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 <zacecob@xxxxxxxxxxxxxx>
Signed-off-by: Yonghong Song <yonghong.song@xxxxxxxxx>
---
kernel/bpf/verifier.c | 93 +++++++++++++++++++++++++++++++++++++++++--
1 file changed, 89 insertions(+), 4 deletions(-)
Changelogs:
v2 -> v3:
- Change sdiv/smod (r/r, r%r) patched insn to be more efficient
for default case.
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..69b8d91f5136 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_sdiv[] = {
+ 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_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 */
@@ -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_OP(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_OP(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_OP(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_OP(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)
--
2.43.5
