[PATCH 5.4 002/165] bpf: Fix passing modified ctx to ld/abs/ind instruction

Greg Kroah-Hartman
2020-01-11T10:20:35Z

From: Daniel Borkmann <daniel@iogearbox.net>

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@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200106215157.3553-1-daniel@iogearbox.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
kernel/bpf/verifier.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)

--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -6039,6 +6039,7 @@ 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 = cur_regs(env);
+ static const int ctx_reg = BPF_REG_6;
u8 mode = BPF_MODE(insn->code);
int i, err;

@@ -6072,7 +6073,7 @@ static int check_ld_abs(struct bpf_verif
}

/* check whether implicit source operand (register R6) is readable */
- err = check_reg_arg(env, BPF_REG_6, SRC_OP);
+ err = check_reg_arg(env, ctx_reg, SRC_OP);
if (err)
return err;

@@ -6091,7 +6092,7 @@ static int check_ld_abs(struct bpf_verif
return -EINVAL;
}

- if (regs[BPF_REG_6].type != PTR_TO_CTX) {
+ if (regs[ctx_reg].type != PTR_TO_CTX) {
verbose(env,
"at the time of BPF_LD_ABS|IND R6 != pointer to skb\n");
return -EINVAL;
@@ -6104,6 +6105,10 @@ static int check_ld_abs(struct bpf_verif
return err;
}

+ err = check_ctx_reg(env, &regs[ctx_reg], ctx_reg);
+ if (err < 0)
+ return err;
+
/* reset caller saved regs to unreadable */
for (i = 0; i < CALLER_SAVED_REGS; i++) {
mark_reg_not_init(env, regs, caller_saved[i]);