Re: [PATCH] optee: remove address tag in check_mem_type()

[Volodymyr Babchuk]

Hello Jens,

Jens Wiklander writes:

> On Mon, Oct 12, 2020 at 11:26 AM Volodymyr Babchuk
> <Volodymyr_Babchuk@xxxxxxxx> wrote:
>>
>> Before passing 'start' to find_vma() we need to remove
>> tags from it to get sane results.
>>
>> Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>
>> ---
>>  drivers/tee/optee/call.c | 2 ++
>>  1 file changed, 2 insertions(+)
>
> Would you mind giving a bit more background to this? For example in
> which contexts this function does or doesn't work as expected? Do you
> have any special use cases that don't work, etc? This is not a new
> regression, it's rather a problem we've always had, right?

Yes, sorry. I had to clarify in the commit description. Issue was found
on Android. Android uses pointer tagging [1], so MSB of user pointers
contain tags. As a result, passing raw user address to find_vma() leads
to NULL result, as it only traverses RB tree and does not alter passed
address in any way.

Code in mm/gup.c already strips tags and maybe, it is better to call
untagged_addr() inside of find_vma(). I'm not sure. Probably, we need
some help from MM maintainers.

Anyways, this patched fixed issue with register_shm failing in our use
case.

[1] https://source.android.com/devices/tech/debug/tagged-pointers


-- 
Volodymyr Babchuk at EPAM