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: 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>

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index e2ee403865ebee6e265cfa3f35f5f0fad78436fe..3acbf45cb7fb5432c0295dff1d7b9bfe61892939 100644 (file)
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -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
        }