netdev - Re: SOCKET_FILTER regression - eBPF can't subtract when attached from unprivileged user

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Date:   Fri, 1 Mar 2019 15:04:42 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Arthur Fabre <afabre@...udflare.com>, marek@...udflare.com
Cc:     ast@...nel.org, netdev@...r.kernel.org
Subject: Re: SOCKET_FILTER regression - eBPF can't subtract when attached from
 unprivileged user

On 03/01/2019 12:39 PM, Arthur Fabre wrote:
> I can reproduce this on 4.19.0-3-amd64 both with, and without the JIT enabled.
> 
> Dumping the "root" and "non-root" programs with bpftool,
> the subtraction instructions differ:
> 
> "non-root":
>    0: (85) call bpf_ktime_get_ns#74944
>    1: (bf) r7 = r0
>    2: (85) call bpf_ktime_get_ns#74944
>    3: (bf) r6 = r0
>    4: (bf) r8 = r6
>    5: (b4) w11 = -1
>    6: (1f) r11 -= r8
>    7: (4f) r11 |= r8
>    8: (87) r11 = -r11
>    9: (c7) r11 s>>= 63
>   10: (5f) r8 &= r11
> 
> "root":
>    0: (85) call bpf_ktime_get_ns#74944
>    1: (bf) r7 = r0
>    2: (85) call bpf_ktime_get_ns#74944
>    3: (bf) r6 = r0
>    4: (bf) r8 = r6
> 
> The remainder of the instructions are for writing the results in the map,
> and the instructions are identical.
> 
> I believe the extra instructions come from "fixup_bpf_calls" in the verifier:
> 
>     if (isneg)
>         *patch++ = BPF_ALU64_IMM(BPF_MUL, off_reg, -1);
>     *patch++ = BPF_MOV32_IMM(BPF_REG_AX, aux->alu_limit - 1);
>     *patch++ = BPF_ALU64_REG(BPF_SUB, BPF_REG_AX, off_reg);
>     *patch++ = BPF_ALU64_REG(BPF_OR, BPF_REG_AX, off_reg);
>     *patch++ = BPF_ALU64_IMM(BPF_NEG, BPF_REG_AX, 0);
>     *patch++ = BPF_ALU64_IMM(BPF_ARSH, BPF_REG_AX, 63);
>     if (issrc) {
>         *patch++ = BPF_ALU64_REG(BPF_AND, BPF_REG_AX,
>                      off_reg);
>         insn->src_reg = BPF_REG_AX;
>     } else {
>         *patch++ = BPF_ALU64_REG(BPF_AND, off_reg,
>                      BPF_REG_AX);
>     }
> 
> This was introduced by "bpf: prevent out of bounds speculation on pointer arithmetic"
> (https://patchwork.ozlabs.org/patch/1039606/).
> I don't yet understand what's going on.

Ok, sigh, fix is this, sorry about the braino:

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index cdd2cb01f789..5b3cd384df1d 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -7629,7 +7629,8 @@ static int fixup_bpf_calls(struct bpf_verifier_env *env)
                        u32 off_reg;

                        aux = &env->insn_aux_data[i + delta];
-                       if (!aux->alu_state)
+                       if (!aux->alu_state ||
+                           aux->alu_state == BPF_ALU_NON_POINTER)
                                continue;

                        isneg = aux->alu_state & BPF_ALU_NEG_VALUE;

And this also makes the test work again:

foo@...t:/root/d0bb75a8c62cc35bec2b342054084aab-7cc37a3a93c8b4028e977f3131feaf7f8705e6a7$ ./ebpf-bug
0 ->                    0 0x0000000000000000
1 ->          54645145816 0x0000000cb91ac0d8
2 ->          54645145860 0x0000000cb91ac104
3 ->                   44 0x000000000000002c
foo@...t:/root/d0bb75a8c62cc35bec2b342054084aab-7cc37a3a93c8b4028e977f3131feaf7f8705e6a7$ exit
root@...t:~/d0bb75a8c62cc35bec2b342054084aab-7cc37a3a93c8b4028e977f3131feaf7f8705e6a7# ./ebpf-bug
0 ->                    0 0x0000000000000000
1 ->          57984017624 0x0000000d801de4d8
2 ->          57984017673 0x0000000d801de509
3 ->                   49 0x0000000000000031

I'll cook it as proper patch in a bit along with a test case.

Thanks for reporting!
Daniel