Re: [PATCH 3.13 044/187] x86_64/entry/xen: Do not invoke espfix64 on Xen

Ben Hutchings
2014-09-17T11:26:07Z

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On Mon, 2014-09-15 at 15:07 -0700, Kamal Mostafa wrote:
> 3.13.11.7 -stable review patch. If anyone has any objections, please let me know.
>
> ------------------
>
> From: Andy Lutomirski <luto@amacapital.net>
>
> commit 7209a75d2009dbf7745e2fd354abf25c3deb3ca3 upstream.
>
> This moves the espfix64 logic into native_iret. To make this work,
> it gets rid of the native patch for INTERRUPT_RETURN:
> INTERRUPT_RETURN on native kernels is now 'jmp native_iret'.
>
> This changes the 16-bit SS behavior on Xen from OOPSing to leaking
> some bits of the Xen hypervisor's RSP (I think).
>
> [ hpa: this is a nonzero cost on native, but probably not enough to
> measure. Xen needs to fix this in their own code, probably doing
> something equivalent to espfix64. ]
>
> Signed-off-by: Andy Lutomirski <luto@amacapital.net>
> Link: http://lkml.kernel.org/r/7b8f1d8ef6597cb16ae004a43c56980a7de3cf94.1406129132.git.luto@amacapital.net
> Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
> Signed-off-by: Kamal Mostafa <kamal@canonical.com>
[...]

There's another fix related to espfix on Xen, commit 8762e5092828
("x86/espfix/xen: Fix allocation of pages for paravirt page tables").
But I'm not sure whether it's actually needed after this.

Ben.

--
Ben Hutchings
Beware of programmers who carry screwdrivers. - Leonard Brandwein

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