Messages in this thread |  | | From | Michael Ellerman <> | Subject | Re: objtool clac/stac handling change.. | Date | Thu, 02 Jul 2020 23:34:31 +1000 |
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Linus Torvalds <torvalds@linux-foundation.org> writes: > On Wed, Jul 1, 2020 at 12:59 PM Al Viro <viro@zeniv.linux.org.uk> 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.
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.
cheers
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