2015-11-05 04:44:08

by Z Lim

Subject: [PATCH] arm64: bpf: fix mod-by-zero case

Turns out in the case of modulo by zero in a BPF program:
A = A % X; (X == 0)
the expected behavior is to terminate with return value 0.

The bug in JIT is exposed by a new test case [1].

[1] https://lkml.org/lkml/2015/11/4/499

Signed-off-by: Zi Shen Lim <[email protected]>
Reported-by: Yang Shi <[email protected]>
Reported-by: Xi Wang <[email protected]>
CC: Alexei Starovoitov <[email protected]>
CC: Catalin Marinas <[email protected]>
Fixes: e54bcde3d69d ("arm64: eBPF JIT compiler")
Cc: <[email protected]> # 3.18+
---
This patch applies on top of "arm64: bpf: fix fiv-by-zero case" [2].

[2] https://lkml.org/lkml/2015/11/4/25

arch/arm64/net/bpf_jit_comp.c | 21 +++++++++++++--------
1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
index 9ae6f23..6217f80 100644
--- a/arch/arm64/net/bpf_jit_comp.c
+++ b/arch/arm64/net/bpf_jit_comp.c
@@ -269,6 +269,8 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx)
break;
case BPF_ALU | BPF_DIV | BPF_X:
case BPF_ALU64 | BPF_DIV | BPF_X:
+ case BPF_ALU | BPF_MOD | BPF_X:
+ case BPF_ALU64 | BPF_MOD | BPF_X:
{
const u8 r0 = bpf2a64[BPF_REG_0];

@@ -281,16 +283,19 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx)
check_imm26(jmp_offset);
emit(A64_B(jmp_offset), ctx);
/* else */
- emit(A64_UDIV(is64, dst, dst, src), ctx);
+ switch (BPF_OP(code)) {
+ case BPF_DIV:
+ emit(A64_UDIV(is64, dst, dst, src), ctx);
+ break;
+ case BPF_MOD:
+ ctx->tmp_used = 1;
+ emit(A64_UDIV(is64, tmp, dst, src), ctx);
+ emit(A64_MUL(is64, tmp, tmp, src), ctx);
+ emit(A64_SUB(is64, dst, dst, tmp), ctx);
+ break;
+ }
break;
}
- case BPF_ALU | BPF_MOD | BPF_X:
- case BPF_ALU64 | BPF_MOD | BPF_X:
- ctx->tmp_used = 1;
- emit(A64_UDIV(is64, tmp, dst, src), ctx);
- emit(A64_MUL(is64, tmp, tmp, src), ctx);
- emit(A64_SUB(is64, dst, dst, tmp), ctx);
- break;
case BPF_ALU | BPF_LSH | BPF_X:
case BPF_ALU64 | BPF_LSH | BPF_X:
emit(A64_LSLV(is64, dst, dst, src), ctx);
--
1.9.1

2015-11-06 17:00:22

by Catalin Marinas

Subject: Re: [PATCH] arm64: bpf: fix mod-by-zero case

On Wed, Nov 04, 2015 at 08:43:59PM -0800, Zi Shen Lim wrote:
> Turns out in the case of modulo by zero in a BPF program:
> A = A % X; (X == 0)
> the expected behavior is to terminate with return value 0.
>
> The bug in JIT is exposed by a new test case [1].
>
> [1] https://lkml.org/lkml/2015/11/4/499
>
> Signed-off-by: Zi Shen Lim <[email protected]>
> Reported-by: Yang Shi <[email protected]>
> Reported-by: Xi Wang <[email protected]>
> CC: Alexei Starovoitov <[email protected]>
> CC: Catalin Marinas <[email protected]>
> Fixes: e54bcde3d69d ("arm64: eBPF JIT compiler")
> Cc: <[email protected]> # 3.18+
> ---
> This patch applies on top of "arm64: bpf: fix fiv-by-zero case" [2].

Both patches applied. Thanks.

--
Catalin