Re: [PATCHv14 01/17] x86/mm: Rework address range check in get_user() and put_user()

[Linus Torvalds]

On Wed, Jan 18, 2023 at 8:49 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> > We'll still return -EFAULT, of course, we're just getting rid of the
> >
> >         WARN_ONCE(trapnr == X86_TRAP_GP,
> >                 "General protection fault in user access.
> > Non-canonical address?");
> >
> > issue that comes from not being so exact about the address limit any more.
>
> Ah indeed, so for !LAM we'd now print the message were we would not
> before (the whole TASK_SIZE_MAX+ range).

Yeah.

We could just remove that warning entirely, but it has been useful for
syzbot catching random user addresses that weren't caught by
"access_ok()" when people did bad bad things (ie using the
non-checking "__copy_from_user()" and friends).

I'm not sure how much that warning is worth any more - and for
get_user() and put_user() itself it buys us nothing, since by
definition _those_ do the range checking. Christoph getting rid of the
set_fs() model simplified a lot of our user address checking.

But I think it's easier to just keep that existing warning about "how
did you get a non-canonical address here" for other user accesses, and
just make get/put_user() use that _ASM_EXTABLE() version that doesn't
do it.

               Linus