On Thu, Jan 4, 2018 at 9:33 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: > On Thu, Jan 4, 2018 at 12:43 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: >> >>> On Jan 4, 2018, at 12:29 PM, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: >>> >>>> On Thu, Jan 4, 2018 at 12:16 PM, Thomas Voegtle <tv@xxxxxxxx> wrote: >>>> >>>> Attached a screenshot. >>>> Is that useful? Are there some debug options I can add? >>> >>> Not much of an oops, because the SIGSEGV happens in user space. The >>> only reason you get any kernel stack printout at all is because 'init' >>> dying will make the kernel print that out. >>> >>> The segfault address for init looks like the fixmap area to me (first >>> byte in the last page of the fixmap?). "Error 5" means that it's a >>> user-space read that got a protection fault. So it's not a LDT of GDT >>> update or anything like that, it's a normal access from user space (or >>> a qemu emulation bug, but that sounds unlikely). >>> >>> Is that the vsyscall page? >>> >>> Adding Luto to the participants. I think he noticed one of the >>> vsyscall patches missing earlier in the 4.9 series. Maybe the 4.4 >>> series had something similar.. >>> >> >> That's almost certainly it. >> >> I'll try to find some time today or tomorrow to add a proper selftest. >> > > Give this a shot: > > https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/pti&id=17c5ebeb2e00879b0af1a9c32bf37ecdd9b9b31b > > Boot with each of vsyscall=none, vsyscall=native, and vsyscall=emulate > and run both the 32-bit and 64-bit variants of that test. All six > combinations should pass. But I bet they don't on 4.4. With my 4.4.110-rc1 under QEMU -cpu=host (Xeon E5-2690 v3) vsyscall=emulate: # ./test_vsyscall_64 ... [RUN] Checking read access to the vsyscall page [FAIL] We don't have read access, but we should vsyscall=native: # ./test_vsyscall_64 ... [RUN] Checking read access to the vsyscall page [FAIL] We don't have read access, but we should Everything else passes. Note that test_vsyscall_32 warns: # ./test_vsyscall_32 Warning: failed to find getcpu in vDSO ... -Kees -- Kees Cook Pixel Security