[PATCH bpf-next v2 1/2] bpf: Fix a umin > umax reg bound error
From: Xu Kuohai
Date: Tue Mar 14 2023 - 03:35:51 EST
From: Xu Kuohai <xukuohai@xxxxxxxxxx>
After commit 3f50f132d840 ("bpf: Verifier, do explicit ALU32 bounds tracking"),
the following bpf prog is rejected:
0: (61) r2 = *(u32 *)(r1 +0) ; R2_w=pkt(off=0,r=0,imm=0)
1: (61) r3 = *(u32 *)(r1 +4) ; R3_w=pkt_end(off=0,imm=0)
2: (bf) r1 = r2
3: (07) r1 += 1
4: (2d) if r1 > r3 goto pc+8
5: (71) r1 = *(u8 *)(r2 +0) ; R1_w=scalar(umax=255,var_off=(0x0; 0xff))
6: (18) r0 = 0x7fffffffffffff10
8: (0f) r1 += r0 ; R1_w=scalar(umin=0x7fffffffffffff10,umax=0x800000000000000f)
9: (18) r0 = 0x8000000000000000
11: (07) r0 += 1
12: (ad) if r0 < r1 goto pc-2
13: (b7) r0 = 0
14: (95) exit
And the verifier log says:
[...]
from 12 to 11: R0_w=-9223372036854775794 R1=scalar(umin=9223372036854775823,umax=9223372036854775823,var_off=(0x8000000000000000; 0xffffffff))
11: (07) r0 += 1 ; R0_w=-9223372036854775793
12: (ad) if r0 < r1 goto pc-2 ; R0_w=-9223372036854775793 R1=scalar(umin=9223372036854775823,umax=9223372036854775823,var_off=(0x8000000000000000; 0xffffffff))
13: safe
from 12 to 11: R0_w=-9223372036854775793 R1=scalar(umin=9223372036854775824,umax=9223372036854775823,var_off=(0x8000000000000000; 0xffffffff))
11: (07) r0 += 1 ; R0_w=-9223372036854775792
12: (ad) if r0 < r1 goto pc-2 ; R0_w=-9223372036854775792 R1=scalar(umin=9223372036854775824,umax=9223372036854775823,var_off=(0x8000000000000000; 0xffffffff))
13: safe
[...]
What can be seen here is that r1->umin grows blindly and becomes bigger
than r1->umax. The reason is because the loop does not terminate, when
r0 increases to r1->umax_value, the following code in reg_set_min_max()
sets r1->umin_value to r1->umax_value + 1 blindly:
case BPF_JGT:
{
if (is_jmp32) {
[...]
} else {
u64 false_umax = opcode == BPF_JGT ? val : val - 1;
u64 true_umin = opcode == BPF_JGT ? val + 1 : val;
false_reg->umax_value = min(false_reg->umax_value, false_umax);
true_reg->umin_value = max(true_reg->umin_value, true_umin);
}
break;
}
Why the loop does not terminate is because tnum_is_const(src_reg->var_off)
always returns false, causing is_branch_taken() to be skipped:
if (src_reg->type == SCALAR_VALUE &&
!is_jmp32 && tnum_is_const(src_reg->var_off)) {
pred = is_branch_taken(dst_reg, // could not reach here
src_reg->var_off.value,
opcode,
is_jmp32);
}
Why tnum_is_const(src_reg->var_off) always returns false is because
r1->umin_value starts increasing from 0x7fffffffffffff10, always bigger
than U32_MAX, causing the __reg_combine_64_into_32() to mark the lower
32 bits unbounded, i.e. not a constant.
To fix it:
1. avoid increasing reg lower bound to a value bigger than the upper bound,
or decreasing reg upper bound to a value smaller than the lower bound.
2. set 32-bit min/max values to the lower 32 bits of the 64-bit min/max values
when the 64-bit min/max values are equal.
Fixes: 3f50f132d840 ("bpf: Verifier, do explicit ALU32 bounds tracking")
Signed-off-by: Xu Kuohai <xukuohai@xxxxxxxxxx>
---
kernel/bpf/verifier.c | 143 +++++++++++++++++++++++++++---------------
1 file changed, 93 insertions(+), 50 deletions(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 2bbd89279070..b775b50353d6 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2223,14 +2223,21 @@ static bool __reg64_bound_u32(u64 a)
static void __reg_combine_64_into_32(struct bpf_reg_state *reg)
{
+ s64 smin = reg->smin_value;
+ s64 smax = reg->smax_value;
+ u64 umin = reg->umin_value;
+ u64 umax = reg->umax_value;
+
__mark_reg32_unbounded(reg);
- if (__reg64_bound_s32(reg->smin_value) && __reg64_bound_s32(reg->smax_value)) {
- reg->s32_min_value = (s32)reg->smin_value;
- reg->s32_max_value = (s32)reg->smax_value;
+ if ((__reg64_bound_s32(smin) && __reg64_bound_s32(smax)) ||
+ smin == smax) {
+ reg->s32_min_value = (s32)smin;
+ reg->s32_max_value = (s32)smax;
}
- if (__reg64_bound_u32(reg->umin_value) && __reg64_bound_u32(reg->umax_value)) {
- reg->u32_min_value = (u32)reg->umin_value;
- reg->u32_max_value = (u32)reg->umax_value;
+ if ((__reg64_bound_u32(umin) && __reg64_bound_u32(umax)) ||
+ umin == umax) {
+ reg->u32_min_value = (u32)umin;
+ reg->u32_max_value = (u32)umax;
}
reg_bounds_sync(reg);
}
@@ -12828,6 +12835,62 @@ static int is_pkt_ptr_branch_taken(struct bpf_reg_state *dst_reg,
return -1;
}
+static void reg_inc_u32_min(struct bpf_reg_state *reg, u32 val)
+{
+ reg->u32_min_value = max(reg->u32_min_value, val);
+ if (reg->u32_min_value > reg->u32_max_value)
+ reg->u32_min_value = reg->u32_max_value;
+}
+
+static void reg_dec_u32_max(struct bpf_reg_state *reg, u32 val)
+{
+ reg->u32_max_value = min(reg->u32_max_value, val);
+ if (reg->u32_max_value < reg->u32_min_value)
+ reg->u32_max_value = reg->u32_min_value;
+}
+
+static void reg_inc_s32_min(struct bpf_reg_state *reg, s32 val)
+{
+ reg->s32_min_value = max(reg->s32_min_value, val);
+ if (reg->s32_min_value > reg->s32_max_value)
+ reg->s32_min_value = reg->s32_max_value;
+}
+
+static void reg_dec_s32_max(struct bpf_reg_state *reg, s32 val)
+{
+ reg->s32_max_value = min(reg->s32_max_value, val);
+ if (reg->s32_max_value < reg->s32_min_value)
+ reg->s32_max_value = reg->s32_min_value;
+}
+
+static void reg_inc_u64_min(struct bpf_reg_state *reg, u64 val)
+{
+ reg->umin_value = max(reg->umin_value, val);
+ if (reg->umin_value > reg->umax_value)
+ reg->umin_value = reg->umax_value;
+}
+
+static void reg_dec_u64_max(struct bpf_reg_state *reg, u64 val)
+{
+ reg->umax_value = min(reg->umax_value, val);
+ if (reg->umax_value < reg->umin_value)
+ reg->umax_value = reg->umin_value;
+}
+
+static void reg_inc_s64_min(struct bpf_reg_state *reg, s64 val)
+{
+ reg->smin_value = max(reg->smin_value, val);
+ if (reg->smin_value > reg->smax_value)
+ reg->smin_value = reg->smax_value;
+}
+
+static void reg_dec_s64_max(struct bpf_reg_state *reg, s64 val)
+{
+ reg->smax_value = min(reg->smax_value, val);
+ if (reg->smax_value < reg->smin_value)
+ reg->smax_value = reg->smin_value;
+}
+
/* Adjusts the register min/max values in the case that the dst_reg is the
* variable register that we are working on, and src_reg is a constant or we're
* simply doing a BPF_K check.
@@ -12898,76 +12961,56 @@ static void reg_set_min_max(struct bpf_reg_state *true_reg,
case BPF_JGE:
case BPF_JGT:
{
- if (is_jmp32) {
- u32 false_umax = opcode == BPF_JGT ? val32 : val32 - 1;
- u32 true_umin = opcode == BPF_JGT ? val32 + 1 : val32;
+ bool neq = (opcode == BPF_JGT);
- false_reg->u32_max_value = min(false_reg->u32_max_value,
- false_umax);
- true_reg->u32_min_value = max(true_reg->u32_min_value,
- true_umin);
+ if (is_jmp32) {
+ reg_dec_u32_max(false_reg, neq ? val32 : val32 - 1);
+ reg_inc_u32_min(true_reg, neq ? val32 + 1 : val32);
} else {
- u64 false_umax = opcode == BPF_JGT ? val : val - 1;
- u64 true_umin = opcode == BPF_JGT ? val + 1 : val;
-
- false_reg->umax_value = min(false_reg->umax_value, false_umax);
- true_reg->umin_value = max(true_reg->umin_value, true_umin);
+ reg_dec_u64_max(false_reg, neq ? val : val - 1);
+ reg_inc_u64_min(true_reg, neq ? val + 1 : val);
}
break;
}
case BPF_JSGE:
case BPF_JSGT:
{
- if (is_jmp32) {
- s32 false_smax = opcode == BPF_JSGT ? sval32 : sval32 - 1;
- s32 true_smin = opcode == BPF_JSGT ? sval32 + 1 : sval32;
+ bool neq = (opcode == BPF_JSGT);
- false_reg->s32_max_value = min(false_reg->s32_max_value, false_smax);
- true_reg->s32_min_value = max(true_reg->s32_min_value, true_smin);
+ if (is_jmp32) {
+ reg_dec_s32_max(false_reg, neq ? sval32 : sval32 - 1);
+ reg_inc_s32_min(true_reg, neq ? sval32 + 1 : sval32);
} else {
- s64 false_smax = opcode == BPF_JSGT ? sval : sval - 1;
- s64 true_smin = opcode == BPF_JSGT ? sval + 1 : sval;
-
- false_reg->smax_value = min(false_reg->smax_value, false_smax);
- true_reg->smin_value = max(true_reg->smin_value, true_smin);
+ reg_dec_s64_max(false_reg, neq ? sval : sval - 1);
+ reg_inc_s64_min(true_reg, neq ? sval + 1 : sval);
}
break;
}
case BPF_JLE:
case BPF_JLT:
{
- if (is_jmp32) {
- u32 false_umin = opcode == BPF_JLT ? val32 : val32 + 1;
- u32 true_umax = opcode == BPF_JLT ? val32 - 1 : val32;
+ bool neq = (opcode == BPF_JLT);
- false_reg->u32_min_value = max(false_reg->u32_min_value,
- false_umin);
- true_reg->u32_max_value = min(true_reg->u32_max_value,
- true_umax);
+ if (is_jmp32) {
+ reg_inc_u32_min(false_reg, neq ? val32 : val32 + 1);
+ reg_dec_u32_max(true_reg, neq ? val32 - 1 : val32);
} else {
- u64 false_umin = opcode == BPF_JLT ? val : val + 1;
- u64 true_umax = opcode == BPF_JLT ? val - 1 : val;
-
- false_reg->umin_value = max(false_reg->umin_value, false_umin);
- true_reg->umax_value = min(true_reg->umax_value, true_umax);
+ reg_inc_u64_min(false_reg, neq ? val : val + 1);
+ reg_dec_u64_max(true_reg, neq ? val - 1 : val);
}
break;
}
case BPF_JSLE:
case BPF_JSLT:
{
- if (is_jmp32) {
- s32 false_smin = opcode == BPF_JSLT ? sval32 : sval32 + 1;
- s32 true_smax = opcode == BPF_JSLT ? sval32 - 1 : sval32;
+ bool neq = (opcode == BPF_JSLT);
- false_reg->s32_min_value = max(false_reg->s32_min_value, false_smin);
- true_reg->s32_max_value = min(true_reg->s32_max_value, true_smax);
+ if (is_jmp32) {
+ reg_inc_s32_min(false_reg, neq ? sval32 : sval32 + 1);
+ reg_dec_s32_max(true_reg, neq ? sval32 - 1 : sval32);
} else {
- s64 false_smin = opcode == BPF_JSLT ? sval : sval + 1;
- s64 true_smax = opcode == BPF_JSLT ? sval - 1 : sval;
-
- false_reg->smin_value = max(false_reg->smin_value, false_smin);
- true_reg->smax_value = min(true_reg->smax_value, true_smax);
+ reg_inc_s64_min(false_reg, neq ? sval : sval + 1);
+ reg_dec_s64_max(true_reg, neq ? sval - 1 : sval);
}
break;
}
--
2.30.2