x86/speculation: PR_SPEC_FORCE_DISABLE enforcement for indirect branches.

commit 4d8df8cbb9156b0a0ab3f802b80cb5db57acc0bf upstream.

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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 330c6e3..acbf3db 100644 (file)
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1261,11 +1261,14 @@ 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);