[PATCH stable 6.6.y v2 2/3] bpf: make the verifier tracks the "not equal" for regs

[Zhenzhong Wu <jt26wzz@gmail.com>]

From: Menglong Dong <menglong8.dong@gmail.com>

[ Upstream commit d028f87517d6775dccff4ddbca2740826f9e53f1 ]

We can derive useful information for BPF_JNE when one side is a constant
and the constant is exactly at the edge of the other register range.

For example, a > 0 can be compiled as a jump if a == 0. The equal branch
marks the register as known zero, but the fallthrough branch also needs to
preserve that the register is not zero. Without this, the range can remain
[0, max] and later verifier state pruning can keep an impossible scalar
path.

The upstream fix lives in regs_refine_cond_op(). The 6.6.y verifier still
uses the older reg_set_min_max() layout, so express the same branch-edge
refinement there: for BPF_JEQ, preserve the known-equal true branch and
exclude the constant from false_reg; for BPF_JNE, preserve the known-equal
false branch and exclude the constant from true_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>
[ zhenzhong: backport to 6.6.y reg_set_min_max() layout. ]
Signed-off-by: Zhenzhong Wu <jt26wzz@gmail.com>
---
 kernel/bpf/verifier.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 5f94bff12..de4f46796 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -14169,18 +14169,50 @@ 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