x86/speculation: Allow enabling STIBP with legacy IBRS

stable inclusion
from stable-v5.10.173
commit abfed855f05863d292de2d0ebab4656791bab9c8
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I6V7TU
CVE: CVE-2023-1998

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=abfed855f05863d292de2d0ebab4656791bab9c8



--------------------------------

commit 6921ed90 upstream.

When plain IBRS is enabled (not enhanced IBRS), the logic in
spectre_v2_user_select_mitigation() determines that STIBP is not needed.

The IBRS bit implicitly protects against cross-thread branch target
injection. However, with legacy IBRS, the IBRS bit is cleared on
returning to userspace for performance reasons which leaves userspace
threads vulnerable to cross-thread branch target injection against which
STIBP protects.

Exclude IBRS from the spectre_v2_in_ibrs_mode() check to allow for
enabling STIBP (through seccomp/prctl() by default or always-on, if
selected by spectre_v2_user kernel cmdline parameter).

  [ bp: Massage. ]

Fixes: 7c693f54 ("x86/speculation: Add spectre_v2=ibrs option to support Kernel IBRS")
Reported-by: José Oliveira <joseloliveira11@gmail.com>
Reported-by: Rodrigo Branco <rodrigo@kernelhacking.com>
Signed-off-by: KP Singh <kpsingh@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20230220120127.1975241-1-kpsingh@kernel.org
Link: https://lore.kernel.org/r/20230221184908.2349578-1-kpsingh@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Wei Li <liwei391@huawei.com>
(cherry picked from commit 811a507c)