On Sat, Jun 5, 2021 at 10:55 AM Yonghong Song [off-list ref] wrote:

On 6/5/21 8:01 AM, Kurt Manucredo wrote:

quoted

Syzbot detects a shift-out-of-bounds in ___bpf_prog_run()
kernel/bpf/core.c:1414:2.

This is not enough. We need more information on why this happens
so we can judge whether the patch indeed fixed the issue.

quoted

I propose: In adjust_scalar_min_max_vals() move boundary check up to avoid
missing them and return with error when detected.

Reported-and-tested-by: syzbot+bed360704c521841c85d@syzkaller.appspotmail.com
Signed-off-by: Kurt Manucredo <redacted>
---

https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231

Changelog:
----------
v4 - Fix shift-out-of-bounds in adjust_scalar_min_max_vals.
      Fix commit message.
v3 - Make it clearer what the fix is for.
v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary
      check in check_alu_op() in verifier.c.
v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary
      check in ___bpf_prog_run().

thanks

kind regards

Kurt

  kernel/bpf/verifier.c | 30 +++++++++---------------------
  1 file changed, 9 insertions(+), 21 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 94ba5163d4c5..ed0eecf20de5 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c

@@ -7510,6 +7510,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
      u32_min_val = src_reg.u32_min_value;
      u32_max_val = src_reg.u32_max_value;

+     if ((opcode == BPF_LSH || opcode == BPF_RSH || opcode == BPF_ARSH) &&
+                     umax_val >= insn_bitness) {
+             /* Shifts greater than 31 or 63 are undefined.
+              * This includes shifts by a negative number.
+              */
+             verbose(env, "invalid shift %lld\n", umax_val);
+             return -EINVAL;
+     }

I think your fix is good. I would like to move after

I suspect such change will break valid programs that do shift by register.

the following code though:

         if (!src_known &&
             opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) {
                 __mark_reg_unknown(env, dst_reg);
                 return 0;
         }

quoted

+
      if (alu32) {
              src_known = tnum_subreg_is_const(src_reg.var_off);
              if ((src_known &&

@@ -7592,39 +7601,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
              scalar_min_max_xor(dst_reg, &src_reg);
              break;
      case BPF_LSH:
-             if (umax_val >= insn_bitness) {
-                     /* Shifts greater than 31 or 63 are undefined.
-                      * This includes shifts by a negative number.
-                      */
-                     mark_reg_unknown(env, regs, insn->dst_reg);
-                     break;
-             }

I think this is what happens. For the above case, we simply
marks the dst reg as unknown and didn't fail verification.
So later on at runtime, the shift optimization will have wrong
shift value (> 31/64). Please correct me if this is not right
analysis. As I mentioned in the early please write detailed
analysis in commit log.

The large shift is not wrong. It's just undefined.
syzbot has to ignore such cases.
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