Re: [PATCH 1/6] x86: fix duplicated X86_BUG(9) macro

[Andy Lutomirski]

On Jul 1, 2016 2:23 AM, "Borislav Petkov" <bp@xxxxxxxxx> wrote:
>
> On Thu, Jun 30, 2016 at 05:12:10PM -0700, Dave Hansen wrote:
> >
> > From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> >
> > epufeatures.h currently defines X86_BUG(9) twice on 32-bit:
> >
> >       #define X86_BUG_NULL_SEG        X86_BUG(9) /* Nulling a selector preserves the base */
> >       ...
> >       #ifdef CONFIG_X86_32
> >       #define X86_BUG_ESPFIX          X86_BUG(9) /* "" IRET to 16-bit SS corrupts ESP/RSP high bits */
> >       #endif
> >
> > I think what happened was that this added the X86_BUG_ESPFIX, but
> > in an #ifdef below most of the bugs:
> >
> >       [58a5aac5] x86/entry/32: Introduce and use X86_BUG_ESPFIX instead of paravirt_enabled
> >
> > Then this came along and added X86_BUG_NULL_SEG, but collided
> > with the earlier one that did the bug below the main block
> > defining all the X86_BUG()s.
> >
> >       [7a5d6704] x86/cpu: Probe the behavior of nulling out a segment at boot time
> >
> > Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > Acked-by: Andy Lutomirski <luto@xxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > ---
> >
> >  b/arch/x86/include/asm/cpufeatures.h |    6 ++----
> >  1 file changed, 2 insertions(+), 4 deletions(-)
> >
> > diff -puN arch/x86/include/asm/cpufeatures.h~knl-leak-10-fix-x86-bugs-macros arch/x86/include/asm/cpufeatures.h
> > --- a/arch/x86/include/asm/cpufeatures.h~knl-leak-10-fix-x86-bugs-macros      2016-06-30 17:10:41.215185869 -0700
> > +++ b/arch/x86/include/asm/cpufeatures.h      2016-06-30 17:10:41.218186005 -0700
> > @@ -301,10 +301,6 @@
> >  #define X86_BUG_FXSAVE_LEAK  X86_BUG(6) /* FXSAVE leaks FOP/FIP/FOP */
> >  #define X86_BUG_CLFLUSH_MONITOR      X86_BUG(7) /* AAI65, CLFLUSH required before MONITOR */
> >  #define X86_BUG_SYSRET_SS_ATTRS      X86_BUG(8) /* SYSRET doesn't fix up SS attrs */
> > -#define X86_BUG_NULL_SEG     X86_BUG(9) /* Nulling a selector preserves the base */
> > -#define X86_BUG_SWAPGS_FENCE X86_BUG(10) /* SWAPGS without input dep on GS */
> > -
> > -
> >  #ifdef CONFIG_X86_32
> >  /*
> >   * 64-bit kernels don't use X86_BUG_ESPFIX.  Make the define conditional
>
> So I'd remove the "#ifdef CONFIG_X86_32" ifdeffery too and make that bit
> unconditional - so what, we have enough free bits. But I'd leave the
> comment to still avoid the confusion :)
>

I put the ifdef there to prevent anyone from accidentally using it in
a 64-bit code path, not to save a bit.  We could put in the middle of
the list to make the mistake much less likely to be repeated, I
suppose.