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

From: Pablo Neira Ayuso <[email-masked]>
Date: Wed, 21 Jun 2023 13:12:14 +0200

On Wed, Jun 21, 2023 at 12:20:44PM +0200, Florent Revest 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]
> [    4.628610] CPU: 1 PID: 1 Comm: swapper/0 Not tainted
> 6.4.0-rc4-00244-gab39b113e747 #47
> [    4.628610] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009),
> BIOS 1.16.0-debian-1.16.0-5 04/01/2014
> [    4.628610] Call Trace:
> [    4.628610]  <TASK>
> [    4.636584] i801_smbus 0000:00:1f.3: SMBus using PCI interrupt
> [    4.628610]  dump_stack_lvl+0x97/0xd0
> [    4.638738] i2c i2c-0: 1/1 memory slots populated (from DMI)
> [    4.628610]  print_report+0x17e/0x570
> [    4.640118] i2c i2c-0: Memory type 0x07 not supported yet, not
> instantiating SPD
> [    4.628610]  ? __virt_addr_valid+0xe4/0x160
> [    4.628610]  kasan_report+0x169/0x1a0
> [    4.628610]  ? nf_conntrack_helper_register+0x129/0x2f0
> [    4.628610]  nf_conntrack_helper_register+0x129/0x2f0
> [    4.628610]  nf_conntrack_helpers_register+0x24/0x60
> [    4.628610]  nf_conntrack_ftp_init+0x114/0x140
> [    4.628610]  ? __pfx_nf_conntrack_ftp_init+0x10/0x10
> [    4.628610]  do_one_initcall+0xe6/0x310
> [    4.628610]  ? kasan_set_track+0x61/0x80
> [    4.628610]  ? kasan_set_track+0x4f/0x80
> [    4.628610]  ? __kasan_kmalloc+0x72/0x90
> [    4.628610]  ? __kmalloc+0xa7/0x1a0
> [    4.628610]  ? do_initcalls+0x1b/0x70
> [    4.628610]  ? kernel_init_freeable+0x174/0x1e0
> [    4.628610]  ? kernel_init+0x18/0x1b0
> [    4.628610]  ? ret_from_fork+0x29/0x50
> [    4.628610]  ? sysvec_apic_timer_interrupt+0xe/0x80
> [    4.628610]  ? asm_sysvec_apic_timer_interrupt+0x1a/0x20
> [    4.628610]  ? __pfx_ignore_unknown_bootoption+0x10/0x10
> [    4.628610]  ? next_arg+0x20b/0x250
> [    4.628610]  ? strlen+0x21/0x40
> [    4.628610]  ? parse_args+0xc7/0x5f0
> [    4.628610]  do_initcall_level+0xa6/0x140
> [    4.628610]  do_initcalls+0x3e/0x70
> [    4.628610]  kernel_init_freeable+0x174/0x1e0
> [    4.628610]  ? __pfx_kernel_init+0x10/0x10
> [    4.628610]  kernel_init+0x18/0x1b0
> [    4.628610]  ? __pfx_kernel_init+0x10/0x10
> [    4.628610]  ret_from_fork+0x29/0x50
> [    4.628610]  </TASK>
> [    4.628610]
> [    4.628610] Allocated by task 1:
> [    4.628610]  kasan_set_track+0x4f/0x80
> [    4.628610]  __kasan_kmalloc+0x72/0x90
> [    4.628610]  __kmalloc_node+0xa7/0x190
> [    4.628610]  kvmalloc_node+0x44/0x120
> [    4.628610]  nf_ct_alloc_hashtable+0x5b/0xe0
> [    4.628610]  nf_conntrack_helper_init+0x1f/0x60
> [    4.628610]  nf_conntrack_init_start+0x1c9/0x2d0
> [    4.628610]  nf_conntrack_standalone_init+0xb/0xa0
> [    4.628610]  do_one_initcall+0xe6/0x310
> [    4.628610]  do_initcall_level+0xa6/0x140
> [    4.628610]  do_initcalls+0x3e/0x70
> [    4.628610]  kernel_init_freeable+0x174/0x1e0
> [    4.628610]  kernel_init+0x18/0x1b0
> [    4.628610]  ret_from_fork+0x29/0x50
> [    4.628610]
> [    4.628610] Freed by task 1:
> [    4.628610]  kasan_set_track+0x4f/0x80
> [    4.628610]  kasan_save_free_info+0x2b/0x50
> [    4.628610]  ____kasan_slab_free+0x116/0x1a0
> [    4.628610]  __kmem_cache_free+0xc4/0x200
> [    4.628610]  nf_conntrack_init_start+0x29c/0x2d0
> [    4.628610]  nf_conntrack_standalone_init+0xb/0xa0
> [    4.628610]  do_one_initcall+0xe6/0x310
> [    4.628610]  do_initcall_level+0xa6/0x140
> [    4.628610]  do_initcalls+0x3e/0x70
> [    4.628610]  kernel_init_freeable+0x174/0x1e0
> [    4.628610]  kernel_init+0x18/0x1b0
> [    4.628610]  ret_from_fork+0x29/0x50
> [    4.628610]
> [    4.628610] The buggy address belongs to the object at ffff888102d24000
> [    4.628610]  which belongs to the cache kmalloc-4k of size 4096
> [    4.628610] The buggy address is located 0 bytes inside of
> [    4.628610]  freed 4096-byte region [ffff888102d24000, ffff888102d25000)
> [    4.628610]
> [    4.628610] The buggy address belongs to the physical page:
> [    4.628610] page:000000001eb64ba1 refcount:1 mapcount:0
> mapping:0000000000000000 index:0x0 pfn:0x102d20
> [    4.628610] head:000000001eb64ba1 order:3 entire_mapcount:0
> nr_pages_mapped:0 pincount:0
> [    4.628610] flags: 0x200000000010200(slab|head|node=0|zone=2)
> [    4.628610] page_type: 0xffffffff()
> [    4.628610] raw: 0200000000010200 ffff888100043040 dead000000000122
> 0000000000000000
> [    4.628610] raw: 0000000000000000 0000000000040004 00000001ffffffff
> 0000000000000000
> [    4.628610] page dumped because: kasan: bad access detected
> ...
> 
> > nf_conntrack_ftp depends on nf_conntrack.
> >
> > If nf_conntrack fails to load, how can nf_conntrack_ftp be loaded?
> 
> Is this maybe only true of dynamically loaded kmods ? With
> CONFIG_NF_CONNTRACK_FTP=y, it seems to me that nf_conntrack_ftp_init()
> will be called as an __init function, independently of whether
> nf_conntrack_init_start() succeeded or not. Am I missing something ?

No idea, nf_conntrack init path invokes nf_conntrack_helper_init()
which initializes the helper hashtable.

How is it that you can nf_conntrack_helpers_register() call before the
initialization path of nf_conntrack is run, that I don't know.