Re: [PATCH 03/16] x86/entry/32: Leave the kernel via the trampoline stack

[Brian Gerst]

On Wed, Jan 17, 2018 at 5:57 AM, Brian Gerst <brgerst@xxxxxxxxx> wrote:
> On Wed, Jan 17, 2018 at 1:24 AM, Joerg Roedel <joro@xxxxxxxxxx> wrote:
>> On Tue, Jan 16, 2018 at 02:48:43PM -0800, Andy Lutomirski wrote:
>>> On Tue, Jan 16, 2018 at 8:36 AM, Joerg Roedel <joro@xxxxxxxxxx> wrote:
>>> > +       /* Restore user %edi and user %fs */
>>> > +       movl (%edi), %edi
>>> > +       popl %fs
>>>
>>> Yikes!  We're not *supposed* to be able to observe an asynchronous
>>> descriptor table change, but if the LDT changes out from under you,
>>> this is going to blow up badly.  It would be really nice if you could
>>> pull this off without percpu access or without needing to do this
>>> dance where you load user FS, then kernel FS, then user FS.  If that's
>>> not doable, then you should at least add exception handling -- look at
>>> the other 'pop %fs' instructions in entry_32.S.
>>
>> You are right! This also means I need to do the 'popl %fs' before the
>> cr3-switch. I'll fix it in the next version.
>>
>> I have no real idea on how to switch back to the entry stack without
>> access to per_cpu variables. I also can't access the cpu_entry_area for
>> the cpu yet, because for that we need to be on the entry stack already.
>
> Switch to the trampoline stack before loading user segments.

But then again, you could take a fault on the trampoline stack if you
get a bad segment.  Perhaps just pushing the new stack pointer onto
the process stack before user segment loads will be the right move.

--
Brian Gerst