bpf: make the verifier tracks the "not equal" for regs

We can derive some new information for BPF_JNE in regs_refine_cond_op().
Take following code for example:

  /* The type of "a" is u32 */
  if (a > 0 && a < 100) {
    /* the range of the register for a is [0, 99], not [1, 99],
     * and will cause the following error:
     *
     *   invalid zero-sized read
     *
     * as a can be 0.
     */
    bpf_skb_store_bytes(skb, xx, xx, a, 0);
  }

In the code above, "a > 0" will be compiled to "jmp xxx if a == 0". In the
TRUE branch, the dst_reg will be marked as known to 0. However, in the
fallthrough(FALSE) branch, the dst_reg will not be handled, which makes
the [min, max] for a is [0, 99], not [1, 99].

For BPF_JNE, we can reduce the range of the dst reg if the src reg is a
const and is exactly the edge of the dst reg.

Signed-off-by: Menglong Dong <menglong8.dong@gmail.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Link: https://lore.kernel.org/r/20231219134800.1550388-2-menglong8.dong@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 4ceec8c2a484145494e7e9c6f063b26dcda0233c..df1cae459c77131508d50e654613cbca2dfc89fd 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -14336,7 +14336,43 @@ again:
                }
                break;
        case BPF_JNE:
-               /* we don't derive any new information for inequality yet */
+               if (!is_reg_const(reg2, is_jmp32))
+                       swap(reg1, reg2);
+               if (!is_reg_const(reg2, is_jmp32))
+                       break;
+
+               /* try to recompute the bound of reg1 if reg2 is a const and
+                * is exactly the edge of reg1.
+                */
+               val = reg_const_value(reg2, is_jmp32);
+               if (is_jmp32) {
+                       /* u32_min_value is not equal to 0xffffffff at this point,
+                        * because otherwise u32_max_value is 0xffffffff as well,
+                        * in such a case both reg1 and reg2 would be constants,
+                        * jump would be predicted and reg_set_min_max() won't
+                        * be called.
+                        *
+                        * Same reasoning works for all {u,s}{min,max}{32,64} cases
+                        * below.
+                        */
+                       if (reg1->u32_min_value == (u32)val)
+                               reg1->u32_min_value++;
+                       if (reg1->u32_max_value == (u32)val)
+                               reg1->u32_max_value--;
+                       if (reg1->s32_min_value == (s32)val)
+                               reg1->s32_min_value++;
+                       if (reg1->s32_max_value == (s32)val)
+                               reg1->s32_max_value--;
+               } else {
+                       if (reg1->umin_value == (u64)val)
+                               reg1->umin_value++;
+                       if (reg1->umax_value == (u64)val)
+                               reg1->umax_value--;
+                       if (reg1->smin_value == (s64)val)
+                               reg1->smin_value++;
+                       if (reg1->smax_value == (s64)val)
+                               reg1->smax_value--;
+               }
                break;
        case BPF_JSET:
                if (!is_reg_const(reg2, is_jmp32))