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

[Andy Lutomirski]

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".

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? */