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

[Pablo Neira Ayuso]

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.