christophe leroy <christophe.le...@c-s.fr> writes: > Le 05/06/2017 à 12:45, Michael Ellerman a écrit : >> Christophe LEROY <christophe.le...@c-s.fr> writes: >> >>> Le 02/06/2017 à 11:26, Michael Ellerman a écrit : >>>> Christophe Leroy <christophe.le...@c-s.fr> writes: >>>> >>>>> Only the get_user() in store_updates_sp() has to be done outside >>>>> the mm semaphore. All the comparison can be done within the semaphore, >>>>> so only when really needed. >>>>> >>>>> As we got a DSI exception, the address pointed by regs->nip is >>>>> obviously valid, otherwise we would have had a instruction exception. >>>>> So __get_user() can be used instead of get_user() >>>> >>>> I don't think that part is true. >>>> >>>> You took a DSI so there *was* an instruction at NIP, but since then it >>>> may have been unmapped by another thread. >>>> >>>> So I don't think you can assume the get_user() will succeed. >>> >>> The difference between get_user() and __get_user() is that get_user() >>> performs an access_ok() in addition. >>> >>> Doesn't access_ok() only checks whether addr is below TASK_SIZE to >>> ensure it is a valid user address ? >> >> Yeah more or less, via some gross macros. >> >> I was actually not that worried about the switch from get_user() to >> __get_user(), but rather that you removed the check of the return value. >> ie. >> >> - if (get_user(inst, (unsigned int __user *)regs->nip)) >> - return 0; >> >> Became: >> >> if (is_write && user_mode(regs)) >> - store_update_sp = store_updates_sp(regs); >> + __get_user(inst, (unsigned int __user *)regs->nip); >> >> >> I think dropping the access_ok() probably is alright, because the NIP >> must (should!) have been in userspace, though as Ben says it's always >> good to be paranoid. >> >> But ignoring that the address can fault at all is wrong AFAICS. > > I see what you mean now. > > Indeed, > > - unsigned int inst; > > Became > > + unsigned int inst = 0; > > Since __get_user() doesn't modify 'inst' in case of error, 'inst' > remains 0, and store_updates_sp(0) return false. That was the idea behind.
Ugh. OK, my bad. Though it is a little subtle. How about: @@ -286,10 +290,13 @@ int do_page_fault(struct pt_regs *regs, unsigned long address, /* * We want to do this outside mmap_sem, because reading code around nip * can result in fault, which will cause a deadlock when called with - * mmap_sem held + * mmap_sem held. We don't need to check if get_user() fails, if it does + * it won't modify inst, and an inst of 0 will return false from + * store_updates_sp(). */ + inst = 0; if (is_write && is_user) - store_update_sp = store_updates_sp(regs); + get_user(inst, (unsigned int __user *)regs->nip); if (is_user) flags |= FAULT_FLAG_USER; Then this one can go in. cheers