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SubjectRe: [PATCH 03/16] x86/entry/32: Leave the kernel via the trampoline stack
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@8bytes.org> 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.


Joerg

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