Commit 07e1d88a authored by Andy Lutomirski's avatar Andy Lutomirski Committed by Ingo Molnar
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x86/fsgsbase/64: Fix ptrace() to read the FS/GS base accurately 



On 64-bit kernels ptrace can read the FS/GS base using the register access
APIs (PTRACE_PEEKUSER, etc.) or PTRACE_ARCH_PRCTL.

Make both of these mechanisms return the actual FS/GS base.

This will improve debuggability by providing the correct information
to ptracer such as GDB.

[ chang: Rebased and revised patch description. ]
[ mingo: Revised the changelog some more. ]

Signed-off-by: default avatarAndy Lutomirski <luto@kernel.org>
Signed-off-by: default avatarChang S. Bae <chang.seok.bae@intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>
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/1537312139-5580-2-git-send-email-chang.seok.bae@intel.com


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

arch/x86/kernel/ptrace.c

+52 −10
Original line number Diff line number Diff line
@@ -39,6 +39,7 @@ 
#include <asm/hw_breakpoint.h>

#include <asm/traps.h>

#include <asm/syscall.h>

#include <asm/mmu_context.h>



#include "tls.h"
@@ -342,6 +343,49 @@ 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)
@@ -435,18 +479,16 @@ static unsigned long getreg(struct task_struct *task, unsigned long offset) 


#ifdef CONFIG_X86_64

	case offsetof(struct user_regs_struct, fs_base): {

		/*

		 * XXX: This will not behave as expected if called on

		 * current or if fsindex != 0.

		 */

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

			return task->thread.fsbase;

		else

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

	}

	case offsetof(struct user_regs_struct, gs_base): {

		/*

		 * XXX: This will not behave as expected if called on

		 * current or if fsindex != 0.

		 */

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

			return task->thread.gsbase;

		else

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

	}

#endif

	}

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