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SubjectRe: [PATCH] x86/entry/32: Fix setup of CS high bits
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On 13.10.18 17:12, Andy Lutomirski wrote:
> On Sat, Oct 13, 2018 at 3:02 AM Jan Kiszka <jan.kiszka@web.de> wrote:
>>
>> From: Jan Kiszka <jan.kiszka@siemens.com>
>>
>> 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@siemens.com>
>> ---
>> 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@kernel.org>
> 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

--
Siemens AG, Corporate Technology, CT RDA IOT SES-DE
Corporate Competence Center Embedded Linux

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