On Mon, Feb 20, 2023 at 3:39 AM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>
> On Mon, Feb 20, 2023 at 11:52:27AM +0100, Greg KH wrote:
> > On Mon, Feb 20, 2023 at 11:39:30AM +0100, KP Singh wrote:
> > > With the introduction of KERNEL_IBRS, STIBP is no longer needed
> > > to prevent cross thread training in the kernel space. When KERNEL_IBRS
> > > was added, it also disabled the user-mode protections for spectre_v2.
> > > KERNEL_IBRS does not mitigate cross thread training in the userspace.
> > >
> > > In order to demonstrate the issue, one needs to avoid syscalls in the
> > > victim as syscalls can shorten the window size due to
> > > a user -> kernel -> user transition which sets the
> > > IBRS bit when entering kernel space and clearing any training the
> > > attacker may have done.
> > >
> > > Allow users to select a spectre_v2_user mitigation (STIBP always on,
> > > opt-in via prctl) when KERNEL_IBRS is enabled.
> > >
> > > Reported-by: José Oliveira <joseloliveira11@xxxxxxxxx>
> > > Reported-by: Rodrigo Branco <rodrigo@xxxxxxxxxxxxxxxxx>
> > > Reviewed-by: Alexandra Sandulescu <aesa@xxxxxxxxxx>
> > > Reviewed-by: Jim Mattson <jmattson@xxxxxxxxxx>
> > > Fixes: 7c693f54c873 ("x86/speculation: Add spectre_v2=ibrs option to support Kernel IBRS")
> > > Cc: stable@xxxxxxxxxxxxxxx
> > > Signed-off-by: KP Singh <kpsingh@xxxxxxxxxx>
> > > ---
> > > arch/x86/kernel/cpu/bugs.c | 25 +++++++++++++++++--------
> > > 1 file changed, 17 insertions(+), 8 deletions(-)
> >
> > As this is posted publicly, there's no need to send it to
> > security@xxxxxxxxxx, it doesn't need to be involved.
>
> Also, since this seems intended to be public, please add lkml to Cc on
> the next revision.
Sure.
>
> --
> Josh
