Subject: Re: [PATCH nf] netfilter: conntrack: Avoid nf_ct_helper_hash uses after free

From: Florian Westphal <[email-masked]>
Date: Wed, 21 Jun 2023 13:14:54 +0200

Florent Revest <[email-masked]> wrote:
> On Tue, Jun 20, 2023 at 8:35 AM Pablo Neira Ayuso <[email-masked]> wrote:
> >
> > On Thu, Jun 15, 2023 at 05:29:18PM +0200, Florent Revest wrote:
> > > If register_nf_conntrack_bpf() fails (for example, if the .BTF section
> > > contains an invalid entry), nf_conntrack_init_start() calls
> > > nf_conntrack_helper_fini() as part of its cleanup path and
> > > nf_ct_helper_hash gets freed.
> > >
> > > Further netfilter modules like netfilter_conntrack_ftp don't check
> > > whether nf_conntrack initialized correctly and call
> > > nf_conntrack_helpers_register() which accesses the freed
> > > nf_ct_helper_hash and causes a uaf.
> > >
> > > This patch guards nf_conntrack_helper_register() from accessing
> > > freed/uninitialized nf_ct_helper_hash maps and fixes a boot-time
> > > use-after-free.
> >
> > How could this possibly happen?
> 
> Here is one way to reproduce this bug:
> 
>   # Use nf/main
>   git clone git://git.kernel.org/pub/scm/linux/kernel/git/netfilter/nf.git
>   cd nf
> 
>   # Start from a minimal config
>   make LLVM=1 LLVM_IAS=0 defconfig
> 
>   # Enable KASAN, BTF and nf_conntrack_ftp
>   scripts/config -e KASAN -e BPF_SYSCALL -e DEBUG_INFO -e
> DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT -e DEBUG_INFO_BTF -e
> NF_CONNTRACK_FTP
>   make LLVM=1 LLVM_IAS=0 olddefconfig
> 
>   # Build without the LLVM integrated assembler
>   make LLVM=1 LLVM_IAS=0 -j `nproc`
> 
> (Note that the use of LLVM_IAS=0, KASAN and BTF is just to trigger a
> bug in BTF that will be fixed by
> https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/commit/?id=9724160b3942b0a967b91a59f81da5593f28b8ba
> Independently of that specific BTF bug, it shows how an error in
> nf_conntrack_bpf can cause a boot-time uaf in netfilter)
> 
> Then, booting gives me:
> 
> [    4.624666] BPF: [13893] FUNC asan.module_ctor
> [    4.625611] BPF: type_id=1
> [    4.626176] BPF:
> [    4.626601] BPF: Invalid name
> [    4.627208] BPF:
> [    4.627723] ==================================================================
> [    4.628610] BUG: KASAN: slab-use-after-free in
> nf_conntrack_helper_register+0x129/0x2f0
> [    4.628610] Read of size 8 at addr ffff888102d24000 by task swapper/0/1
> [    4.628610]

Isn't that better than limping along?

in this case an initcall is failing and I think panic is preferrable
to a kernel that behaves like NF_CONNTRACK_FTP=n.

AFAICS this problem is specific to NF_CONNTRACK_FTP=y
(or any other helper module, for that matter).

If you disagree please resend with a commit message that
makes it clear that this is only relevant for the 'builtin' case.