[tip: x86/urgent] x86/speculation: PR_SPEC_FORCE_DISABLE enforcement for indirect branches.

[tip: x86/urgent] x86/speculation: PR_SPEC_FORCE_DISABLE enforcement for indirect branches.

[tip-bot2 for Anthony Steinhauser]

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);