Re: [PATCH 4/4] uprobe: only do FOLL_SPLIT_PMD for uprobe register

[Song Liu <songliubraving@fb.com>]

> On Oct 16, 2019, at 5:10 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> 
> On 10/16, Song Liu wrote:
>> 
>> --- a/kernel/events/uprobes.c
>> +++ b/kernel/events/uprobes.c
>> @@ -474,14 +474,17 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
>> 	struct vm_area_struct *vma;
>> 	int ret, is_register, ref_ctr_updated = 0;
>> 	bool orig_page_huge = false;
>> +	unsigned int gup_flags = FOLL_FORCE;
>> 
>> 	is_register = is_swbp_insn(&opcode);
>> 	uprobe = container_of(auprobe, struct uprobe, arch);
>> 
>> retry:
>> +	if (is_register)
>> +		gup_flags |= FOLL_SPLIT_PMD;
>> 	/* Read the page with vaddr into memory */
>> -	ret = get_user_pages_remote(NULL, mm, vaddr, 1,
>> -			FOLL_FORCE | FOLL_SPLIT_PMD, &old_page, &vma, NULL);
>> +	ret = get_user_pages_remote(NULL, mm, vaddr, 1, gup_flags,
>> +				    &old_page, &vma, NULL);
>> 	if (ret <= 0)
>> 		return ret;
>> 
>> @@ -489,6 +492,9 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
>> 	if (ret <= 0)
>> 		goto put_old;
>> 
>> +	WARN(!is_register && PageCompound(old_page),
>> +	     "uprobe unregister should never work on compound page\n");
> 
> But this can happen with the change above. You can't know if *vaddr was
> previously changed by install_breakpoint() or not.

> If not, verify_opcode() should likely save us, but we can't rely on it.
> Say, someone can write "int3" into vm_file at uprobe->offset.

I think this won't really happen. With is_register == false, we already 
know opcode is not "int3", so current call must be from set_orig_insn(). 
Therefore, old_page must be installed by uprobe, and cannot be compound.

The other way is not guaranteed. With is_register == true, it is still
possible current call is from set_orig_insn(). However, we do not rely
on this path. 

Does this make sense? Or did I miss anything?

> 
> And I am not sure it is safe to continue in this case, I'd suggest to
> return -EWHATEVER to avoid the possible crash.

I think we can return -ESOMETHING here to be safe. However, if the 
analysis above makes sense, it is not necessary. 

Thanks,
Song