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SubjectRe: [PATCH v2 3/8] x86/enter: Use IBRS on syscall and interrupts
On Fri, Jan 05, 2018 at 06:12:18PM -0800, Tim Chen wrote:
> From: Andrea Arcangeli <aarcange@redhat.com>
>
> Set IBRS upon kernel entrance via syscall and interrupts. Clear it
> upon exit. IBRS protects against unsafe indirect branching predictions
> in the kernel.
>
> The NMI interrupt save/restore of IBRS state was based on Andrea
> Arcangeli's implementation.
> Here's an explanation by Dave Hansen on why we save IBRS state for NMI.
>
> The normal interrupt code uses the 'error_entry' path which uses the
> Code Segment (CS) of the instruction that was interrupted to tell
> whether it interrupted the kernel or userspace and thus has to switch
> IBRS, or leave it alone.
>
> The NMI code is different. It uses 'paranoid_entry' because it can
> interrupt the kernel while it is running with a userspace IBRS (and %GS
> and CR3) value, but has a kernel CS. If we used the same approach as
> the normal interrupt code, we might do the following;
>
> SYSENTER_entry
> <-------------- NMI HERE
> IBRS=1
> do_something()
> IBRS=0
> SYSRET
>
> The NMI code might notice that we are running in the kernel and decide
> that it is OK to skip the IBRS=1. This would leave it running
> unprotected with IBRS=0, which is bad.
>
> However, if we unconditionally set IBRS=1, in the NMI, we might get the
> following case:
>
> SYSENTER_entry
> IBRS=1
> do_something()
> IBRS=0
> <-------------- NMI HERE (set IBRS=1)
> SYSRET
>
> and we would return to userspace with IBRS=1. Userspace would run
> slowly until we entered and exited the kernel again.
>
> Instead of those two approaches, we chose a third one where we simply
> save the IBRS value in a scratch register (%r13) and then restore that
> value, verbatim.

That's one helluva commit message. This is how you write commit
messages!

> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
> ---
> arch/x86/entry/entry_64.S | 23 +++++++++++++++++++++++
> arch/x86/entry/entry_64_compat.S | 8 ++++++++
> 2 files changed, 31 insertions(+)

Reviewed-by: Borislav Petkov <bp@suse.de>

--
Regards/Gruss,
Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.

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