[lkml]   [2018]   [Jan]   [7]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
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 <>
> 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
> 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)
> 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

> Signed-off-by: Andrea Arcangeli <>
> Signed-off-by: Tim Chen <>
> ---
> arch/x86/entry/entry_64.S | 23 +++++++++++++++++++++++
> arch/x86/entry/entry_64_compat.S | 8 ++++++++
> 2 files changed, 31 insertions(+)

Reviewed-by: Borislav Petkov <>


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

 \ /
  Last update: 2018-01-14 23:16    [W:0.256 / U:9.932 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site