From 6b3e64c237c072797a9ec918654a60e3a46488e2 Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Sun, 25 Nov 2018 19:33:55 +0100 Subject: x86/speculation: Add seccomp Spectre v2 user space protection mode If 'prctl' mode of user space protection from spectre v2 is selected on the kernel command-line, STIBP and IBPB are applied on tasks which restrict their indirect branch speculation via prctl. SECCOMP enables the SSBD mitigation for sandboxed tasks already, so it makes sense to prevent spectre v2 user space to user space attacks as well. The Intel mitigation guide documents how STIPB works: Setting bit 1 (STIBP) of the IA32_SPEC_CTRL MSR on a logical processor prevents the predicted targets of indirect branches on any logical processor of that core from being controlled by software that executes (or executed previously) on another logical processor of the same core. Ergo setting STIBP protects the task itself from being attacked from a task running on a different hyper-thread and protects the tasks running on different hyper-threads from being attacked. While the document suggests that the branch predictors are shielded between the logical processors, the observed performance regressions suggest that STIBP simply disables the branch predictor more or less completely. Of course the document wording is vague, but the fact that there is also no requirement for issuing IBPB when STIBP is used points clearly in that direction. The kernel still issues IBPB even when STIBP is used until Intel clarifies the whole mechanism. IBPB is issued when the task switches out, so malicious sandbox code cannot mistrain the branch predictor for the next user space task on the same logical processor. Signed-off-by: Jiri Kosina Signed-off-by: Thomas Gleixner Reviewed-by: Ingo Molnar Cc: Peter Zijlstra Cc: Andy Lutomirski Cc: Linus Torvalds Cc: Tom Lendacky Cc: Josh Poimboeuf Cc: Andrea Arcangeli Cc: David Woodhouse Cc: Tim Chen Cc: Andi Kleen Cc: Dave Hansen Cc: Casey Schaufler Cc: Asit Mallick Cc: Arjan van de Ven Cc: Jon Masters Cc: Waiman Long Cc: Greg KH Cc: Dave Stewart Cc: Kees Cook Cc: stable@vger.kernel.org Link: https://lkml.kernel.org/r/20181125185006.051663132@linutronix.de --- Documentation/admin-guide/kernel-parameters.txt | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) (limited to 'Documentation') diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index a9b98a4e8789..f405281bb202 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -4241,9 +4241,16 @@ per thread. The mitigation control state is inherited on fork. + seccomp + - Same as "prctl" above, but all seccomp + threads will enable the mitigation unless + they explicitly opt out. + auto - Kernel selects the mitigation depending on the available CPU features and vulnerability. - Default is prctl. + + Default mitigation: + If CONFIG_SECCOMP=y then "seccomp", otherwise "prctl" Not specifying this option is equivalent to spectre_v2_user=auto. -- cgit v1.2.3