This is a note to let you know that I've just added the patch titled bpf: Fix passing modified ctx to ld/abs/ind instruction to the 4.9-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: bpf-fix-passing-modified-ctx-to-ld-abs-ind-instruction.patch and it can be found in the queue-4.9 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From 6d4f151acf9a4f6fab09b615f246c717ddedcf0c Mon Sep 17 00:00:00 2001 From: Daniel Borkmann <daniel@xxxxxxxxxxxxx> Date: Mon, 6 Jan 2020 22:51:57 +0100 Subject: bpf: Fix passing modified ctx to ld/abs/ind instruction From: Daniel Borkmann <daniel@xxxxxxxxxxxxx> commit 6d4f151acf9a4f6fab09b615f246c717ddedcf0c upstream. Anatoly has been fuzzing with kBdysch harness and reported a KASAN slab oob in one of the outcomes: [...] [ 77.359642] BUG: KASAN: slab-out-of-bounds in bpf_skb_load_helper_8_no_cache+0x71/0x130 [ 77.360463] Read of size 4 at addr ffff8880679bac68 by task bpf/406 [ 77.361119] [ 77.361289] CPU: 2 PID: 406 Comm: bpf Not tainted 5.5.0-rc2-xfstests-00157-g2187f215eba #1 [ 77.362134] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014 [ 77.362984] Call Trace: [ 77.363249] dump_stack+0x97/0xe0 [ 77.363603] print_address_description.constprop.0+0x1d/0x220 [ 77.364251] ? bpf_skb_load_helper_8_no_cache+0x71/0x130 [ 77.365030] ? bpf_skb_load_helper_8_no_cache+0x71/0x130 [ 77.365860] __kasan_report.cold+0x37/0x7b [ 77.366365] ? bpf_skb_load_helper_8_no_cache+0x71/0x130 [ 77.366940] kasan_report+0xe/0x20 [ 77.367295] bpf_skb_load_helper_8_no_cache+0x71/0x130 [ 77.367821] ? bpf_skb_load_helper_8+0xf0/0xf0 [ 77.368278] ? mark_lock+0xa3/0x9b0 [ 77.368641] ? kvm_sched_clock_read+0x14/0x30 [ 77.369096] ? sched_clock+0x5/0x10 [ 77.369460] ? sched_clock_cpu+0x18/0x110 [ 77.369876] ? bpf_skb_load_helper_8+0xf0/0xf0 [ 77.370330] ___bpf_prog_run+0x16c0/0x28f0 [ 77.370755] __bpf_prog_run32+0x83/0xc0 [ 77.371153] ? __bpf_prog_run64+0xc0/0xc0 [ 77.371568] ? match_held_lock+0x1b/0x230 [ 77.371984] ? rcu_read_lock_held+0xa1/0xb0 [ 77.372416] ? rcu_is_watching+0x34/0x50 [ 77.372826] sk_filter_trim_cap+0x17c/0x4d0 [ 77.373259] ? sock_kzfree_s+0x40/0x40 [ 77.373648] ? __get_filter+0x150/0x150 [ 77.374059] ? skb_copy_datagram_from_iter+0x80/0x280 [ 77.374581] ? do_raw_spin_unlock+0xa5/0x140 [ 77.375025] unix_dgram_sendmsg+0x33a/0xa70 [ 77.375459] ? do_raw_spin_lock+0x1d0/0x1d0 [ 77.375893] ? unix_peer_get+0xa0/0xa0 [ 77.376287] ? __fget_light+0xa4/0xf0 [ 77.376670] __sys_sendto+0x265/0x280 [ 77.377056] ? __ia32_sys_getpeername+0x50/0x50 [ 77.377523] ? lock_downgrade+0x350/0x350 [ 77.377940] ? __sys_setsockopt+0x2a6/0x2c0 [ 77.378374] ? sock_read_iter+0x240/0x240 [ 77.378789] ? __sys_socketpair+0x22a/0x300 [ 77.379221] ? __ia32_sys_socket+0x50/0x50 [ 77.379649] ? mark_held_locks+0x1d/0x90 [ 77.380059] ? trace_hardirqs_on_thunk+0x1a/0x1c [ 77.380536] __x64_sys_sendto+0x74/0x90 [ 77.380938] do_syscall_64+0x68/0x2a0 [ 77.381324] entry_SYSCALL_64_after_hwframe+0x49/0xbe [ 77.381878] RIP: 0033:0x44c070 [...] After further debugging, turns out while in case of other helper functions we disallow passing modified ctx, the special case of ld/abs/ind instruction which has similar semantics (except r6 being the ctx argument) is missing such check. Modified ctx is impossible here as bpf_skb_load_helper_8_no_cache() and others are expecting skb fields in original position, hence, add check_ctx_reg() to reject any modified ctx. Issue was first introduced back in f1174f77b50c ("bpf/verifier: rework value tracking"). Fixes: f1174f77b50c ("bpf/verifier: rework value tracking") Reported-by: Anatoly Trosinenko <anatoly.trosinenko@xxxxxxxxx> Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx> Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx> Link: https://lore.kernel.org/bpf/20200106215157.3553-1-daniel@xxxxxxxxxxxxx Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- kernel/bpf/verifier.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-) --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -2422,6 +2422,7 @@ static bool may_access_skb(enum bpf_prog static int check_ld_abs(struct bpf_verifier_env *env, struct bpf_insn *insn) { struct bpf_reg_state *regs = env->cur_state.regs; + static const int ctx_reg = BPF_REG_6; u8 mode = BPF_MODE(insn->code); struct bpf_reg_state *reg; int i, err; @@ -2439,11 +2440,11 @@ static int check_ld_abs(struct bpf_verif } /* check whether implicit source operand (register R6) is readable */ - err = check_reg_arg(regs, BPF_REG_6, SRC_OP); + err = check_reg_arg(regs, ctx_reg, SRC_OP); if (err) return err; - if (regs[BPF_REG_6].type != PTR_TO_CTX) { + if (regs[ctx_reg].type != PTR_TO_CTX) { verbose("at the time of BPF_LD_ABS|IND R6 != pointer to skb\n"); return -EINVAL; } @@ -2455,6 +2456,10 @@ static int check_ld_abs(struct bpf_verif return err; } + err = check_ctx_reg(env, ®s[ctx_reg], ctx_reg); + if (err < 0) + return err; + /* reset caller saved regs to unreadable */ for (i = 0; i < CALLER_SAVED_REGS; i++) { reg = regs + caller_saved[i]; Patches currently in stable-queue which might be from daniel@xxxxxxxxxxxxx are queue-4.9/bpf-fix-passing-modified-ctx-to-ld-abs-ind-instruction.patch