Messages in this thread |  | | Subject | Re: objtool clac/stac handling change.. | From | Christophe Leroy <> | Date | Thu, 2 Jul 2020 17:13:28 +0200 |
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Le 02/07/2020 à 15:34, Michael Ellerman a écrit : > 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.
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
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