bpf: Simplify alu_limit masking for pointer arithmetic

BugLink: https://bugs.launchpad.net/bugs/1921710
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>
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index c5b596f41adae2b5f6dafec7587e94db8f2acff5..78abf6cc6a1abc171dbeca2cf76b93662da911c8 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -5398,16 +5398,16 @@ 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:
@@ -11083,7 +11083,7 @@ 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);