[PATCH bpf-next 03/13] bpf: enhance BPF_JEQ/BPF_JNE is_branch_taken logic

Andrii Nakryiko
2023-11-03T00:08:46Z

Use 32-bit subranges to prune some 64-bit BPF_JEQ/BPF_JNE conditions
that otherwise would be "inconclusive" (i.e., is_branch_taken() would
return -1). This can happen, for example, when registers are initialized
as 64-bit u64/s64, then compared for inequality as 32-bit subregisters,
and then followed by 64-bit equality/inequality check. That 32-bit
inequality can establish some pattern for lower 32 bits of a register
(e.g., s< 0 condition determines whether the bit #31 is zero or not),
while overall 64-bit value could be anything (according to a value range
representation).

This is not a fancy quirky special case, but actually a handling that's
necessary to prevent correctness issue with BPF verifier's range
tracking: set_range_min_max() assumes that register ranges are
non-overlapping, and if that condition is not guaranteed by
is_branch_taken() we can end up with invalid ranges, where min > max.

[0] https://lore.kernel.org/bpf/CACkBjsY2q1_fUohD7hRmKGqv1MV=eP2f6XK8kjkYNw7BaiF8iQ@mail.gmail.com/

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
kernel/bpf/verifier.c | 24 ++++++++++++++++++++++++
1 file changed, 24 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 2627461164ed..8691cacd3ad3 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -14214,6 +14214,18 @@ static int is_scalar_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_sta
return 0;
if (smin1 > smax2 || smax1 < smin2)
return 0;
+ if (!is_jmp32) {
+ /* if 64-bit ranges are inconclusive, see if we can
+ * utilize 32-bit subrange knowledge to eliminate
+ * branches that can't be taken a priori
+ */
+ if (reg1->u32_min_value > reg2->u32_max_value ||
+ reg1->u32_max_value < reg2->u32_min_value)
+ return 0;
+ if (reg1->s32_min_value > reg2->s32_max_value ||
+ reg1->s32_max_value < reg2->s32_min_value)
+ return 0;
+ }
break;
case BPF_JNE:
/* constants, umin/umax and smin/smax checks would be
@@ -14226,6 +14238,18 @@ static int is_scalar_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_sta
return 1;
if (smin1 > smax2 || smax1 < smin2)
return 1;
+ if (!is_jmp32) {
+ /* if 64-bit ranges are inconclusive, see if we can
+ * utilize 32-bit subrange knowledge to eliminate
+ * branches that can't be taken a priori
+ */
+ if (reg1->u32_min_value > reg2->u32_max_value ||
+ reg1->u32_max_value < reg2->u32_min_value)
+ return 1;
+ if (reg1->s32_min_value > reg2->s32_max_value ||
+ reg1->s32_max_value < reg2->s32_min_value)
+ return 1;
+ }
break;
case BPF_JSET:
if (!is_reg_const(reg2, is_jmp32)) {
--
2.34.1