bpf: fix integer overflows

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.

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>

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index c561b986bab0ebf886000ea34e377ea789138a3b..1632bb13ad8aed8cfeba2ccc69cfa458d02540bb 100644 (file)
--- 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 982bd9ec721ae5fa9af11ddb534e4b78aefcef2d..86dfe6b5c2436c4dc608feebb4fe62e56039e604 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1819,6 +1819,41 @@ 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(env, "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(env, "%s pointer offset %d is not allowed\n",
+                       reg_type_str[type], reg->off);
+               return false;
+       }
+
+       if (smin == S64_MIN) {
+               verbose(env, "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(env, "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
@@ -1887,6 +1922,10 @@ 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
@@ -2017,6 +2056,9 @@ 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);
@@ -2046,6 +2088,12 @@ 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) ||