Linux-Doc Archive on lore.kernel.org
 help / color / Atom feed
From: Sasha Levin <sashal@kernel.org>
To: tglx@linutronix.de, luto@kernel.org, ak@linux.intel.com
Cc: corbet@lwn.net, mingo@redhat.com, bp@alien8.de, x86@kernel.org,
	shuah@kernel.org, gregkh@linuxfoundation.org,
	tony.luck@intel.com, chang.seok.bae@intel.com,
	dave.hansen@linux.intel.com, peterz@infradead.org,
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	jarkko.sakkinen@linux.intel.com,
	Ravi Shankar <ravi.v.shankar@intel.com>,
	"H . Peter Anvin" <hpa@zytor.com>,
	Sasha Levin <sashal@kernel.org>
Subject: [PATCH v13 13/16] x86/elf: Enumerate kernel FSGSBASE capability in AT_HWCAP2
Date: Thu, 28 May 2020 16:13:59 -0400
Message-ID: <20200528201402.1708239-14-sashal@kernel.org> (raw)
In-Reply-To: <20200528201402.1708239-1-sashal@kernel.org>

From: Andi Kleen <ak@linux.intel.com>

The kernel needs to explicitly enable FSGSBASE. So, the application needs
to know if it can safely use these instructions. Just looking at the CPUID
bit is not enough because it may be running in a kernel that does not
enable the instructions.

One way for the application would be to just try and catch the SIGILL.
But that is difficult to do in libraries which may not want to overwrite
the signal handlers of the main application.

Enumerate the enabled FSGSBASE capability in bit 1 of AT_HWCAP2 in the ELF
aux vector. AT_HWCAP2 is already used by PPC for similar purposes.

The application can access it open coded or by using the getauxval()
function in newer versions of glibc.

[ tglx: Massaged changelog ]

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Ravi Shankar <ravi.v.shankar@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Link: https://lkml.kernel.org/r/1557309753-24073-18-git-send-email-chang.seok.bae@intel.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 arch/x86/include/uapi/asm/hwcap2.h | 3 +++
 arch/x86/kernel/cpu/common.c       | 4 +++-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/uapi/asm/hwcap2.h b/arch/x86/include/uapi/asm/hwcap2.h
index 8b2effe6efb8..5fdfcb47000f 100644
--- a/arch/x86/include/uapi/asm/hwcap2.h
+++ b/arch/x86/include/uapi/asm/hwcap2.h
@@ -5,4 +5,7 @@
 /* MONITOR/MWAIT enabled in Ring 3 */
 #define HWCAP2_RING3MWAIT		(1 << 0)
 
+/* Kernel allows FSGSBASE instructions available in Ring 3 */
+#define HWCAP2_FSGSBASE			BIT(1)
+
 #endif
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 0d480cbadc7d..b5a086ea3425 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1495,8 +1495,10 @@ static void identify_cpu(struct cpuinfo_x86 *c)
 	setup_umip(c);
 
 	/* Enable FSGSBASE instructions if available. */
-	if (cpu_has(c, X86_FEATURE_FSGSBASE))
+	if (cpu_has(c, X86_FEATURE_FSGSBASE)) {
 		cr4_set_bits(X86_CR4_FSGSBASE);
+		elf_hwcap2 |= HWCAP2_FSGSBASE;
+	}
 
 	/*
 	 * The vendor-specific functions might have changed features.
-- 
2.25.1


  parent reply index

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-28 20:13 [PATCH v13 00/16] Enable FSGSBASE instructions Sasha Levin
2020-05-28 20:13 ` [PATCH v13 01/16] x86/ptrace: Prevent ptrace from clearing the FS/GS selector Sasha Levin
2020-05-28 20:13 ` [PATCH v13 02/16] x86/cpu: Add 'unsafe_fsgsbase' to enable CR4.FSGSBASE Sasha Levin
2020-05-28 20:13 ` [PATCH v13 03/16] x86/fsgsbase/64: Add intrinsics for FSGSBASE instructions Sasha Levin
2020-05-28 20:13 ` [PATCH v13 04/16] x86/fsgsbase/64: Enable FSGSBASE instructions in helper functions Sasha Levin
2020-05-28 20:13 ` [PATCH v13 05/16] x86/process/64: Use FSBSBASE in switch_to() if available Sasha Levin
2020-05-28 20:13 ` [PATCH v13 06/16] x86/process/64: Make save_fsgs() public available Sasha Levin
2020-05-28 20:13 ` [PATCH v13 07/16] x86/process/64: Use FSGSBASE instructions on thread copy and ptrace Sasha Levin
2020-05-28 20:13 ` [PATCH v13 08/16] x86/speculation/swapgs: Check FSGSBASE in enabling SWAPGS mitigation Sasha Levin
2020-05-28 20:13 ` [PATCH v13 09/16] x86/entry/64: Switch CR3 before SWAPGS in paranoid entry Sasha Levin
2020-05-28 20:13 ` [PATCH v13 10/16] x86/entry/64: Introduce the FIND_PERCPU_BASE macro Sasha Levin
2020-05-28 20:13 ` [PATCH v13 11/16] x86/entry/64: Handle FSGSBASE enabled paranoid entry/exit Sasha Levin
2020-05-28 20:13 ` [PATCH v13 12/16] x86/cpu: Enable FSGSBASE on 64bit by default and add a chicken bit Sasha Levin
2020-05-28 20:13 ` Sasha Levin [this message]
2020-05-28 20:14 ` [PATCH v13 14/16] Documentation/x86/64: Add documentation for GS/FS addressing mode Sasha Levin
2020-05-28 20:14 ` [PATCH v13 15/16] selftests/x86/fsgsbase: Test GS selector on ptracer-induced GS base write Sasha Levin
2020-05-29 14:40   ` Shuah Khan
2020-05-28 20:14 ` [PATCH v13 16/16] selftests/x86/fsgsbase: Test ptracer-induced GS base write with FSGSBASE Sasha Levin
2020-05-29 14:42   ` Shuah Khan
2020-06-18 14:18 ` [PATCH v13 00/16] Enable FSGSBASE instructions Thomas Gleixner
2020-06-18 18:08   ` Andy Lutomirski
2020-06-18 18:25     ` Thomas Gleixner

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20200528201402.1708239-14-sashal@kernel.org \
    --to=sashal@kernel.org \
    --cc=ak@linux.intel.com \
    --cc=bp@alien8.de \
    --cc=chang.seok.bae@intel.com \
    --cc=corbet@lwn.net \
    --cc=dave.hansen@linux.intel.com \
    --cc=gregkh@linuxfoundation.org \
    --cc=hpa@zytor.com \
    --cc=jarkko.sakkinen@linux.intel.com \
    --cc=linux-doc@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=luto@kernel.org \
    --cc=mingo@redhat.com \
    --cc=peterz@infradead.org \
    --cc=ravi.v.shankar@intel.com \
    --cc=shuah@kernel.org \
    --cc=tglx@linutronix.de \
    --cc=tony.luck@intel.com \
    --cc=x86@kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-Doc Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-doc/0 linux-doc/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-doc linux-doc/ https://lore.kernel.org/linux-doc \
		linux-doc@vger.kernel.org
	public-inbox-index linux-doc

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-doc


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git