Re: [PATCH v4 7/7] ACPI / x86: introduce acpi_os_readable() support

[Andy Lutomirski]

On Tue, Dec 15, 2015 at 4:25 PM, Zheng, Lv <lv.zheng@xxxxxxxxx> wrote:
> Hi, Andy and Yu
>
>> From: Zheng, Lv
>> Sent: Tuesday, December 15, 2015 4:52 PM
>>
>> Hi,
>>
>> > From: Chen, Yu C
>> > Sent: Tuesday, December 15, 2015 2:13 PM
>> >
>> > Hi, Andy
>> >
>> > > From: Andy Lutomirski [mailto:luto@xxxxxxxxxxxxxx]
>> > > Sent: Tuesday, December 15, 2015 7:28 AM
>> > >
>> > > On Wed, Dec 2, 2015 at 6:43 PM, Lv Zheng <lv.zheng@xxxxxxxxx> wrote:
>> > > > From: Chen Yu <yu.c.chen@xxxxxxxxx>
>> > > >
>> > > > This patch implements acpi_os_readable(). The function is used by
>> > > > ACPICA AML debugger to validate user specified pointers for dumping
>> > > > the memory as ACPICA descriptor objects.
>> > > >
>> > [cut]
>> > > >
>> > > > +bool __acpi_memory_readable(void *pointer, size_t length) {
>> > > > +       unsigned long obj_start, obj_end;
>> > > > +       unsigned long start_pfn, end_pfn;
>> > >
>> > > What does "readable" mean in this context?
>>
>> [Lv Zheng]
>> The function is used by ACPICA "dump" command.
>> It accepts an arbitrary address, and tries to dump the memory block specified
>> by the address as an acpi_object.
>> You can try: "help dump" in the interactive mode to confirm.
>> While acpi_object is actually all created by kmalloc.
> [Lv Zheng]
> This statement might be wrong, Let me Cc Bob to confirm.
>
> Hi, Bob
>
> Do we have statically allocated acpi_objects?
> If we have, we need to change this patch to allow high map .data/.bss segments addresses to pass this check.
>

I think that hpa or Borislav [cc'd] could address the memory map
details better than I could.  However, this functionality seems
strange.

Are these physical addresses or virtual addresses that are being
dumped?  In either case, ISTM that using something iike page_is_ram
might be a lot simpler.

--Andy

> Thanks and best regards
> -Lv
>
>> So we just check if the specified memory block belongs to the kernel heap.
>> The readable/writeable is not so meaningful here as the kernel heap should
>> always be both readable and writeable.
>>
>> We do a lot of checks in this function in order to allow it to:
>> 1. return true if "pointer" belongs to kernel heap when "length" is 0;
>> 2. return false if "pointer" doesn't belong to kernel heap when "length" is 0;
>> 3. return true if "pointer ~ pointer+length-1" belongs to a kernel heap range;
>> 4. return false if "pointer ~ pointer+length-1" doesn't belong to any kernel heap
>> range.
>>
>> These checks are weak, but can help to avoid panics if users specify wrong
>> pointers for the "dump" command.
>>
>> > 'readable' means :  the address provided by the user,
>> > is a dynamically allocated  virtual address -
>> > because  the  acpi address space are allocated by 'kmalloc',
>> > acpi debugger must check if this address is a valid 'kmalloc'
>> > address before accessing it.
>> >
>> > This function does the sanity check that, the vitual address is a:
>> > 1.   dynamically allocated address (beyond PAGE_OFFSET , but lower
>> > than high_memory, VMALLOC_START, eg)
>> > 2.  besides, the physical address must be direct-mapped(so it would not be a
>> > hole).
>>
>> [Lv Zheng]
>> There is a special case (possibly hackish) on x86_64.
>> x86_64 kernel maps kernel image twice.
>> One is called as high map and the other is called as low map.
>>
>> Since we use __pa() to convert a virtual address,
>> If the virtual address belongs to the high map range, __pa() which takes care of
>> converting high map addresses actually returns a physical address where there
>> should also be low map mappings ready for it.
>> Thus the converted PFN from the result of __pa() will be treated as valid.
>>
>> But this doesn't mean there is a high map for this virtual address.
>> x86_64 kernel drops several pages from high map in cleanup_highmap().
>> So accessing a virtual address that belongs to the holes whose page mappings
>> have been dropped in this function could still result in panic due to no mappings.
>> By enforcing this check, we can avoid such a case.
>> Actually no acpi_object's virtual address will belong to high map range.
>>
>> Thanks and best regards
>> -Lv



-- 
Andy Lutomirski
AMA Capital Management, LLC
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