Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::3:7 as permitted sender) client-ip=2620:137:e000::3:7;
From:   Xin Li <xin3.li@intel.com>
To:     linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-edac@vger.kernel.org, linux-hyperv@vger.kernel.org,
        kvm@vger.kernel.org, xen-devel@lists.xenproject.org
Cc:     tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com,
        luto@kernel.org, pbonzini@redhat.com, seanjc@google.com,
        peterz@infradead.org, jgross@suse.com, ravi.v.shankar@intel.com,
        mhiramat@kernel.org, andrew.cooper3@citrix.com,
        jiangshanlai@gmail.com, nik.borisov@suse.com, shan.kang@intel.com
Subject: [PATCH v13 18/35] x86/fred: Disallow the swapgs instruction when FRED is enabled
Date:   Tue,  5 Dec 2023 02:50:07 -0800
Message-ID: <20231205105030.8698-19-xin3.li@intel.com>
In-Reply-To: <20231205105030.8698-1-xin3.li@intel.com>
References: <20231205105030.8698-1-xin3.li@intel.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Precedence: bulk

From: "H. Peter Anvin (Intel)" <hpa@zytor.com>

SWAPGS is no longer needed thus NOT allowed with FRED because FRED
transitions ensure that an operating system can _always_ operate
with its own GS base address:
- For events that occur in ring 3, FRED event delivery swaps the GS
  base address with the IA32_KERNEL_GS_BASE MSR.
- ERETU (the FRED transition that returns to ring 3) also swaps the
  GS base address with the IA32_KERNEL_GS_BASE MSR.

And the operating system can still setup the GS segment for a user
thread without the need of loading a user thread GS with:
- Using LKGS, available with FRED, to modify other attributes of the
  GS segment without compromising its ability always to operate with
  its own GS base address.
- Accessing the GS segment base address for a user thread as before
  using RDMSR or WRMSR on the IA32_KERNEL_GS_BASE MSR.

Note, LKGS loads the GS base address into the IA32_KERNEL_GS_BASE MSR
instead of the GS segment's descriptor cache. As such, the operating
system never changes its runtime GS base address.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Tested-by: Shan Kang <shan.kang@intel.com>
Signed-off-by: Xin Li <xin3.li@intel.com>
---

Change since v12:
* Use the ASCII char ' (char number 0x27), instead of its unicode char
  (Borislav Petkov).

Change since v8:
* Explain why writing directly to the IA32_KERNEL_GS_BASE MSR is
  doing the right thing (Thomas Gleixner).
---
 arch/x86/kernel/process_64.c | 27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index b924477c5ba8..7f66c0b14de6 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -166,7 +166,29 @@ static noinstr unsigned long __rdgsbase_inactive(void)
 
 	lockdep_assert_irqs_disabled();
 
-	if (!cpu_feature_enabled(X86_FEATURE_XENPV)) {
+	/*
+	 * SWAPGS is no longer needed thus NOT allowed with FRED because
+	 * FRED transitions ensure that an operating system can _always_
+	 * operate with its own GS base address:
+	 * - For events that occur in ring 3, FRED event delivery swaps
+	 *   the GS base address with the IA32_KERNEL_GS_BASE MSR.
+	 * - ERETU (the FRED transition that returns to ring 3) also swaps
+	 *   the GS base address with the IA32_KERNEL_GS_BASE MSR.
+	 *
+	 * And the operating system can still setup the GS segment for a
+	 * user thread without the need of loading a user thread GS with:
+	 * - Using LKGS, available with FRED, to modify other attributes
+	 *   of the GS segment without compromising its ability always to
+	 *   operate with its own GS base address.
+	 * - Accessing the GS segment base address for a user thread as
+	 *   before using RDMSR or WRMSR on the IA32_KERNEL_GS_BASE MSR.
+	 *
+	 * Note, LKGS loads the GS base address into the IA32_KERNEL_GS_BASE
+	 * MSR instead of the GS segment’s descriptor cache. As such, the
+	 * operating system never changes its runtime GS base address.
+	 */
+	if (!cpu_feature_enabled(X86_FEATURE_FRED) &&
+	    !cpu_feature_enabled(X86_FEATURE_XENPV)) {
 		native_swapgs();
 		gsbase = rdgsbase();
 		native_swapgs();
@@ -191,7 +213,8 @@ static noinstr void __wrgsbase_inactive(unsigned long gsbase)
 {
 	lockdep_assert_irqs_disabled();
 
-	if (!cpu_feature_enabled(X86_FEATURE_XENPV)) {
+	if (!cpu_feature_enabled(X86_FEATURE_FRED) &&
+	    !cpu_feature_enabled(X86_FEATURE_XENPV)) {
 		native_swapgs();
 		wrgsbase(gsbase);
 		native_swapgs();
-- 
2.43.0