[PATCH v2 1/2] arm64: ftrace: don't validate branch via PLT in ftrace_make_nop()

[Ard Biesheuvel]

On 5 June 2017 at 17:15, Will Deacon <will.deacon at arm.com> wrote:
> On Tue, May 30, 2017 at 01:52:19PM +0000, Ard Biesheuvel wrote:
>> When turning branch instructions into NOPs, we attempt to validate the
>> action by comparing the old value at the call site with the opcode of
>> a direct relative branch instruction pointing at the old target.
>>
>> However, these call sites are statically initialized to call _mcount(),
>> and may be redirected via a PLT entry if the module is loaded far away
>> from the kernel text, leading to false negatives and spurious errors.
>>
>> So skip the validation if CONFIG_ARM64_MODULE_PLTS is configured.
>>
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
>> ---
>>  arch/arm64/kernel/ftrace.c | 9 ++++++---
>>  1 file changed, 6 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
>> index 40ad08ac569a..a8db6857cad6 100644
>> --- a/arch/arm64/kernel/ftrace.c
>> +++ b/arch/arm64/kernel/ftrace.c
>> @@ -84,12 +84,15 @@ int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
>>                   unsigned long addr)
>>  {
>>       unsigned long pc = rec->ip;
>> -     u32 old, new;
>> +     u32 old = 0, new;
>>
>> -     old = aarch64_insn_gen_branch_imm(pc, addr, AARCH64_INSN_BRANCH_LINK);
>> +     if (!IS_ENABLED(CONFIG_ARM64_MODULE_PLTS))
>> +             old = aarch64_insn_gen_branch_imm(pc, addr,
>> +                                               AARCH64_INSN_BRANCH_LINK);
>>       new = aarch64_insn_gen_nop();
>>
>> -     return ftrace_modify_code(pc, old, new, true);
>> +     return ftrace_modify_code(pc, old, new,
>> +                               !IS_ENABLED(CONFIG_ARM64_MODULE_PLTS));
>
> Even in the case of PLTs, the instruction is still a branch instruction,
> so could we validate the basic opcode fields rather than disable the check
> altogether?
>

That shouldn't be too hard.