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

[Linus Torvalds]

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:

> --- 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.

                 Linus