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