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bpf: Simplify alu_limit masking for pointer arithmetic 

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>
This commit is contained in:
Piotr Krysiuk 2021-03-16 08:26:25 +01:00 committed by Daniel Borkmann
parent 10d2bb2e6b
commit b5871dca25
1 changed files with 5 additions and 5 deletions
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10
kernel/bpf/verifier.c
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@ -5870,16 +5870,16 @@ 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:
@ -11668,7 +11668,7 @@ 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);