Re: [RFC][PATCH] ftrace/x86: Emulate call function while updating in breakpoint handler

[Peter Zijlstra]

On Wed, May 01, 2019 at 12:03:52PM -0700, Linus Torvalds wrote:
> On Wed, May 1, 2019 at 6:11 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > Here goes, compile tested only...
> 
> Ugh, two different threads. This has the same bug (same source) as the
> one Steven posted:

This is what Steve started from; lets continue in the other thread.

> > --- a/arch/x86/entry/entry_32.S
> > +++ b/arch/x86/entry/entry_32.S
> > @@ -1479,6 +1479,13 @@ ENTRY(int3)
> >         ASM_CLAC
> >         pushl   $-1                             # mark this as an int
> >
> > +       testl   $SEGMENT_RPL_MASK, PT_CS(%esp)
> > +       jnz     .Lfrom_usermode_no_gap
> > +       .rept 6
> > +       pushl   5*4(%esp)
> > +       .endr
> > +.Lfrom_usermode_no_gap:
> 
> This will corrupt things horribly if you still use vm86 mode. Checking
> CS RPL is simply not correct.

I'll go fix; I never really understood that vm86 crud and I cobbled this
32bit thing together based on the 64bit version (that Josh did a while
ago).