linux-kernel - [tip: x86/urgent] Documentation/hw-vuln: Document the interaction between IBRS and STIBP

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Message-ID: <167752793989.5837.12551324006381461991.tip-bot2@tip-bot2>
Date:   Mon, 27 Feb 2023 19:58:59 -0000
From:   "tip-bot2 for KP Singh" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     KP Singh <kpsingh@...nel.org>,
        "Borislav Petkov (AMD)" <bp@...en8.de>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: x86/urgent] Documentation/hw-vuln: Document the interaction
 between IBRS and STIBP

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

Commit-ID:     e02b50ca442e88122e1302d4dbc1b71a4808c13f
Gitweb:        https://git.kernel.org/tip/e02b50ca442e88122e1302d4dbc1b71a4808c13f
Author:        KP Singh <kpsingh@...nel.org>
AuthorDate:    Mon, 27 Feb 2023 07:05:41 +01:00
Committer:     Borislav Petkov (AMD) <bp@...en8.de>
CommitterDate: Mon, 27 Feb 2023 19:02:47 +01:00

Documentation/hw-vuln: Document the interaction between IBRS and STIBP

Explain why STIBP is needed with legacy IBRS as currently implemented
(KERNEL_IBRS) and why STIBP is not needed when enhanced IBRS is enabled.

Fixes: 7c693f54c873 ("x86/speculation: Add spectre_v2=ibrs option to support Kernel IBRS")
Signed-off-by: KP Singh <kpsingh@...nel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
Link: https://lore.kernel.org/r/20230227060541.1939092-2-kpsingh@kernel.org
---
 Documentation/admin-guide/hw-vuln/spectre.rst | 21 +++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/Documentation/admin-guide/hw-vuln/spectre.rst b/Documentation/admin-guide/hw-vuln/spectre.rst
index 3fe6511..4d186f5 100644
--- a/Documentation/admin-guide/hw-vuln/spectre.rst
+++ b/Documentation/admin-guide/hw-vuln/spectre.rst
@@ -479,8 +479,16 @@ Spectre variant 2
    On Intel Skylake-era systems the mitigation covers most, but not all,
    cases. See :ref:`[3] <spec_ref3>` for more details.
 
-   On CPUs with hardware mitigation for Spectre variant 2 (e.g. Enhanced
-   IBRS on x86), retpoline is automatically disabled at run time.
+   On CPUs with hardware mitigation for Spectre variant 2 (e.g. IBRS
+   or enhanced IBRS on x86), retpoline is automatically disabled at run time.
+
+   Systems which support enhanced IBRS (eIBRS) enable IBRS protection once at
+   boot, by setting the IBRS bit, and they're automatically protected against
+   Spectre v2 variant attacks, including cross-thread branch target injections
+   on SMT systems (STIBP). In other words, eIBRS enables STIBP too.
+
+   Legacy IBRS systems clear the IBRS bit on exit to userspace and
+   therefore explicitly enable STIBP for that
 
    The retpoline mitigation is turned on by default on vulnerable
    CPUs. It can be forced on or off by the administrator
@@ -504,9 +512,12 @@ Spectre variant 2
    For Spectre variant 2 mitigation, individual user programs
    can be compiled with return trampolines for indirect branches.
    This protects them from consuming poisoned entries in the branch
-   target buffer left by malicious software.  Alternatively, the
-   programs can disable their indirect branch speculation via prctl()
-   (See :ref:`Documentation/userspace-api/spec_ctrl.rst <set_spec_ctrl>`).
+   target buffer left by malicious software.
+
+   On legacy IBRS systems, at return to userspace, implicit STIBP is disabled
+   because the kernel clears the IBRS bit. In this case, the userspace programs
+   can disable indirect branch speculation via prctl() (See
+   :ref:`Documentation/userspace-api/spec_ctrl.rst <set_spec_ctrl>`).
    On x86, this will turn on STIBP to guard against attacks from the
    sibling thread when the user program is running, and use IBPB to
    flush the branch target buffer when switching to/from the program.