Re: [git pull] uaccess-related bits of vfs.git

Brian Gerst
2017-05-13T13:47:02Z

On Sat, May 13, 2017 at 8:05 AM, Adam Borowski <kilobyte@angband.pl> wrote:
> On Sat, May 13, 2017 at 07:57:45AM +0100, Al Viro wrote:
>> On Fri, May 12, 2017 at 06:00:44PM -0700, Linus Torvalds wrote:
>> > But the *first* thing I'd like to do would be to just get rid of
>> > __get_user/__put_user as a thing. It really does generate nasty code,
>> > and we might as well just make it do that function call.
>>
>> But IMO the first step is not to convert __get_user()/__put_user() to
>> aliases of get_user()/put_user() - it's to get rid of their infestations
>> outside of arch/*. They are concentrated in relatively few places, so
>> we can deal with those individually and then just fucking ban their use
>> outside of arch/*. That's easy enough to watch for - simple git grep will do,
>> so anything creeping back will be immediately spotted.
>
> As someone from the peanuts gallery, I took a look for __put_user() in my
> usual haunt, drivers/tty/vt/
>
> * use 1: con_[gs]et_trans_*():
> Copies a linear array of 256 bytes/shorts, one by one.
> The obvious patch has 9 insertions(+), 22 deletions(-).
>
> * use 2: con_[gs]et_unimap():
> Ditto, up to 65535*2 shorts, also in a nice linear array.
>
> * use 3: tioclinux():
> Does a __put into a place that was checked only for read. This got me
> frightened as it initially looked like something that can allow an user to
> write where they shouldn't. Fortunately, it turns out the first argument
> to access_ok() is ignored on every single architecture -- why does it even
> exist then? I imagined it's there for some odd arch that allows writes
> when in privileged mode, but unlike what the docs say, VERIFY_WRITE is
> exactly same as VERIFY_READ.

The original i386 (no longer supported) ignored the write-protect bit
when in supervisor mode.

--
Brian Gerst