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From de31796c052e47c99b1bb342bc70aa826733e862 Mon Sep 17 00:00:00 2001
From: Daniel Borkmann <daniel@iogearbox.net>
Date: Fri, 22 Dec 2017 16:23:11 +0100
Subject: [PATCH] bpf: fix integer overflows
From: Alexei Starovoitov <ast@kernel.org>
[ Upstream commit bb7f0f989ca7de1153bd128a40a71709e339fa03 ]
There were various issues related to the limited size of integers used in
the verifier:
- `off + size` overflow in __check_map_access()
- `off + reg->off` overflow in check_mem_access()
- `off + reg->var_off.value` overflow or 32-bit truncation of
`reg->var_off.value` in check_mem_access()
- 32-bit truncation in check_stack_boundary()
Make sure that any integer math cannot overflow by not allowing
pointer math with large values.
Also reduce the scope of "scalar op scalar" tracking.
CVE: CVE-2017-17854
Upstream-Status: Backport [https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=linux-4.14.y&id=de31796c052e47c99b1bb342bc70aa826733e862]
Fixes: f1174f77b50c ("bpf/verifier: rework value tracking")
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andreas Wellving <andreas.wellving@enea.com>
---
include/linux/bpf_verifier.h | 4 +--
kernel/bpf/verifier.c | 48 ++++++++++++++++++++++++++++++++++++
2 files changed, 50 insertions(+), 2 deletions(-)
diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 5d6de3b57758..73bec75b74c8 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -15,11 +15,11 @@
* In practice this is far bigger than any realistic pointer offset; this limit
* ensures that umax_value + (int)off + (int)size cannot overflow a u64.
*/
-#define BPF_MAX_VAR_OFF (1ULL << 31)
+#define BPF_MAX_VAR_OFF (1 << 29)
/* Maximum variable size permitted for ARG_CONST_SIZE[_OR_ZERO]. This ensures
* that converting umax_value to int cannot overflow.
*/
-#define BPF_MAX_VAR_SIZ INT_MAX
+#define BPF_MAX_VAR_SIZ (1 << 29)
/* Liveness marks, used for registers and spilled-regs (in stack slots).
* Read marks propagate upwards until they find a write mark; they record that
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 5a30eda17c4f..c5ff809e86d0 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1789,6 +1789,41 @@ static bool signed_sub_overflows(s64 a, s64 b)
return res > a;
}
+static bool check_reg_sane_offset(struct bpf_verifier_env *env,
+ const struct bpf_reg_state *reg,
+ enum bpf_reg_type type)
+{
+ bool known = tnum_is_const(reg->var_off);
+ s64 val = reg->var_off.value;
+ s64 smin = reg->smin_value;
+
+ if (known && (val >= BPF_MAX_VAR_OFF || val <= -BPF_MAX_VAR_OFF)) {
+ verbose("math between %s pointer and %lld is not allowed\n",
+ reg_type_str[type], val);
+ return false;
+ }
+
+ if (reg->off >= BPF_MAX_VAR_OFF || reg->off <= -BPF_MAX_VAR_OFF) {
+ verbose("%s pointer offset %d is not allowed\n",
+ reg_type_str[type], reg->off);
+ return false;
+ }
+
+ if (smin == S64_MIN) {
+ verbose("math between %s pointer and register with unbounded min value is not allowed\n",
+ reg_type_str[type]);
+ return false;
+ }
+
+ if (smin >= BPF_MAX_VAR_OFF || smin <= -BPF_MAX_VAR_OFF) {
+ verbose("value %lld makes %s pointer be out of bounds\n",
+ smin, reg_type_str[type]);
+ return false;
+ }
+
+ return true;
+}
+
/* Handles arithmetic on a pointer and a scalar: computes new min/max and var_off.
* Caller should also handle BPF_MOV case separately.
* If we return -EACCES, caller may want to try again treating pointer as a
@@ -1854,6 +1889,10 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
dst_reg->type = ptr_reg->type;
dst_reg->id = ptr_reg->id;
+ if (!check_reg_sane_offset(env, off_reg, ptr_reg->type) ||
+ !check_reg_sane_offset(env, ptr_reg, ptr_reg->type))
+ return -EINVAL;
+
switch (opcode) {
case BPF_ADD:
/* We can take a fixed offset as long as it doesn't overflow
@@ -1984,6 +2023,9 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
return -EACCES;
}
+ if (!check_reg_sane_offset(env, dst_reg, ptr_reg->type))
+ return -EINVAL;
+
__update_reg_bounds(dst_reg);
__reg_deduce_bounds(dst_reg);
__reg_bound_offset(dst_reg);
@@ -2013,6 +2055,12 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
src_known = tnum_is_const(src_reg.var_off);
dst_known = tnum_is_const(dst_reg->var_off);
+ if (!src_known &&
+ opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) {
+ __mark_reg_unknown(dst_reg);
+ return 0;
+ }
+
switch (opcode) {
case BPF_ADD:
if (signed_add_overflows(dst_reg->smin_value, smin_val) ||
--
2.20.1
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