linux-kernel - Re: [PATCH] optee: remove address tag in check_mem

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Date:   Mon, 12 Oct 2020 12:24:32 +0000
From:   Volodymyr Babchuk <Volodymyr_Babchuk@...m.com>
To:     Jens Wiklander <jens.wiklander@...aro.org>
CC:     "tee-dev@...ts.linaro.org" <tee-dev@...ts.linaro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] optee: remove address tag in check_mem_type()

Hello Jens,

Jens Wiklander writes:

> On Mon, Oct 12, 2020 at 11:26 AM Volodymyr Babchuk
> <Volodymyr_Babchuk@...m.com> 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@...m.com>
>> ---
>>  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