Re: objtool clac/stac handling change..

[Christophe Leroy]

Le 02/07/2020 Ã 15:34, Michael Ellerman a ÃcritÂ:
> Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes:
> > On Wed, Jul 1, 2020 at 12:59 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> > > On Wed, Jul 01, 2020 at 12:04:36PM -0700, Linus Torvalds wrote:
> > > > That's actually for the access granting. Shutting the access down ends
> > > > up always doing the same thing anyway..
> > >
> > > #define user_read_access_end            prevent_current_read_from_user
> > > #define user_write_access_end           prevent_current_write_to_user
> > > static inline void prevent_current_read_from_user(void)
> > > {
> > >          prevent_user_access(NULL, NULL, ~0UL, KUAP_CURRENT_READ);
> > > }
> > >
> > > static inline void prevent_current_write_to_user(void)
> > > {
> > >          prevent_user_access(NULL, NULL, ~0UL, KUAP_CURRENT_WRITE);
> > > }
> > >
> > > and prevent_user_access() has instances that do care about the direction...
> >
> > Go and look closer.
> >
> > There are three cases:
> >
> >   (a) the 32-bit book3s case. It looks like it cares, but when you look
> > closer, it ends up not caring about the read side, and saving the
> > "which address to I allow user writes to" in current->thread.kuap
> >
> >   (b) the nohash 32-bit case - doesn't care
> >
> >   (c) the 64-bit books case - doesn't care
> >
> > So yes, in the (a) case it does make a difference between reads and
> > writes, but at least as far as I can tell, it ignores the read case,
> > and has code to avoid the unnecessary "disable user writes" case when
> > there was only a read enable done.
>
> Yeah that's my understanding too.
>
> Christophe is the expert on that code so I'll defer to him if I'm wrong.
>
> > Now, it's possible that I'm wrong, but the upshot of that is that even
> > on powerpc, I think that if we just made the rule be that "taking a
> > user exception should automatically do the 'user_access_end()' for us"
> > is trivial.
>
> I think we can do something to make it work.
>
> We don't have an equivalent of x86's ex_handler_uaccess(), so it's not
> quite as easy as whacking a user_access_end() in there.

Isn't it something easy to do in bad_page_fault() ?

Not exactly a call to user_access_end() but altering regs->kuap so that 
user access is not restored on exception exit.

> Probably the simplest option for us is to just handle it in our
> unsafe_op_wrap(). I'll try and come up with something tomorrow.

unsafe_op_wrap() is not used anymore for unsafe_put_user() as we are now 
using asm goto.

Christophe