On Thu, Sep 12, 2024 at 3:53 PM Yonghong Song <yonghong.song@xxxxxxxxx> wrote: > > > On 9/12/24 11:17 AM, Andrii Nakryiko wrote: > > On Wed, Sep 11, 2024 at 9:00 PM Yonghong Song <yonghong.song@xxxxxxxxx> wrote: > >> Zac Ecob reported a problem where a bpf program may cause kernel crash due > >> to the following error: > >> Oops: divide error: 0000 [#1] PREEMPT SMP KASAN PTI > >> > >> The failure is due to the below signed divide: > >> LLONG_MIN/-1 where LLONG_MIN equals to -9,223,372,036,854,775,808. > >> LLONG_MIN/-1 is supposed to give a positive number 9,223,372,036,854,775,808, > >> but it is impossible since for 64-bit system, the maximum positive > >> number is 9,223,372,036,854,775,807. On x86_64, LLONG_MIN/-1 will > >> cause a kernel exception. On arm64, the result for LLONG_MIN/-1 is > >> LLONG_MIN. > >> > >> Further investigation found all the following sdiv/smod cases may trigger > >> an exception when bpf program is running on x86_64 platform: > >> - LLONG_MIN/-1 for 64bit operation > >> - INT_MIN/-1 for 32bit operation > >> - LLONG_MIN%-1 for 64bit operation > >> - INT_MIN%-1 for 32bit operation > >> where -1 can be an immediate or in a register. > >> > >> On arm64, there are no exceptions: > >> - LLONG_MIN/-1 = LLONG_MIN > >> - INT_MIN/-1 = INT_MIN > >> - LLONG_MIN%-1 = 0 > >> - INT_MIN%-1 = 0 > >> where -1 can be an immediate or in a register. > >> > >> Insn patching is needed to handle the above cases and the patched codes > >> produced results aligned with above arm64 result. > >> > >> [1] https://lore.kernel.org/bpf/tPJLTEh7S_DxFEqAI2Ji5MBSoZVg7_G-Py2iaZpAaWtM961fFTWtsnlzwvTbzBzaUzwQAoNATXKUlt0LZOFgnDcIyKCswAnAGdUF3LBrhGQ=@protonmail.com/ > >> > >> Reported-by: Zac Ecob <zacecob@xxxxxxxxxxxxxx> > >> Signed-off-by: Yonghong Song <yonghong.song@xxxxxxxxx> > >> --- > >> kernel/bpf/verifier.c | 84 ++++++++++++++++++++++++++++++++++++++++--- > >> 1 file changed, 80 insertions(+), 4 deletions(-) > >> > >> Changelogs: > >> v1 -> v2: > >> - Handle more crash cases like 32bit operation and modules. > >> - Add more tests to test new cases. > >> > >> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > >> index f35b80c16cda..ad7f51302c70 100644 > >> --- a/kernel/bpf/verifier.c > >> +++ b/kernel/bpf/verifier.c > >> @@ -20499,13 +20499,46 @@ static int do_misc_fixups(struct bpf_verifier_env *env) > >> /* Convert BPF_CLASS(insn->code) == BPF_ALU64 to 32-bit ALU */ > >> insn->code = BPF_ALU | BPF_OP(insn->code) | BPF_SRC(insn->code); > >> > >> - /* Make divide-by-zero exceptions impossible. */ > >> + /* Make sdiv/smod divide-by-minus-one exceptions impossible. */ > >> + if ((insn->code == (BPF_ALU64 | BPF_MOD | BPF_K) || > >> + insn->code == (BPF_ALU64 | BPF_DIV | BPF_K) || > >> + insn->code == (BPF_ALU | BPF_MOD | BPF_K) || > >> + insn->code == (BPF_ALU | BPF_DIV | BPF_K)) && > >> + insn->off == 1 && insn->imm == -1) { > >> + bool is64 = BPF_CLASS(insn->code) == BPF_ALU64; > >> + bool isdiv = BPF_OP(insn->code) == BPF_DIV; > >> + struct bpf_insn *patchlet; > >> + struct bpf_insn chk_and_div[] = { > >> + BPF_RAW_INSN((is64 ? BPF_ALU64 : BPF_ALU) | > >> + BPF_OP(BPF_NEG) | BPF_K, insn->dst_reg, > >> + 0, 0, 0), > >> + }; > >> + struct bpf_insn chk_and_mod[] = { > >> + BPF_ALU32_REG(BPF_XOR, insn->dst_reg, insn->dst_reg), > >> + }; > >> + > >> + patchlet = isdiv ? chk_and_div : chk_and_mod; > > nit: "chk_and_" part in the name is misleading, it's more like > > "safe_div" and "safe_mod". Oh, and it's "sdiv" and "smod" specific, so > > probably not a bad idea to have that in the name as well. > > good idea. Will use chk_and_sdiv and chk_and_smod. > > > > >> + cnt = isdiv ? ARRAY_SIZE(chk_and_div) : ARRAY_SIZE(chk_and_mod); > >> + > >> + new_prog = bpf_patch_insn_data(env, i + delta, patchlet, cnt); > >> + if (!new_prog) > >> + return -ENOMEM; > >> + > >> + delta += cnt - 1; > >> + env->prog = prog = new_prog; > >> + insn = new_prog->insnsi + i + delta; > >> + goto next_insn; > >> + } > >> + > >> + /* Make divide-by-zero and divide-by-minus-one exceptions impossible. */ > >> if (insn->code == (BPF_ALU64 | BPF_MOD | BPF_X) || > >> insn->code == (BPF_ALU64 | BPF_DIV | BPF_X) || > >> insn->code == (BPF_ALU | BPF_MOD | BPF_X) || > >> insn->code == (BPF_ALU | BPF_DIV | BPF_X)) { > >> bool is64 = BPF_CLASS(insn->code) == BPF_ALU64; > >> bool isdiv = BPF_OP(insn->code) == BPF_DIV; > >> + bool is_sdiv = isdiv && insn->off == 1; > >> + bool is_smod = !isdiv && insn->off == 1; > >> struct bpf_insn *patchlet; > >> struct bpf_insn chk_and_div[] = { > >> /* [R,W]x div 0 -> 0 */ > >> @@ -20525,10 +20558,53 @@ static int do_misc_fixups(struct bpf_verifier_env *env) > >> BPF_JMP_IMM(BPF_JA, 0, 0, 1), > >> BPF_MOV32_REG(insn->dst_reg, insn->dst_reg), > >> }; > >> + struct bpf_insn chk_and_sdiv[] = { > >> + /* [R,W]x sdiv 0 -> 0 */ > >> + BPF_RAW_INSN((is64 ? BPF_JMP : BPF_JMP32) | > >> + BPF_JNE | BPF_K, insn->src_reg, > >> + 0, 2, 0), > >> + BPF_ALU32_REG(BPF_XOR, insn->dst_reg, insn->dst_reg), > >> + BPF_JMP_IMM(BPF_JA, 0, 0, 4), > >> + /* LLONG_MIN sdiv -1 -> LLONG_MIN > >> + * INT_MIN sdiv -1 -> INT_MIN > >> + */ > >> + BPF_RAW_INSN((is64 ? BPF_JMP : BPF_JMP32) | > >> + BPF_JNE | BPF_K, insn->src_reg, > >> + 0, 2, -1), > >> + /* BPF_NEG(LLONG_MIN) == -LLONG_MIN == LLONG_MIN */ > >> + BPF_RAW_INSN((is64 ? BPF_ALU64 : BPF_ALU) | > >> + BPF_OP(BPF_NEG) | BPF_K, insn->dst_reg, > >> + 0, 0, 0), > >> + BPF_JMP_IMM(BPF_JA, 0, 0, 1), > > I don't know how much it actually matters, but it feels like common > > safe case should be as straight-line-executed as possible, no? > > > > So maybe it's better to rearrange to roughly this (where rX is the > > divisor register): > > > > if rX == 0 goto L1 > > if rX == -1 goto L2 > > rY /= rX > > goto L3 > > L1: /* zero case */ > > rY = 0 /* fallthrough, negation doesn't hurt, but less jumping */ > > L2: /* negative one case (or zero) */ > > rY = -rY > > L3: > > ... the rest of the program code ... > > My previous patched insn try to clearly separate rX == 0 and > rX == -1 case. It has 2 insns including 2 cond jmps, 2 uncond jmps and > one 3 alu operations. The above one removed one uncond jmp, which > is indeed better. > > > > > > > Those two branches for common case are still annoyingly inefficient, I > > wonder if we should do > > > > rX += 1 /* [-1, 0] -> [0, 1] > > if rX <=(unsigned) 1 goto L1 > > rX -= 1 /* restore original divisor */ > > rY /= rX /* common case */ > > goto L3 > > L1: > > if rX == 0 goto L2 /* jump if originally -1 */ > > rY = 0 /* division by zero case */ > > L2: /* fallthrough */ > > rY = -rY > > rX -= 1 /* restore original divisor */ > > L3: > > ... continue with the rest ... > > > > > > It's a bit trickier to follow, but should be faster in a common case. > > > > WDYT? Too much too far? > > This is even better. The above rX -= 1 can be removed if we use > BPF_REG_AX as the temporary register. For example, > > tmp = rX > tmp += 1 /* [-1, 0] -> [0, 1] > if tmp <=(unsigned) 1 goto L1 > rY /= rX /* common case */ > goto L3 > L1: > if tmp == 0 goto L2 /* jump if originally -1 */ > rY = 0 /* division by zero case */ > L2: /* fallthrough */ > rY = -rY > L3: > ... continue with the rest ... > > Maybe we can do even better > > tmp = rX > tmp += 1 /* [-1, 0] -> [0, 1] > if tmp >(unsigned) 1 goto L2 > if tmp == 0 goto L1 > rY = 0 > L1: > rY = -rY; > goto L3 > L2: > rY /= rX > L3: > > Could this be even better by reducing one uncond jmp in the fast path? Yep, makes sense to me. Go for it (as far as I'm concerned). > > > > > > >> + *insn, > >> + }; > >> + struct bpf_insn chk_and_smod[] = { > >> + /* [R,W]x mod 0 -> [R,W]x */ > >> + BPF_RAW_INSN((is64 ? BPF_JMP : BPF_JMP32) | > >> + BPF_JNE | BPF_K, insn->src_reg, > >> + 0, 2, 0), > >> + BPF_MOV32_REG(insn->dst_reg, insn->dst_reg), > >> + BPF_JMP_IMM(BPF_JA, 0, 0, 4), > >> + /* [R,W]x mod -1 -> 0 */ > >> + BPF_RAW_INSN((is64 ? BPF_JMP : BPF_JMP32) | > >> + BPF_JNE | BPF_K, insn->src_reg, > >> + 0, 2, -1), > >> + BPF_ALU32_REG(BPF_XOR, insn->dst_reg, insn->dst_reg), > >> + BPF_JMP_IMM(BPF_JA, 0, 0, 1), > >> + *insn, > >> + }; > >> > > Same idea here, keep the common case as straight as possible. > > Sure. Will do. > > > > >> - patchlet = isdiv ? chk_and_div : chk_and_mod; > >> - cnt = isdiv ? ARRAY_SIZE(chk_and_div) : > >> - ARRAY_SIZE(chk_and_mod) - (is64 ? 2 : 0); > >> + if (is_sdiv) { > >> + patchlet = chk_and_sdiv; > >> + cnt = ARRAY_SIZE(chk_and_sdiv); > >> + } else if (is_smod) { > >> + patchlet = chk_and_smod; > >> + cnt = ARRAY_SIZE(chk_and_smod); > >> + } else { > >> + patchlet = isdiv ? chk_and_div : chk_and_mod; > >> + cnt = isdiv ? ARRAY_SIZE(chk_and_div) : > >> + ARRAY_SIZE(chk_and_mod) - (is64 ? 2 : 0); > >> + } > >> > >> new_prog = bpf_patch_insn_data(env, i + delta, patchlet, cnt); > >> if (!new_prog) > >> -- > >> 2.43.5 > >>
