[RFC PATCH 6.1.y 2/2] bpf: make the verifier tracks the "not equal" for regs

[Zhenzhong Wu]

[ Upstream commit d028f87517d6775dccff4ddbca2740826f9e53f1 ]

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@xxxxxxxxx>
Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
Acked-by: Shung-Hsi Yu <shung-hsi.yu@xxxxxxxx>
Link: https://lore.kernel.org/r/20231219134800.1550388-2-menglong8.dong@xxxxxxxxx
Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
[ zhenzhong: adapt to 6.1.y reg_set_min_max() layout. The upstream
  change lives in regs_refine_cond_op(); 6.1.y still refines true/false
  branch states in reg_set_min_max(), so apply the not-equal range
  exclusion to BPF_JEQ's false_reg and BPF_JNE's true_reg there. ]
Signed-off-by: Zhenzhong Wu <jt26wzz@xxxxxxxxx>
---
 kernel/bpf/verifier.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 5e029d1..e51f44b 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -9954,18 +9954,50 @@ static void reg_set_min_max(struct bpf_reg_state *true_reg,
 		if (is_jmp32) {
 			__mark_reg32_known(true_reg, val32);
 			true_32off = tnum_subreg(true_reg->var_off);
+			if (false_reg->u32_min_value == val32)
+				false_reg->u32_min_value++;
+			if (false_reg->u32_max_value == val32)
+				false_reg->u32_max_value--;
+			if (false_reg->s32_min_value == sval32)
+				false_reg->s32_min_value++;
+			if (false_reg->s32_max_value == sval32)
+				false_reg->s32_max_value--;
 		} else {
 			___mark_reg_known(true_reg, val);
 			true_64off = true_reg->var_off;
+			if (false_reg->umin_value == val)
+				false_reg->umin_value++;
+			if (false_reg->umax_value == val)
+				false_reg->umax_value--;
+			if (false_reg->smin_value == sval)
+				false_reg->smin_value++;
+			if (false_reg->smax_value == sval)
+				false_reg->smax_value--;
 		}
 		break;
 	case BPF_JNE:
 		if (is_jmp32) {
 			__mark_reg32_known(false_reg, val32);
 			false_32off = tnum_subreg(false_reg->var_off);
+			if (true_reg->u32_min_value == val32)
+				true_reg->u32_min_value++;
+			if (true_reg->u32_max_value == val32)
+				true_reg->u32_max_value--;
+			if (true_reg->s32_min_value == sval32)
+				true_reg->s32_min_value++;
+			if (true_reg->s32_max_value == sval32)
+				true_reg->s32_max_value--;
 		} else {
 			___mark_reg_known(false_reg, val);
 			false_64off = false_reg->var_off;
+			if (true_reg->umin_value == val)
+				true_reg->umin_value++;
+			if (true_reg->umax_value == val)
+				true_reg->umax_value--;
+			if (true_reg->smin_value == sval)
+				true_reg->smin_value++;
+			if (true_reg->smax_value == sval)
+				true_reg->smax_value--;
 		}
 		break;
 	case BPF_JSET:
-- 
2.43.0