Re: [PATCH] arm64, vmcoreinfo : Append 'MAX_USER_VA_BITS' and 'MAX_PHYSMEM_BITS' to vmcoreinfo

[James Morse <james.morse@arm.com>]

Hi guys,

(CC: +Steve, +Kristina) "What's the best way of letting user-space know the MMU
config when 52-bit VA and pointer-auth may be in use?"

On 13/02/2019 19:52, Kazuhito Hagio wrote:
> On 2/13/2019 1:22 PM, James Morse wrote:
>> On 13/02/2019 11:15, Dave Young wrote:
>>> On 02/12/19 at 11:03pm, Kazuhito Hagio wrote:
>>>> On 2/12/2019 2:59 PM, Bhupesh Sharma wrote:
>>>>> BTW, in the makedumpfile enablement patch thread for ARMv8.2 LVA
>>>>> (which I sent out for 52-bit User space VA enablement) (see [0]), Kazu
>>>>> mentioned that the changes look necessary.
>>>>>
>>>>> [0]. http://lists.infradead.org/pipermail/kexec/2019-February/022431.html
>>>>
>>>>>>> The increased 'PTRS_PER_PGD' value for such cases needs to be then
>>>>>>> calculated as is done by the underlying kernel
>>
>> Aha! Nothing to do with which-bits-are-pfn in the tables...
>>
>> You need to know if the top level PGD is 512bytes or bigger. As we use a
>> kmem-cache the adjacent data could be some else's page tables.
>>
>> Is this really a problem though? You can't pull the user-space pgd pointers out
>> of no-where, you must have walked some task_struct and struct_mm's to find them.
>> In which case you would have the VMAs on hand to tell you if its in the mapped
>> user range.
>>
>> It would be good to avoid putting something arch-specific in here if we can at
>> all help it.

>>>>>>> (see
>>>>>>> 'arch/arm64/include/asm/pgtable-hwdef.h' for details):
>>>>>>>
>>>>>>> #define PTRS_PER_PGD          (1 << (MAX_USER_VA_BITS - PGDIR_SHIFT))
>>>>
>>>> Yes, this is the reason why makedumpfile needs the MAX_USER_VA_BITS.
>>>> It is used for pgd_index() also in makedumpfile to walk page tables.
>>>>
>>>> /* to find an entry in a page-table-directory */
>>>> #define pgd_index(addr)         (((addr) >> PGDIR_SHIFT) & (PTRS_PER_PGD - 1))
>>>
>>> Since Dave mentioned crash tool does not need it, but crash should also
>>> travel the pg tables.
> 
> The crash utility is always invoked with vmlinux, so it can read the
> vabits_user variable directly from vmcore, but makedumpfile can not.

(This sounds fragile. That symbol's name may change, it may disappear
completely! ... but I guess crash changes with every kernel release anyway)


>>> If this is really necessary it would be good to describe what will
>>> happen without the patch, eg. some user visible error from an actual test etc.
>>
>> Yes please, it would really help if there was a specific example we could discuss.
> 
> With 52-bit user space and 48-bit kernel space configuration,
> makedumpfile will not be able to convert a virtual kernel address
> to a physical address, and fail to capture a dumpfile, because the
> pgd_index() will return a wrong index.

Got it, thanks!
(all this user stuff had me thinking it was user-space you were trying to walk).

Yes, this is because of commit e842dfb5a2d3 ("arm64: mm: Offset TTBR1 to allow
52-bit PTRS_PER_PGD"). The kernel has offset the ttbr1 value, if you try and
walk it without knowing the offset you get junk.

Ideally we tell you the offset with some 'ttbr1_offset=' in vmcoreinfo, but if
the offsetting code disappears, the kernel would still have to provide
'ttbr1_offset=0' for user-space to keep working.

I'd like to find something future-proof that always has an unambiguous meaning,
and isn't a problem if the kernel variable/symbol/kconfig names change.

With pointer-auth in use too you can't guess which bits are address and which
bits are data.

Taking arch-specific to its extreme, we could expose TCR_EL1, but this is a
problem if we ever switch that per task (some new bits may turn up with a new
feature). Some of those bits vary per cpu too, so we'd have to mask them out in
case user-space tries to conclude something from them.


My current best suggestion is to export:
from core code:
* USER_MMAP_END, the maximum value a user-space can try and mmap().
This would normally be TASK_SIZE, but x86 and powerpc also have support for
larger VA space, and its plumbed into mm slightly differently. We should have
one arch-independent property that covers all these. On arm64 this would be the
runtime va bits for user-space's TTBR. (This assumes the value isn't per-task)

arch specific:
* ARM64_TCR.T1SZ, the va bits mapped by the kernel's TTBR. (We can assume we'll
never flip user/kernel space). This has to be arch specific, it will always have
a value and its meaning comes from the ARM-ARM (so linux can't change it in the
future). It should be the same on every CPU.
* ARM64_TTBR1.BADDR, the pa of the kernel page tables, which implicitly has the
offset. Again this always has a value, and its meaning comes from the ARM-ARM.
If we ever get clever with different page-tables/TCR values on different CPUs,
these two should come from the same CPU.


I think this gives you what you need if user/kernel may both be using
pointer-auth and both may be using 52-bit va. I'm pretty sure the 48:52 bits can
be picked at boot time depending on the kernel kconfig and the hardware support.

Does anyone have a better idea? (or a corner where this won't work?)


Thanks,

James

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