrack module with doubled hashsize parameter, i.e.
> modprobe nf_conntrack hashsize=12345 hashsize=12345
> causes NULL-ptr deref.
>
> If 'hashsize' specified twice, the nf_conntrack_set_hashsize() function
> will be called also twice.
> The first nf_conntrack_set_hashsize() call will set the
> 'nf_conntrack_htable_size' variable:
>
> nf_conntrack_set_hashsize()
> ...
> /* On boot, we can set this without any fancy locking. */
> if (!nf_conntrack_htable_size)
> return param_set_uint(val, kp);
>
> But on the second invocation, the nf_conntrack_htable_size is already set,
> so the nf_conntrack_set_hashsize() will take a different path and call
> the nf_conntrack_hash_resize() function. Which will crash on the attempt
> to dereference 'nf_conntrack_hash' pointer:
>
> BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
> RIP: 0010:nf_conntrack_hash_resize+0x255/0x490 [nf_conntrack]
> Call Trace:
> nf_conntrack_set_hashsize+0xcd/0x100 [nf_conntrack]
> parse_args+0x1f9/0x5a0
> load_module+0x1281/0x1a50
> __se_sys_finit_module+0xbe/0xf0
> do_syscall_64+0x7c/0x390
> entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> Fix this, by checking !nf_conntrack_hash instead of
> !nf_conntrack_htable_size. nf_conntrack_hash will be initialized only
> after the module loaded, so the second invocation of the
> nf_conntrack_set_hashsize() won't crash, it will just reinitialize
> nf_conntrack_htable_size again.

Applied, thanks.