Re: [PATCH bpf-next 1/2] bpf: Fix a sdiv overflow issue

[Yonghong Song <yonghong.song@linux.dev>]

On 9/11/24 10:17 AM, Andrii Nakryiko wrote:
> On Tue, Sep 10, 2024 at 9:40 PM Yonghong Song <yonghong.song@linux.dev> 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.
>>
>> So for 64-bit signed divide (sdiv), some additional insns are patched
>> to check LLONG_MIN/-1 pattern. If such a pattern does exist, the result
>> will be LLONG_MIN. Otherwise, it follows normal sdiv operation.
>>
>>    [1] https://lore.kernel.org/bpf/tPJLTEh7S_DxFEqAI2Ji5MBSoZVg7_G-Py2iaZpAaWtM961fFTWtsnlzwvTbzBzaUzwQAoNATXKUlt0LZOFgnDcIyKCswAnAGdUF3LBrhGQ=@protonmail.com/
>>
>> Reported-by: Zac Ecob <zacecob@protonmail.com>
>> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
>> ---
>>   kernel/bpf/verifier.c | 29 ++++++++++++++++++++++++++---
>>   1 file changed, 26 insertions(+), 3 deletions(-)
>>
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index f35b80c16cda..d77f1a05a065 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -20506,6 +20506,7 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
>>                      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_sdiv64 = is64 && isdiv && insn->off == 1;
>>                          struct bpf_insn *patchlet;
>>                          struct bpf_insn chk_and_div[] = {
>>                                  /* [R,W]x div 0 -> 0 */
>> @@ -20525,10 +20526,32 @@ 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_sdiv64[] = {
>> +                               /* Rx sdiv 0 -> 0 */
>> +                               BPF_RAW_INSN(BPF_JMP | 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, 8),
>> +                               /* LLONG_MIN sdiv -1 -> LLONG_MIN */
>> +                               BPF_RAW_INSN(BPF_JMP | BPF_JNE | BPF_K, insn->src_reg,
>> +                                            0, 6, -1),
>> +                               BPF_LD_IMM64(insn->src_reg, LLONG_MIN),
> wouldn't it be simpler and faster to just check if insn->dst_reg ==
> -1, and if yes, just negate src_reg? Regardless of src_reg value this
> should be correct because by definition division by -1 is a negation.
> WDYT?

Yes. This should work! It utilized special property that -INT_MIN == INT_MIN and
-LLONG_MIN == LLONG_MIN.

For module like Reg%(-1), the result will be always 0 for both 32- or 64-bit operation.

>
>> +                               BPF_RAW_INSN(BPF_JMP | BPF_JNE | BPF_X, insn->dst_reg,
>> +                                            insn->src_reg, 2, 0),
>> +                               BPF_MOV64_IMM(insn->src_reg, -1),
>> +                               BPF_JMP_IMM(BPF_JA, 0, 0, 2),
>> +                               BPF_MOV64_IMM(insn->src_reg, -1),
>> +                               *insn,
>> +                       };
>>
>> -                       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_sdiv64) {
>> +                               patchlet = chk_and_sdiv64;
>> +                               cnt = ARRAY_SIZE(chk_and_sdiv64);
>> +                       } 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
>>