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Peter Anvin (Intel)" Sender: tip-bot2@linutronix.de Reply-to: linux-kernel@vger.kernel.org To: linux-tip-commits@vger.kernel.org Subject: [tip: x86/fred] x86/fred: Disallow the swapgs instruction when FRED is enabled Cc: "H. Peter Anvin (Intel)" , Xin Li , Thomas Gleixner , Shan Kang , x86@kernel.org, linux-kernel@vger.kernel.org In-Reply-To: <20231205105030.8698-19-xin3.li@intel.com> References: <20231205105030.8698-19-xin3.li@intel.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Message-ID: <170620688794.398.9560084584740644544.tip-bot2@tip-bot2> Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails Precedence: bulk X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: 1784440800557275242 X-GMAIL-MSGID: 1789087944394178700 The following commit has been merged into the x86/fred branch of tip: Commit-ID: 45ba25d49059ddab201a22d62c9364eab283856f Gitweb: https://git.kernel.org/tip/45ba25d49059ddab201a22d62c9364eab283856f Author: H. Peter Anvin (Intel) AuthorDate: Tue, 05 Dec 2023 02:50:07 -08:00 Committer: Thomas Gleixner CommitterDate: Thu, 25 Jan 2024 19:10:31 +01:00 x86/fred: Disallow the swapgs instruction when FRED is enabled 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) Signed-off-by: Xin Li Signed-off-by: Thomas Gleixner Tested-by: Shan Kang Link: https://lore.kernel.org/r/20231205105030.8698-19-xin3.li@intel.com --- 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 0f78b58..4f87f59 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();