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From: "Chang S. Bae" <>
To: Andy Lutomirski <>,
	"H . Peter Anvin" <>,
	Thomas Gleixner <>,
	Ingo Molnar <>
Cc: Andi Kleen <>,
	Dave Hansen <>,
	Markus T Metzger <>,
	"Ravi V . Shankar" <>,
	"Chang S . Bae" <>,
Subject: [PATCH V2 15/15] x86/fsgsbase/64: Add documentation for FSGSBASE
Date: Thu, 31 May 2018 10:58:45 -0700	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

From: Andi Kleen <>

v2: Minor updates to documentation requested in review.
v3: Update for new gcc and various improvements.

Signed-off-by: Andi Kleen <>
[chang: Minor edit and include descriptions for entry
changes by FSGSBASE]
Signed-off-by: Chang S. Bae <>
Cc: Andy Lutomirski <>
Cc: H. Peter Anvin <>
Cc: Dave Hansen <>
Cc: Thomas Gleixner <>
Cc: Ingo Molnar <>
 Documentation/x86/entry_64.txt |   9 ++++
 Documentation/x86/fsgs.txt     | 104 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 113 insertions(+)
 create mode 100644 Documentation/x86/fsgs.txt

diff --git a/Documentation/x86/entry_64.txt b/Documentation/x86/entry_64.txt
index c1df8eb..9ff38d9 100644
--- a/Documentation/x86/entry_64.txt
+++ b/Documentation/x86/entry_64.txt
@@ -102,3 +102,12 @@ We try to only use IST entries and the paranoid entry code for vectors
 that absolutely need the more expensive check for the GS base - and we
 generate all 'normal' entry points with the regular (faster) paranoid=0
+When FSGSBASE enabled, an arbitrary GS base (including negative value)
+is possible in user space. Thus, current GS base does not offer any
+useful guesses of the origin, whether from kernel or from user space.
+Also, finding the proper kernel GS base is efficiently possible;
+doing RDPID/LSL to get CPU number and indexing it into a table will
+take per-CPU base. With that, paranoid entry with FSGSBASE will always
+overwrite GS base with the per-CPU base, while the original GS base is
+stored and restored back to GS base on its paranoid exit.
diff --git a/Documentation/x86/fsgs.txt b/Documentation/x86/fsgs.txt
new file mode 100644
index 0000000..ebe15f3
--- /dev/null
+++ b/Documentation/x86/fsgs.txt
@@ -0,0 +1,104 @@
+Using FS and GS prefixes on 64bit x86 linux
+The x86 architecture supports segment prefixes per instruction to add an
+offset to an address.  On 64bit x86, these are mostly nops, except for FS
+and GS.
+This offers an efficient way to reference a global pointer.
+The compiler has to generate special code to use these base registers,
+or they can be accessed with inline assembler.
+	mov %gs:offset,%reg
+	mov %fs:offset,%reg
+On 64bit code, FS is used to address the thread local segment (TLS), declared using
+__thread.  The compiler then automatically generates the correct prefixes and
+relocations to access these values.
+FS is normally managed by the runtime code or the threading library
+Overwriting it can break a lot of things (including syscalls and gdb),
+but it can make sense to save/restore it for threading purposes.
+GS is freely available, but may need special (compiler or inline assembler)
+code to use.
+Traditionally 64bit FS and GS could be set by the arch_prctl system call
+	arch_prctl(ARCH_SET_GS, value)
+	arch_prctl(ARCH_SET_FS, value)
+[There was also an older method using modify_ldt(), inherited from 32bit,
+but this is not discussed here.]
+However using a syscall is problematic for user space threading libraries
+that want to context switch in user space. The whole point of them
+is avoiding the overhead of a syscall. It's also cleaner for compilers
+wanting to use the extra register to use instructions to write
+it, or read it directly to compute addresses and offsets.
+Newer Intel CPUs (Ivy Bridge and later) added new instructions to directly
+access these registers quickly from user context
+	RDFSBASE %reg	read the FS base	(or _readfsbase_u64)
+	RDGSBASE %reg	read the GS base	(or _readgsbase_u64)
+	WRFSBASE %reg	write the FS base	(or _writefsbase_u64)
+	WRGSBASE %reg	write the GS base	(or _writegsbase_u64)
+If you use the intrinsics include <immintrin.h> and set the -mfsgsbase option.
+The instructions are supported by the CPU when the "fsgsbase" string is shown in
+/proc/cpuinfo (or directly retrieved through the CPUID instruction,
+7:0 (ebx), word 9, bit 0)
+The instructions are only available to 64bit binaries.
+In addition the kernel needs to explicitly enable these instructions, as it
+may otherwise not correctly context switch the state. Newer Linux
+kernels enable this. When the kernel did not enable the instruction
+they will fault with an #UD exception.
+An FSGSBASE enabled kernel can be detected by checking the AT_HWCAP2
+bitmask in the aux vector. When the HWCAP2_FSGSBASE bit is set the
+kernel supports RDFSGSBASE.
+	#include <sys/auxv.h>
+	#include <elf.h>
+	/* Will be eventually in asm/hwcap.h */
+	#define HWCAP2_FSGSBASE        (1 << 1)
+        unsigned val = getauxval(AT_HWCAP2);
+        if (val & HWCAP2_FSGSBASE) {
+                asm("wrgsbase %0" :: "r" (ptr));
+        }
+No extra CPUID check needed as the kernel will not set this bit if the CPU
+does not support it.
+gcc 6 will have special support to directly access data relative
+to fs/gs using the __seg_fs and __seg_gs address space pointer
+#ifndef __SEG_GS
+#error "Need gcc 6 or later"
+struct gsdata {
+	int a;
+	int b;
+} gsdata = { 1, 2 };
+int __seg_gs *valp = 0;		/* offset relative to GS */
+	/* Check if kernel supports FSGSBASE as above */
+	/* Set up new GS */
+	asm("wrgsbase %0" :: "r" (&gsdata));
+	/* Now the global pointer can be used normally */
+	printf("gsdata.a = %d\n", valp->a);
+Andi Kleen

  parent reply	other threads:[~2018-05-31 17:59 UTC|newest]

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-31 17:58 [PATCH V2 00/15] x86: Enable FSGSBASE instructions Chang S. Bae
2018-05-31 17:58 ` [PATCH V2 01/15] x86/fsgsbase/64: Introduce FS/GS base helper functions Chang S. Bae
2018-05-31 20:14   ` Andy Lutomirski
2018-05-31 21:03     ` Bae, Chang Seok
2018-05-31 17:58 ` [PATCH V2 02/15] x86/fsgsbase/64: Make ptrace read FS/GS base accurately Chang S. Bae
2018-05-31 20:14   ` Andy Lutomirski
2018-05-31 20:31     ` hpa
2018-05-31 20:38       ` Andy Lutomirski
2018-05-31 17:58 ` [PATCH V2 03/15] x86/fsgsbase/64: Use FS/GS base helpers in core dump Chang S. Bae
2018-05-31 20:15   ` Andy Lutomirski
2018-05-31 21:03     ` Bae, Chang Seok
2018-05-31 17:58 ` [PATCH V2 04/15] x86/fsgsbase/64: Factor out load FS/GS segments from __switch_to Chang S. Bae
2018-05-31 20:16   ` Andy Lutomirski
2018-05-31 17:58 ` [PATCH V2 05/15] x86/vdso: Move out the CPU number store Chang S. Bae
2018-05-31 20:25   ` Andy Lutomirski
2018-05-31 21:06     ` Bae, Chang Seok
2018-06-05  7:02   ` [lkp-robot] [x86/vdso] f52001961d: BUG:kernel_hang_in_early-boot_stage,last_printk:Probing_EDD(edd=off_to_disable)...ok kernel test robot
2018-05-31 17:58 ` [PATCH V2 06/15] taint: Add taint for insecure Chang S. Bae
2018-05-31 20:25   ` Andy Lutomirski
2018-05-31 20:50     ` hpa
2018-05-31 17:58 ` [PATCH V2 07/15] x86/fsgsbase/64: Add 'unsafe_fsgsbase' to enable CR4.FSGSBASE Chang S. Bae
2018-05-31 17:58 ` [PATCH V2 08/15] x86/fsgsbase/64: Add intrinsics/macros for FSGSBASE instructions Chang S. Bae
2018-05-31 17:58 ` [PATCH V2 09/15] x86/fsgsbase/64: Enable FSGSBASE instructions in helper functions Chang S. Bae
2018-05-31 17:58 ` [PATCH V2 10/15] x86/fsgsbase/64: Preserve FS/GS state in __switch_to if FSGSBASE is on Chang S. Bae
2018-05-31 17:58 ` [PATCH V2 11/15] x86/fsgsbase/64: When copying a thread, use FSGSBASE if enabled Chang S. Bae
2018-05-31 17:58 ` [PATCH V2 12/15] x86/fsgsbase/64: Use per-CPU base as GS base on paranoid_entry Chang S. Bae
2018-05-31 17:58 ` [PATCH V2 13/15] x86/fsgsbase/64: Enable FSGSBASE by default and add a chicken bit Chang S. Bae
2018-05-31 17:58 ` [PATCH V2 14/15] x86/elf: Enumerate kernel FSGSBASE capability in AT_HWCAP2 Chang S. Bae
2018-05-31 17:58 ` Chang S. Bae [this message]
2018-05-31 20:37 ` [PATCH V2 00/15] x86: Enable FSGSBASE instructions Andy Lutomirski
2018-05-31 21:11   ` Bae, Chang Seok

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