Re: [syzbot] BUG: unable to handle kernel paging request in get_desc

[Sean Christopherson]

On Fri, Nov 04, 2022, Dmitry Vyukov wrote:
> On Fri, 4 Nov 2022 at 08:28, 'Sean Christopherson' via syzkaller-bugs
> <syzkaller-bugs@xxxxxxxxxxxxxxxx> wrote:
> >
> > On Fri, Nov 04, 2022, syzbot wrote:
> > > Hello,
> > >
> > > syzbot found the following issue on:
> > >
> > > HEAD commit:    81214a573d19 Add linux-next specific files for 20221103
> > > git tree:       linux-next
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=132019de880000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=cdc625e9234bac0
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=ffb4f000dc2872c93f62
> > > compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12dd52ca880000
> > >
> > > Downloadable assets:
> > > disk image: https://storage.googleapis.com/syzbot-assets/5d4dda497754/disk-81214a57.raw.xz
> > > vmlinux: https://storage.googleapis.com/syzbot-assets/9658efff160a/vmlinux-81214a57.xz
> > > kernel image: https://storage.googleapis.com/syzbot-assets/3711180f2565/bzImage-81214a57.xz
> > >
> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: syzbot+ffb4f000dc2872c93f62@xxxxxxxxxxxxxxxxxxxxxxxxx
> > >
> > > BUG: unable to handle page fault for address: fffffbc5a1c22e00
> > > #PF: supervisor read access in kernel mode
> > > #PF: error_code(0x0000) - not-present page
> > > PGD 23ffe4067 P4D 23ffe4067 PUD 13ff2d067 PMD 13ff2c067 PTE 0
> > > Oops: 0000 [#1] PREEMPT SMP KASAN
> > > CPU: 0 PID: 5368 Comm: syz-executor.2 Not tainted 6.1.0-rc3-next-20221103-syzkaller #0
> > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/11/2022
> > > RIP: 0010:get_desc+0x128/0x460 arch/x86/lib/insn-eval.c:660
> >
> > I'm pretty sure this is the same thing as
> >
> >   BUG: unable to handle kernel paging request in vmx_handle_exit_irqoff
> >
> > I'll verify and get a patch posted shortly.
> 
> This repro does not create any VMs, it's just:
> 
> iopl(0x3)
> rt_sigreturn()
> 
> Do you still think it's related to the vmx_handle_exit_irqoff issue?

Yes, the issue is that the shadow for the read-only IDT mapping in the CPU entry
area isn't populated (commit 9fd429c28073 ("x86/kasan: Map shadow for percpu pages
on demand") is to blame).  The bug manifests anytime software manually does an IDT
lookup.