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

Yonghong Song <yonghong.song@xxxxxxxxx> · Wed, 11 Sep 2024 10:32:51 -0700

On 9/11/24 10:17 AM, Andrii Nakryiko wrote:
On Tue, Sep 10, 2024 at 9:40 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.

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@xxxxxxxxxxxxxx>
Signed-off-by: Yonghong Song <yonghong.song@xxxxxxxxx>
---
  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








