bpf: Simplify alu_limit masking for pointer arithmetic

commit b5871dca250cd391885218b99cc015aca1a51aea upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 393f1209a776ff3d422b4cdf9dfbcbf6660cef49..a83f2feee4263ef88b689b579a38a26485df8822 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2035,16 +2035,16 @@ static int retrieve_ptr_limit(const struct bpf_reg_state *ptr_reg,
        case PTR_TO_STACK:
                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:
@@ -4802,7 +4802,7 @@ 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);