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x86/fsgsbase/64: Fix ptrace() to read the FS/GS base accurately

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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: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Chang 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: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Andy Lutomirski 2018-09-18 16:08:52 -07:00 committed by Ingo Molnar
parent edfbeecd92
commit 07e1d88ada
1 changed files with 52 additions and 10 deletions
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62
arch/x86/kernel/ptrace.c
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@ -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.
*/
return task->thread.fsbase;
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.
*/
return task->thread.gsbase;
if (task->thread.gsindex == 0)
return task->thread.gsbase;
else
return task_seg_base(task, task->thread.gsindex);
}
#endif
}