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Message-Id: <20200330160324.15259-3-daniel@iogearbox.net>
Date: Mon, 30 Mar 2020 18:03:23 +0200
From: Daniel Borkmann <daniel@...earbox.net>
To: alexei.starovoitov@...il.com
Cc: jannh@...gle.com, yhs@...com, john.fastabend@...il.com,
bpf@...r.kernel.org, netdev@...r.kernel.org,
Daniel Borkmann <daniel@...earbox.net>
Subject: [PATCH bpf-next 2/3] bpf: Fix tnum constraints for 32-bit comparisons
From: Jann Horn <jannh@...gle.com>
The BPF verifier tried to track values based on 32-bit comparisons by
(ab)using the tnum state via 581738a681b6 ("bpf: Provide better register
bounds after jmp32 instructions"). The idea is that after a check like
this:
if ((u32)r0 > 3)
exit
We can't meaningfully constrain the arithmetic-range-based tracking, but
we can update the tnum state to (value=0,mask=0xffff'ffff'0000'0003).
However, the implementation from 581738a681b6 didn't compute the tnum
constraint based on the fixed operand, but instead derives it from the
arithmetic-range-based tracking. This means that after the following
sequence of operations:
if (r0 >= 0x1'0000'0001)
exit
if ((u32)r0 > 7)
exit
The verifier assumed that the lower half of r0 is in the range (0, 0)
and apply the tnum constraint (value=0,mask=0xffff'ffff'0000'0000) thus
causing the overall tnum to be (value=0,mask=0x1'0000'0000), which was
incorrect. Provide a fixed implementation.
Fixes: 581738a681b6 ("bpf: Provide better register bounds after jmp32 instructions")
Signed-off-by: Jann Horn <jannh@...gle.com>
Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
---
kernel/bpf/verifier.c | 108 ++++++++++++++++++++++++++++--------------
1 file changed, 72 insertions(+), 36 deletions(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 2a84f73a93a1..6fce6f096c16 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -5678,6 +5678,70 @@ static bool cmp_val_with_extended_s64(s64 sval, struct bpf_reg_state *reg)
reg->smax_value <= 0 && reg->smin_value >= S32_MIN);
}
+/* Constrain the possible values of @reg with unsigned upper bound @bound.
+ * If @is_exclusive, @bound is an exclusive limit, otherwise it is inclusive.
+ * If @is_jmp32, @bound is a 32-bit value that only constrains the low 32 bits
+ * of @reg.
+ */
+static void set_upper_bound(struct bpf_reg_state *reg, u64 bound, bool is_jmp32,
+ bool is_exclusive)
+{
+ if (is_exclusive) {
+ /* There are no values for `reg` that make `reg<0` true. */
+ if (bound == 0)
+ return;
+ bound--;
+ }
+ if (is_jmp32) {
+ /* Constrain the register's value in the tnum representation.
+ * For 64-bit comparisons this happens later in
+ * __reg_bound_offset(), but for 32-bit comparisons, we can be
+ * more precise than what can be derived from the updated
+ * numeric bounds.
+ */
+ struct tnum t = tnum_range(0, bound);
+
+ t.mask |= ~0xffffffffULL; /* upper half is unknown */
+ reg->var_off = tnum_intersect(reg->var_off, t);
+
+ /* Compute the 64-bit bound from the 32-bit bound. */
+ bound += gen_hi_max(reg->var_off);
+ }
+ reg->umax_value = min(reg->umax_value, bound);
+}
+
+/* Constrain the possible values of @reg with unsigned lower bound @bound.
+ * If @is_exclusive, @bound is an exclusive limit, otherwise it is inclusive.
+ * If @is_jmp32, @bound is a 32-bit value that only constrains the low 32 bits
+ * of @reg.
+ */
+static void set_lower_bound(struct bpf_reg_state *reg, u64 bound, bool is_jmp32,
+ bool is_exclusive)
+{
+ if (is_exclusive) {
+ /* There are no values for `reg` that make `reg>MAX` true. */
+ if (bound == (is_jmp32 ? U32_MAX : U64_MAX))
+ return;
+ bound++;
+ }
+ if (is_jmp32) {
+ /* Constrain the register's value in the tnum representation.
+ * For 64-bit comparisons this happens later in
+ * __reg_bound_offset(), but for 32-bit comparisons, we can be
+ * more precise than what can be derived from the updated
+ * numeric bounds.
+ */
+ struct tnum t = tnum_range(bound, U32_MAX);
+
+ t.mask |= ~0xffffffffULL; /* upper half is unknown */
+ reg->var_off = tnum_intersect(reg->var_off, t);
+
+ /* Compute the 64-bit bound from the 32-bit bound. */
+ bound += gen_hi_min(reg->var_off);
+ }
+ reg->umin_value = max(reg->umin_value, bound);
+}
+
/* 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.
@@ -5733,15 +5797,8 @@ static void reg_set_min_max(struct bpf_reg_state *true_reg,
case BPF_JGE:
case BPF_JGT:
{
- u64 false_umax = opcode == BPF_JGT ? val : val - 1;
- u64 true_umin = opcode == BPF_JGT ? val + 1 : val;
-
- if (is_jmp32) {
- false_umax += gen_hi_max(false_reg->var_off);
- true_umin += gen_hi_min(true_reg->var_off);
- }
- false_reg->umax_value = min(false_reg->umax_value, false_umax);
- true_reg->umin_value = max(true_reg->umin_value, true_umin);
+ set_upper_bound(false_reg, val, is_jmp32, opcode == BPF_JGE);
+ set_lower_bound(true_reg, val, is_jmp32, opcode == BPF_JGT);
break;
}
case BPF_JSGE:
@@ -5762,15 +5819,8 @@ static void reg_set_min_max(struct bpf_reg_state *true_reg,
case BPF_JLE:
case BPF_JLT:
{
- u64 false_umin = opcode == BPF_JLT ? val : val + 1;
- u64 true_umax = opcode == BPF_JLT ? val - 1 : val;
-
- if (is_jmp32) {
- false_umin += gen_hi_min(false_reg->var_off);
- true_umax += gen_hi_max(true_reg->var_off);
- }
- false_reg->umin_value = max(false_reg->umin_value, false_umin);
- true_reg->umax_value = min(true_reg->umax_value, true_umax);
+ set_lower_bound(false_reg, val, is_jmp32, opcode == BPF_JLE);
+ set_upper_bound(true_reg, val, is_jmp32, opcode == BPF_JLT);
break;
}
case BPF_JSLE:
@@ -5845,15 +5895,8 @@ static void reg_set_min_max_inv(struct bpf_reg_state *true_reg,
case BPF_JGE:
case BPF_JGT:
{
- u64 false_umin = opcode == BPF_JGT ? val : val + 1;
- u64 true_umax = opcode == BPF_JGT ? val - 1 : val;
-
- if (is_jmp32) {
- false_umin += gen_hi_min(false_reg->var_off);
- true_umax += gen_hi_max(true_reg->var_off);
- }
- false_reg->umin_value = max(false_reg->umin_value, false_umin);
- true_reg->umax_value = min(true_reg->umax_value, true_umax);
+ set_lower_bound(false_reg, val, is_jmp32, opcode == BPF_JGE);
+ set_upper_bound(true_reg, val, is_jmp32, opcode == BPF_JGT);
break;
}
case BPF_JSGE:
@@ -5871,15 +5914,8 @@ static void reg_set_min_max_inv(struct bpf_reg_state *true_reg,
case BPF_JLE:
case BPF_JLT:
{
- u64 false_umax = opcode == BPF_JLT ? val : val - 1;
- u64 true_umin = opcode == BPF_JLT ? val + 1 : val;
-
- if (is_jmp32) {
- false_umax += gen_hi_max(false_reg->var_off);
- true_umin += gen_hi_min(true_reg->var_off);
- }
- false_reg->umax_value = min(false_reg->umax_value, false_umax);
- true_reg->umin_value = max(true_reg->umin_value, true_umin);
+ set_upper_bound(false_reg, val, is_jmp32, opcode == BPF_JLE);
+ set_lower_bound(true_reg, val, is_jmp32, opcode == BPF_JLT);
break;
}
case BPF_JSLE:
--
2.20.1
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