Re: [PATCH] x86/entry/32: Fix setup of CS high bits

[Jan Kiszka]

On 13.10.18 17:12, Andy Lutomirski wrote:
> On Sat, Oct 13, 2018 at 3:02 AM Jan Kiszka <jan.kiszka@xxxxxx> wrote:
> > From: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
> >
> > Even if we are not on an entry stack, we have to initialize the CS high
> > bits because we are unconditionally evaluating them
> > PARANOID_EXIT_TO_KERNEL_MODE. Failing to do so broke the boot on Galileo
> > Gen2 and IOT2000 boards.
> >
> > Fixes: b92a165df17e ("x86/entry/32: Handle Entry from Kernel-Mode on Entry-Stack")
> > Signed-off-by: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
> > ---
> >   arch/x86/entry/entry_32.S | 12 ++++++------
> >   1 file changed, 6 insertions(+), 6 deletions(-)
> >
> > diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
> > index 2767c625a52c..95c94d48ecd2 100644
> > --- a/arch/x86/entry/entry_32.S
> > +++ b/arch/x86/entry/entry_32.S
> > @@ -389,6 +389,12 @@
> >           * that register for the time this macro runs
> >           */
> >
> > +       /*
> > +        * Clear unused upper bits of the dword containing the word-sized CS
> > +        * slot in pt_regs in case hardware didn't clear it for us.
> > +        */
> > +       andl    $(0x0000ffff), PT_CS(%esp)
> > +
>
> Please improve the comment. Since commit:
>
> commit 385eca8f277c4c34f361a4c3a088fd876d29ae21
> Author: Andy Lutomirski <luto@xxxxxxxxxx>
> Date:   Fri Jul 28 06:00:30 2017 -0700
>
>      x86/asm/32: Make pt_regs's segment registers be 16 bits
>
> Those fields are genuinely 16 bit.  So the comment should say
> something like "Those high bits are used for CS_FROM_ENTRY_STACK and
> CS_FROM_USER_CR3".

/*
 * The high bits of the CS dword (__csh) are used for
 * CS_FROM_ENTRY_STACK and CS_FROM_USER_CR3. Clear them in case
 * hardware didn't do this for us.
 */

OK? I will send out v2 with this wording soon.

> Also, can you fold something like this in:
>
> diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
> index 2767c625a52c..358eed8cf62a 100644
> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -171,7 +171,7 @@
>          ALTERNATIVE "jmp .Lend_\@", "", X86_FEATURE_PTI
>          .if \no_user_check == 0
>          /* coming from usermode? */
> -       testl   $SEGMENT_RPL_MASK, PT_CS(%esp)
> +       testb   $SEGMENT_RPL_MASK, PT_CS(%esp)
>          jz      .Lend_\@
>          .endif
>          /* On user-cr3? */

Makes sense, but this looks like a separate patch to me.

Jan

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