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bpf: Fix a sdiv overflow issue
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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. The below are pseudo
codes to handle sdiv/smod exceptions including both divisor -1 and divisor 0
and the divisor is stored in a register.

sdiv:
      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:

smod:
      tmp = rX
      tmp += 1 /* [-1, 0] -> [0, 1]
      if tmp >(unsigned) 1 goto L1
      if tmp == 1 (is64 ? goto L2 : goto L3)
      rY = 0;
      goto L2
  L1:
      rY %= rX
  L2:
      goto L4  // only when !is64
  L3:
      wY = wY  // only when !is64
  L4:

  [1] https://lore.kernel.org/bpf/tPJLTEh7S_DxFEqAI2Ji5MBSoZVg7_G-Py2iaZpAaWtM961fFTWtsnlzwvTbzBzaUzwQAoNATXKUlt0LZOFgnDcIyKCswAnAGdUF3LBrhGQ=@protonmail.com/

Reported-by: Zac Ecob <[email protected]>
Signed-off-by: Yonghong Song <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>
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Yonghong Song authored and Alexei Starovoitov committed Sep 13, 2024
1 parent ea02a94 commit 7dd34d7
Showing 1 changed file with 89 additions and 4 deletions.
93 changes: 89 additions & 4 deletions kernel/bpf/verifier.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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_sdiv[] = {
BPF_RAW_INSN((is64 ? BPF_ALU64 : BPF_ALU) |
BPF_NEG | BPF_K, insn->dst_reg,
0, 0, 0),
};
struct bpf_insn chk_and_smod[] = {
BPF_MOV32_IMM(insn->dst_reg, 0),
};

patchlet = isdiv ? chk_and_sdiv : chk_and_smod;
cnt = isdiv ? ARRAY_SIZE(chk_and_sdiv) : ARRAY_SIZE(chk_and_smod);

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 */
Expand All @@ -20525,10 +20558,62 @@ 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
* LLONG_MIN sdiv -1 -> LLONG_MIN
* INT_MIN sdiv -1 -> INT_MIN
*/
BPF_MOV64_REG(BPF_REG_AX, insn->src_reg),
BPF_RAW_INSN((is64 ? BPF_ALU64 : BPF_ALU) |
BPF_ADD | BPF_K, BPF_REG_AX,
0, 0, 1),
BPF_RAW_INSN((is64 ? BPF_JMP : BPF_JMP32) |
BPF_JGT | BPF_K, BPF_REG_AX,
0, 4, 1),
BPF_RAW_INSN((is64 ? BPF_JMP : BPF_JMP32) |
BPF_JEQ | BPF_K, BPF_REG_AX,
0, 1, 0),
BPF_RAW_INSN((is64 ? BPF_ALU64 : BPF_ALU) |
BPF_MOV | BPF_K, insn->dst_reg,
0, 0, 0),
/* BPF_NEG(LLONG_MIN) == -LLONG_MIN == LLONG_MIN */
BPF_RAW_INSN((is64 ? BPF_ALU64 : BPF_ALU) |
BPF_NEG | BPF_K, insn->dst_reg,
0, 0, 0),
BPF_JMP_IMM(BPF_JA, 0, 0, 1),
*insn,
};
struct bpf_insn chk_and_smod[] = {
/* [R,W]x mod 0 -> [R,W]x */
/* [R,W]x mod -1 -> 0 */
BPF_MOV64_REG(BPF_REG_AX, insn->src_reg),
BPF_RAW_INSN((is64 ? BPF_ALU64 : BPF_ALU) |
BPF_ADD | BPF_K, BPF_REG_AX,
0, 0, 1),
BPF_RAW_INSN((is64 ? BPF_JMP : BPF_JMP32) |
BPF_JGT | BPF_K, BPF_REG_AX,
0, 3, 1),
BPF_RAW_INSN((is64 ? BPF_JMP : BPF_JMP32) |
BPF_JEQ | BPF_K, BPF_REG_AX,
0, 3 + (is64 ? 0 : 1), 1),
BPF_MOV32_IMM(insn->dst_reg, 0),
BPF_JMP_IMM(BPF_JA, 0, 0, 1),
*insn,
BPF_JMP_IMM(BPF_JA, 0, 0, 1),
BPF_MOV32_REG(insn->dst_reg, insn->dst_reg),
};

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) - (is64 ? 2 : 0);
} 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)
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