bpf: restrict unknown scalars of mixed signed bounds for unprivileged

[ commit 9d7eceede769f90b66cfa06ad5b357140d5141ed upstream ]

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 unprivileged 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 <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>

kernel/bpf/verifier.c

@@ -2712,8 +2712,8 @@ 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];
 
@@ -2749,6 +2749,12 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
 			dst);
 		return -EACCES;
 	}
+	if (ptr_reg->type == PTR_TO_MAP_VALUE &&
+	    !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;
+	}
 
 	/* In case of 'scalar += pointer', dst_reg inherits pointer type and id.
 	 * The id may be overwritten later if we create a new variable offset.