On Wed, Jun 21, 2023 at 1:14 PM Florian Westphal <fw@xxxxxxxxx> wrote: > > Florent Revest <revest@xxxxxxxxxxxx> wrote: > > On Tue, Jun 20, 2023 at 8:35 AM Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx> 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? Note that this only panics because KASAN instrumentation notices the use-after-free and makes a lot of noise about it. In a non-debug boot, this would just silently corrupt random memory instead. > in this case an initcall is failing and I think panic is preferrable > to a kernel that behaves like NF_CONNTRACK_FTP=n. In that case, it seems like what you'd want is nf_conntrack_standalone_init() to BUG() instead of returning an error then ? (so you'd never get to NF_CONNTRACK_FTP or any other if nf_conntrack failed to initialize) If this is the prefered behavior, then sure, why not. > AFAICS this problem is specific to NF_CONNTRACK_FTP=y > (or any other helper module, for that matter). Even with NF_CONNTRACK_FTP=m, the initialization failure in nf_conntrack_standalone_init() still happens. Therefore, the helper hashtable gets freed and when the nf_conntrack_ftp.ko module gets insmod-ed, it calls nf_conntrack_helpers_register() and this still causes a use-after-free. > If you disagree please resend with a commit message that > makes it clear that this is only relevant for the 'builtin' case.
