On Thu, Sep 12, 2024 at 11:17 AM Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> 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. > > > + 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 ... > > > 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 ... hmm.. just in case rX is the same register as rY, probably best to restore rX early right at L1: label (and adjust `if rX == 0 goto L2` into `if rX != 0 goto L2`). > > > It's a bit trickier to follow, but should be faster in a common case. > > WDYT? Too much too far? > > > > + *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. > > > - 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 > >
