> On Sep 28, 2020, at 1:52 PM, Nick Desaulniers <ndesaulni...@google.com> wrote: > > On Mon, Sep 28, 2020 at 3:34 AM 'Dmitry Vyukov' via Clang Built Linux > <clang-built-li...@googlegroups.com> wrote: >> >>> On Mon, Sep 28, 2020 at 10:54 AM Borislav Petkov <b...@alien8.de> wrote: >>> >>> On Mon, Sep 28, 2020 at 10:40:19AM +0200, Dmitry Vyukov wrote: >>>> I meant the kernel self-corrupts itself, that just wasn't detected by >>>> KASAN, page protections, etc. >>> >>> Well, Nick already asked this but we're marking all kernel text RO early >>> during boot. So it either is happening before that or something else >>> altogether is going on. > >> On Sun, Sep 27, 2020 at 11:06 PM 'Dmitry Vyukov' via Clang Built Linux >> <clang-built-li...@googlegroups.com> wrote: >> >> Interestingly there is a new crash, which looks similar: >> >> general protection fault in map_vdso >> https://syzkaller.appspot.com/bug?extid=c2ae01c2b1b385384a06 >> >> The code is also with 4 0's: >> Code: 00 00 00 48 b8 00 00 00 00 00 fc ff df 41 57 49 89 ff 41 56 41 >> 55 41 54 55 65 48 8b 2c 25 c0 fe 01 00 48 8d bd 28 04 00 00 53 <48> 00 >> 00 00 00 fa 48 83 ec 10 48 c1 ea 03 80 3c 02 00 0f 85 51 02 >> >> But it happened with gcc. >> >> Also I found this older one: >> general protection fault in map_vdso_randomized >> https://syzkaller.appspot.com/bug?id=8366fd024559946137b9db23b26fd2235d43b383 >> >> which also has code smashed and happened with gcc: >> Code: 00 fc ff df 48 89 f9 48 c1 e9 03 80 3c 01 00 0f 85 eb 00 00 00 >> 65 48 8b 1c 25 c0 fe 01 00 48 8d bb 28 04 00 00 41 2b 54 24 20 <00> 00 >> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 > > If this is related to vdso's, they seem mapped as `r-xp` (not `w): > $ sudo cat /proc/1/maps | grep vdso > 7ffc667f5000-7ffc667f7000 r-xp 00000000 00:00 0 > [vdso] > > map_vdso() in arch/x86/entry/vdso/vma.c doesn't map the VMA as > writable, but it uses VM_MAYWRITE with a comment about GDB setting > breakpoints. > > So it sounds like the page protections on the vdso can be changed at > runtime (via mprotect). Maybe syzkaller is tickling that first? > > map_vdso_randomized() does call map_vdso(). Maybe if we mprotect the > vdso to be writable, it may be easier to spot the write. > >
The kernel shouldn’t be executing the vDSO code. Unless I’ve misread it, Te crash is that the map_vdso() text itself was corrupted. This isn’t the same thing. The VM_MAYWRITE means that a program may CoW the page and write to the copy, which still won’t allow changing the vDSO text or executing it inside the kernel.
