[PATCH bpf 6/9] bpf: restrict unknown scalars of mixed signed bounds for unpriviledged

[Daniel Borkmann]

For unknown scalars of mixed signed bounds, meaning their smin_value is
negative and their smax_value is positive, we need to reject arithmetic
with pointer to map value. For unpriviledged the goal is to mask every
map pointer arithmetic and this cannot reliably be done when it is
unknown at verification time whether the scalar value is negative or
positive. Given this is a corner case, the likelihood of breaking should
be very small.

Signed-off-by: Daniel Borkmann <dan...@iogearbox.net>
Acked-by: Alexei Starovoitov <a...@kernel.org>
---
 kernel/bpf/verifier.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 9ac205d..eebbc03 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -3081,8 +3081,8 @@ static int adjust_ptr_min_max_vals(struct 
bpf_verifier_env *env,
            smin_ptr = ptr_reg->smin_value, smax_ptr = ptr_reg->smax_value;
        u64 umin_val = off_reg->umin_value, umax_val = off_reg->umax_value,
            umin_ptr = ptr_reg->umin_value, umax_ptr = ptr_reg->umax_value;
+       u32 dst = insn->dst_reg, src = insn->src_reg;
        u8 opcode = BPF_OP(insn->code);
-       u32 dst = insn->dst_reg;
 
        dst_reg = &regs[dst];
 
@@ -3115,6 +3115,13 @@ static int adjust_ptr_min_max_vals(struct 
bpf_verifier_env *env,
                verbose(env, "R%d pointer arithmetic on %s prohibited\n",
                        dst, reg_type_str[ptr_reg->type]);
                return -EACCES;
+       case PTR_TO_MAP_VALUE:
+               if (!env->allow_ptr_leaks && !known && (smin_val < 0) != 
(smax_val < 0)) {
+                       verbose(env, "R%d has unknown scalar with mixed signed 
bounds, pointer arithmetic with it prohibited for !root\n",
+                               off_reg == dst_reg ? dst : src);
+                       return -EACCES;
+               }
+               /* fall-through */
        default:
                break;
        }
-- 
2.9.5