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From:   Brian Gerst <brgerst@gmail.com>
Date:   Mon, 5 Mar 2018 09:51:29 -0500
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Subject: Re: [PATCH 07/34] x86/entry/32: Restore segments before int registers
To:     Joerg Roedel <joro@8bytes.org>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, Peter Anvin <hpa@zytor.com>,
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        Andrea Arcangeli <aarcange@redhat.com>,
        Waiman Long <llong@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Joerg Roedel <jroedel@suse.de>
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On Mon, Mar 5, 2018 at 8:12 AM, Joerg Roedel <joro@8bytes.org> wrote:
> On Mon, Mar 05, 2018 at 04:17:45AM -0800, Linus Torvalds wrote:
>>     Restoring the segments can cause exceptions that need to be
>>     handled. With PTI enabled, we still need to be on kernel cr3
>>     when the exception happens. For the cr3-switch we need
>>     at least one integer scratch register, so we can't switch
>>     with the user integer registers already loaded.
>>
>>
>> This fundamentally seems wrong.
>
> Okay, right, with v3 it is wrong, in v2 I still thought I could get away
> without remembering the entry-cr3, but didn't think about the #DB case
> then.
>
> In v3 I added code which remembers the entry-cr3 and handles the
> entry-from-kernel-mode-with-user-cr3 case for all exceptions including
> #DB.
>
>> The things is, we *know* that we will restore two segment registers with the
>> user cr3 already loaded: CS and SS get restored with the final iret.
>
> Yeah, I know, but the iret-exception path is fine because it will
> deliver a SIGILL and doesn't return to the faulting iret.
>
> Anyway, I will remove these restore-reorderings, they are not needed
> anymore.
>
>> So has this been tested with
>>
>>  - single-stepping through sysenter
>>
>>    This takes a DB fault in the first kernel instruction. We're in kernel mode,
>> but with user cr3.
>>
>>  - ptracing and setting CS/SS to something bad
>>
>>    That should test the "exception on iret" case - again in kernel mode, but
>> with user cr3 restored for the return.
>
> The iret-exception case is tested by the ldt_gdt selftest (the
> do_multicpu_tests subtest). But I didn't actually tested single-stepping
> through sysenter yet. I just re-ran the same tests I did with v2 on this
> patch-set.
>
> Regards,
>
>         Joerg
>

For the IRET fault case you will still need to catch it in the
exception code.  See the 64-bit code (.Lerror_bad_iret) for example.
For 32-bit, you could just expand that check to cover the whole exit
prologue after the CR3 switch, including the data segment loads.

I do wonder though, how expensive is a CR3 read?  The SDM implies that
only writes are serializing.  It may be simpler to just
unconditionally check it.

--
Brian Gerst