> On Aug 21, 2017, at 3:33 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > >> On Thu, Aug 17, 2017 at 08:52:38AM -0700, Andy Lutomirski wrote: >> On Thu, Aug 17, 2017 at 3:35 AM, Will Deacon <will.deacon@xxxxxxx> wrote: > >>> I'm still concerned that we're treating perf specially here -- are we >>> absolutely sure that nobody else is going to attempt user accesses off the >>> back of an interrupt? >> >> Reasonably sure? If nothing else, an interrupt taken while mmap_sem() >> is held for write that tries to access user memory is asking for >> serious trouble. There are still a few callers of pagefault_disable() >> and copy...inatomic(), though. > > I'm not immediately seeing how holding mmap_sem for writing is a > problem. > >>> If not, then I'd much prefer a solution that catches >>> anybody doing that with the EFI page table installed, rather than trying >>> to play whack-a-mole like this. >> >> Using a kernel thread solves the problem for real. Anything that >> blindly accesses user memory in kernel thread context is terminally >> broken no matter what. > > So perf-callchain doesn't do it 'blindly', it wants either: > > - user_mode(regs) true, or > - task_pt_regs() set. > > However I'm thinking that if the kernel thread has ->mm == &efi_mm, the > EFI code running could very well have user_mode(regs) being true. > > intel_pmu_pebs_fixup() OTOH 'blindly' assumes that the LBR addresses are > accessible. It bails on error though. So while its careful, it does > attempt to access the 'user' mapping directly. Which should also trigger > with the EFI code. > > And I'm not seeing anything particularly broken with either. The PEBS > fixup relies on the CPU having just executed the code, and if it could > fetch and execute the code, why shouldn't it be able to fetch and read? There are two ways this could be a problem. One is that u privileged user apps shouldn't be able to read from EFI memory. The other is that, if EFI were to have IO memory mapped at a "user" address, perf could end up reading it. > (eXecute implies Read assumed). And like said, it if triggers a fault, > it bails, no worries. > > It really doesn't care if the task is a kernel thread or not. Same for > the unwinder, if we get an interrupt register set that points into > 'userspace' we try and unwind it. -- To unsubscribe from this list: send the line "unsubscribe linux-efi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
