The following commit has been merged into the x86/urgent branch of tip:

Commit-ID: 4d8df8cbb9156b0a0ab3f802b80cb5db57acc0bf Gitweb: https://git.kernel.org/tip/4d8df8cbb9156b0a0ab3f802b80cb5db57acc0bf Author: Anthony Steinhauser asteinhauser@google.com AuthorDate: Sun, 07 Jun 2020 05:44:19 -07:00 Committer: Thomas Gleixner tglx@linutronix.de CommitterDate: Tue, 09 Jun 2020 10:50:55 +02:00

x86/speculation: PR_SPEC_FORCE_DISABLE enforcement for indirect branches.

Currently, it is possible to enable indirect branch speculation even after it was force-disabled using the PR_SPEC_FORCE_DISABLE option. Moreover, the PR_GET_SPECULATION_CTRL command gives afterwards an incorrect result (force-disabled when it is in fact enabled). This also is inconsistent vs. STIBP and the documention which cleary states that PR_SPEC_FORCE_DISABLE cannot be undone.

Fix this by actually enforcing force-disabled indirect branch speculation. PR_SPEC_ENABLE called after PR_SPEC_FORCE_DISABLE now fails with -EPERM as described in the documentation.

Fixes: 9137bb27e60e ("x86/speculation: Add prctl() control for indirect branch speculation") Signed-off-by: Anthony Steinhauser asteinhauser@google.com Signed-off-by: Thomas Gleixner tglx@linutronix.de Cc: stable@vger.kernel.org --- arch/x86/kernel/cpu/bugs.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c index 8d57562..56f573a 100644 --- a/arch/x86/kernel/cpu/bugs.c +++ b/arch/x86/kernel/cpu/bugs.c @@ -1175,11 +1175,14 @@ static int ib_prctl_set(struct task_struct *task, unsigned long ctrl) return 0; /* * Indirect branch speculation is always disabled in strict - * mode. + * mode. It can neither be enabled if it was force-disabled + * by a previous prctl call. + */ if (spectre_v2_user_ibpb == SPECTRE_V2_USER_STRICT || spectre_v2_user_stibp == SPECTRE_V2_USER_STRICT || - spectre_v2_user_stibp == SPECTRE_V2_USER_STRICT_PREFERRED) + spectre_v2_user_stibp == SPECTRE_V2_USER_STRICT_PREFERRED || + task_spec_ib_force_disable(task)) return -EPERM; task_clear_spec_ib_disable(task); task_update_spec_tif(task);