Commit b1378a56 authored by Chang S. Bae's avatar Chang S. Bae Committed by Ingo Molnar
Browse files

x86/fsgsbase/64: Introduce FS/GS base helper functions 



Introduce FS/GS base access functionality via <asm/fsgsbase.h>,
not yet used by anything directly.

Factor out task_seg_base() from x86/ptrace.c and rename it to
x86_fsgsbase_read_task() to make it part of the new helpers.

This will allow us to enhance FSGSBASE support and eventually enable
the FSBASE/GSBASE instructions.

An "inactive" GS base refers to a base saved at kernel entry
and being part of an inactive, non-running/stopped user-task.
(The typical ptrace model.)

Here are the new functions:

  x86_fsbase_read_task()
  x86_gsbase_read_task()
  x86_fsbase_write_task()
  x86_gsbase_write_task()
  x86_fsbase_read_cpu()
  x86_fsbase_write_cpu()
  x86_gsbase_read_cpu_inactive()
  x86_gsbase_write_cpu_inactive()

As an advantage of the unified namespace we can now see all FS/GSBASE
API use in the kernel via the following 'git grep' pattern:

  $ git grep x86_.*sbase

[ mingo: Wrote new changelog. ]

Based-on-code-from: Andy Lutomirski <luto@kernel.org>
Suggested-by: default avatarIngo Molnar <mingo@kernel.org>
Signed-off-by: default avatarChang S. Bae <chang.seok.bae@intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Markus T Metzger <markus.t.metzger@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi Shankar <ravi.v.shankar@intel.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1537312139-5580-3-git-send-email-chang.seok.bae@intel.com


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 07e1d88a

arch/x86/include/asm/fsgsbase.h

0 → 100644
+50 −0
Original line number Diff line number Diff line 
/* SPDX-License-Identifier: GPL-2.0 */

#ifndef _ASM_FSGSBASE_H

#define _ASM_FSGSBASE_H 1



#ifndef __ASSEMBLY__



#ifdef CONFIG_X86_64



#include <asm/msr-index.h>



unsigned long x86_fsgsbase_read_task(struct task_struct *task,

				     unsigned short selector);



/*

 * Read/write a task's fsbase or gsbase. This returns the value that

 * the FS/GS base would have (if the task were to be resumed). These

 * work on current or on a different non-running task.

 */

unsigned long x86_fsbase_read_task(struct task_struct *task);

unsigned long x86_gsbase_read_task(struct task_struct *task);

int x86_fsbase_write_task(struct task_struct *task, unsigned long fsbase);

int x86_gsbase_write_task(struct task_struct *task, unsigned long gsbase);



/* Helper functions for reading/writing FS/GS base */



static inline unsigned long x86_fsbase_read_cpu(void)

{

	unsigned long fsbase;



	rdmsrl(MSR_FS_BASE, fsbase);

	return fsbase;

}



void x86_fsbase_write_cpu(unsigned long fsbase);



static inline unsigned long x86_gsbase_read_cpu_inactive(void)

{

	unsigned long gsbase;



	rdmsrl(MSR_KERNEL_GS_BASE, gsbase);

	return gsbase;

}



void x86_gsbase_write_cpu_inactive(unsigned long gsbase);



#endif /* CONFIG_X86_64 */



#endif /* __ASSEMBLY__ */



#endif /* _ASM_FSGSBASE_H */

arch/x86/kernel/process_64.c

+124 −0
Original line number Diff line number Diff line
@@ -54,6 +54,7 @@ 
#include <asm/vdso.h>

#include <asm/intel_rdt_sched.h>

#include <asm/unistd.h>

#include <asm/fsgsbase.h>

#ifdef CONFIG_IA32_EMULATION

/* Not included via unistd.h */

#include <asm/unistd_32_ia32.h>
@@ -286,6 +287,129 @@ static __always_inline void load_seg_legacy(unsigned short prev_index, 
	}

}



unsigned long x86_fsgsbase_read_task(struct task_struct *task,

				     unsigned short selector)

{

	unsigned short idx = selector >> 3;

	unsigned long base;



	if (likely((selector & SEGMENT_TI_MASK) == 0)) {

		if (unlikely(idx >= GDT_ENTRIES))

			return 0;



		/*

		 * There are no user segments in the GDT with nonzero bases

		 * other than the TLS segments.

		 */

		if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)

			return 0;



		idx -= GDT_ENTRY_TLS_MIN;

		base = get_desc_base(&task->thread.tls_array[idx]);

	} else {

#ifdef CONFIG_MODIFY_LDT_SYSCALL

		struct ldt_struct *ldt;



		/*

		 * If performance here mattered, we could protect the LDT

		 * with RCU.  This is a slow path, though, so we can just

		 * take the mutex.

		 */

		mutex_lock(&task->mm->context.lock);

		ldt = task->mm->context.ldt;

		if (unlikely(idx >= ldt->nr_entries))

			base = 0;

		else

			base = get_desc_base(ldt->entries + idx);

		mutex_unlock(&task->mm->context.lock);

#else

		base = 0;

#endif

	}



	return base;

}



void x86_fsbase_write_cpu(unsigned long fsbase)

{

	/*

	 * Set the selector to 0 as a notion, that the segment base is

	 * overwritten, which will be checked for skipping the segment load

	 * during context switch.

	 */

	loadseg(FS, 0);

	wrmsrl(MSR_FS_BASE, fsbase);

}



void x86_gsbase_write_cpu_inactive(unsigned long gsbase)

{

	/* Set the selector to 0 for the same reason as %fs above. */

	loadseg(GS, 0);

	wrmsrl(MSR_KERNEL_GS_BASE, gsbase);

}



unsigned long x86_fsbase_read_task(struct task_struct *task)

{

	unsigned long fsbase;



	if (task == current)

		fsbase = x86_fsbase_read_cpu();

	else if (task->thread.fsindex == 0)

		fsbase = task->thread.fsbase;

	else

		fsbase = x86_fsgsbase_read_task(task, task->thread.fsindex);



	return fsbase;

}



unsigned long x86_gsbase_read_task(struct task_struct *task)

{

	unsigned long gsbase;



	if (task == current)

		gsbase = x86_gsbase_read_cpu_inactive();

	else if (task->thread.gsindex == 0)

		gsbase = task->thread.gsbase;

	else

		gsbase = x86_fsgsbase_read_task(task, task->thread.gsindex);



	return gsbase;

}



int x86_fsbase_write_task(struct task_struct *task, unsigned long fsbase)

{

	/*

	 * Not strictly needed for %fs, but do it for symmetry

	 * with %gs

	 */

	if (unlikely(fsbase >= TASK_SIZE_MAX))

		return -EPERM;



	preempt_disable();

	task->thread.fsbase = fsbase;

	if (task == current)

		x86_fsbase_write_cpu(fsbase);

	task->thread.fsindex = 0;

	preempt_enable();



	return 0;

}



int x86_gsbase_write_task(struct task_struct *task, unsigned long gsbase)

{

	if (unlikely(gsbase >= TASK_SIZE_MAX))

		return -EPERM;



	preempt_disable();

	task->thread.gsbase = gsbase;

	if (task == current)

		x86_gsbase_write_cpu_inactive(gsbase);

	task->thread.gsindex = 0;

	preempt_enable();



	return 0;

}



int copy_thread_tls(unsigned long clone_flags, unsigned long sp,

		unsigned long arg, struct task_struct *p, unsigned long tls)

{

arch/x86/kernel/ptrace.c

+5 −46
Original line number Diff line number Diff line
@@ -39,7 +39,7 @@ 
#include <asm/hw_breakpoint.h>

#include <asm/traps.h>

#include <asm/syscall.h>

#include <asm/mmu_context.h>

#include <asm/fsgsbase.h>



#include "tls.h"
@@ -343,49 +343,6 @@ static int set_segment_reg(struct task_struct *task, 
	return 0;

}



static unsigned long task_seg_base(struct task_struct *task,

				   unsigned short selector)

{

	unsigned short idx = selector >> 3;

	unsigned long base;



	if (likely((selector & SEGMENT_TI_MASK) == 0)) {

		if (unlikely(idx >= GDT_ENTRIES))

			return 0;



		/*

		 * There are no user segments in the GDT with nonzero bases

		 * other than the TLS segments.

		 */

		if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)

			return 0;



		idx -= GDT_ENTRY_TLS_MIN;

		base = get_desc_base(&task->thread.tls_array[idx]);

	} else {

#ifdef CONFIG_MODIFY_LDT_SYSCALL

		struct ldt_struct *ldt;



		/*

		 * If performance here mattered, we could protect the LDT

		 * with RCU.  This is a slow path, though, so we can just

		 * take the mutex.

		 */

		mutex_lock(&task->mm->context.lock);

		ldt = task->mm->context.ldt;

		if (unlikely(idx >= ldt->nr_entries))

			base = 0;

		else

			base = get_desc_base(ldt->entries + idx);

		mutex_unlock(&task->mm->context.lock);

#else

		base = 0;

#endif

	}



	return base;

}



#endif	/* CONFIG_X86_32 */



static unsigned long get_flags(struct task_struct *task)
@@ -482,13 +439,15 @@ static unsigned long getreg(struct task_struct *task, unsigned long offset) 
		if (task->thread.fsindex == 0)

			return task->thread.fsbase;

		else

			return task_seg_base(task, task->thread.fsindex);

			return x86_fsgsbase_read_task(task,

						      task->thread.fsindex);

	}

	case offsetof(struct user_regs_struct, gs_base): {

		if (task->thread.gsindex == 0)

			return task->thread.gsbase;

		else

			return task_seg_base(task, task->thread.gsindex);

			return x86_fsgsbase_read_task(task,

						      task->thread.gsindex);

	}

#endif

	}

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