Re: [tip: x86/entry] x86/entry: Convert Divide Error to IDTENTRY

[Dmitry Vyukov]

On Mon, Oct 12, 2020 at 10:30 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> On Sun, Oct 11, 2020 at 05:25:22PM +0200, Dmitry Vyukov wrote:
> > On Tue, May 19, 2020 at 9:59 PM tip-bot2 for Thomas Gleixner
> > <tip-bot2@xxxxxxxxxxxxx> wrote:
> > >
> > > The following commit has been merged into the x86/entry branch of tip:
> > >
> > > -DO_ERROR(X86_TRAP_DE,     SIGFPE,  FPE_INTDIV,   IP, "divide error",        divide_error)
> > >
> > > +DEFINE_IDTENTRY(exc_divide_error)
> > > +{
> > > +       do_error_trap(regs, 0, "divide_error", X86_TRAP_DE, SIGFPE,
> > > +                     FPE_INTDIV, error_get_trap_addr(regs));
> > > +}
> >
> > I suppose this is a copy-paste typo and was supposed to be "divide
> > error", right?
> > Otherwise it changes how kernel oopses look like and breaks syzkaller
> > crash parsing, and probably of every other kernel testing system that
> > looks for kernel crashes.
> >
> > syzkaller now says just the following for divide errors, without
> > attribution to function/file/maintainers:
> >
> > kernel panic: Fatal exception (3)
> > FS:  0000000000000000(0000) GS:ffff8880ae500000(0000) knlGS:0000000000000000
> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 00000000004c9428 CR3: 0000000009e8d000 CR4: 00000000001506e0
> > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > Kernel panic - not syncing: Fatal exception in interrupt
> > Kernel Offset: disabled
> > Rebooting in 86400 seconds..
> >
> > I will fix it up in syzkaller. It is now required anyway since this
> > new crash mode is in git history, so needed for bisection and testing
> > of older releases.
> >
> > It is not the first time kernel crash output changes
> > intentionally/unintentionally breaking kernel testing.
> > But I wonder if LKDTM can be turned into actual executable tests that
> > produce pass/fail and fix crash output for different oopses?
> > Marco, you implemented some "output tests" for KCSAN. Can that be
> > extended to other crash types? With some KUnit help? However, I am not
> > sure about hard panics, they may not play well with unit-testing...
>
> A lot of the behavioral tests in LKDTM end up triggering arch-specific
> logging. I decided to avoid trying to consolidate it in favor of
> actually getting the test coverage. :)

What do you mean? Some of these tests would need to be arch-specific,
yes. But I think this is mostly orthogonal to making them PASS/FAIL
tests that can be used during testing.

FTR, syzkaller fix for crash format:
https://github.com/google/syzkaller/commit/c9f222e7b410a336e436f094e24f6465d1cfdc13