bpf: Simplify alu_limit masking for pointer arithmetic

Origin: https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/commit?id=b5871dca250cd391885218b99cc015aca1a51aea

Instead of having the mov32 with aux->alu_limit - 1 immediate, move this
operation to retrieve_ptr_limit() instead to simplify the logic and to
allow for subsequent sanity boundary checks inside retrieve_ptr_limit().
This avoids in future that at the time of the verifier masking rewrite
we'd run into an underflow which would not sign extend due to the nature
of mov32 instruction.

Signed-off-by: Piotr Krysiuk <piotras@gmail.com>
Co-developed-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Gbp-Pq: Topic bugfix/all
Gbp-Pq: Name bpf-Simplify-alu_limit-masking-for-pointer-arithmeti.patch

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index be716ef63986e9223ad33a6e33eaf359bc7172d6..e6b238a5e785dff992cf1fb6bb743c6dd747e466 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -5342,16 +5342,16 @@ static int retrieve_ptr_limit(const struct bpf_reg_state *ptr_reg,
                 */
                off = ptr_reg->off + ptr_reg->var_off.value;
                if (mask_to_left)
-                       *ptr_limit = MAX_BPF_STACK + off + 1;
+                       *ptr_limit = MAX_BPF_STACK + off;
                else
-                       *ptr_limit = -off;
+                       *ptr_limit = -off - 1;
                return 0;
        case PTR_TO_MAP_VALUE:
                if (mask_to_left) {
-                       *ptr_limit = ptr_reg->umax_value + ptr_reg->off + 1;
+                       *ptr_limit = ptr_reg->umax_value + ptr_reg->off;
                } else {
                        off = ptr_reg->smin_value + ptr_reg->off;
-                       *ptr_limit = ptr_reg->map_ptr->value_size - off;
+                       *ptr_limit = ptr_reg->map_ptr->value_size - off - 1;
                }
                return 0;
        default:
@@ -10946,7 +10946,7 @@ static int fixup_bpf_calls(struct bpf_verifier_env *env)
                        off_reg = issrc ? insn->src_reg : insn->dst_reg;
                        if (isneg)
                                *patch++ = BPF_ALU64_IMM(BPF_MUL, off_reg, -1);
-                       *patch++ = BPF_MOV32_IMM(BPF_REG_AX, aux->alu_limit - 1);
+                       *patch++ = BPF_MOV32_IMM(BPF_REG_AX, aux->alu_limit);
                        *patch++ = BPF_ALU64_REG(BPF_SUB, BPF_REG_AX, off_reg);
                        *patch++ = BPF_ALU64_REG(BPF_OR, BPF_REG_AX, off_reg);
                        *patch++ = BPF_ALU64_IMM(BPF_NEG, BPF_REG_AX, 0);