x86/arch_prctl: Fix ARCH_GET_FS and ARCH_GET_GS

ARCH_GET_FS and ARCH_GET_GS attempted to figure out the fsbase and
gsbase respectively from saved thread state.  This was wrong: fsbase
and gsbase live in registers while a thread is running, not in
memory.

For reasons I can't fathom, the fsbase and gsbase code were
different.  Since neither was correct, I didn't try to figure out
what the point of the difference was.

Change it to simply read the MSRs.

The code for reading the base for a remote thread is also completely
wrong if the target thread uses its own descriptors (which is the case
for all 32-bit threaded programs), but fixing that is a different
story.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Reviewed-by: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rudolf Marek <r.marek@assembler.cz>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/c6e7b507c72ca3bdbf6c7a8a3ceaa0334e873bd9.1460075211.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/process_64.c

@@ -566,27 +566,22 @@ long do_arch_prctl(struct task_struct *task, int code, unsigned long addr)
 		break;
 	case ARCH_GET_FS: {
 		unsigned long base;
-		if (task->thread.fsindex == FS_TLS_SEL)
-			base = read_32bit_tls(task, FS_TLS);
-		else if (doit)
+		if (doit)
 			rdmsrl(MSR_FS_BASE, base);
+		else if (task->thread.fsindex == FS_TLS_SEL)
+			base = read_32bit_tls(task, FS_TLS);
 		else
 			base = task->thread.fs;
 		ret = put_user(base, (unsigned long __user *)addr);
 		break;
 	}
 	case ARCH_GET_GS: {
 		unsigned long base;
-		unsigned gsindex;
-		if (task->thread.gsindex == GS_TLS_SEL)
+		if (doit)
+			rdmsrl(MSR_KERNEL_GS_BASE, base);
+		else if (task->thread.gsindex == GS_TLS_SEL)
 			base = read_32bit_tls(task, GS_TLS);
-		else if (doit) {
-			savesegment(gs, gsindex);
-			if (gsindex)
-				rdmsrl(MSR_KERNEL_GS_BASE, base);
-			else
-				base = task->thread.gs;
-		} else
+		else
 			base = task->thread.gs;
 		ret = put_user(base, (unsigned long __user *)addr);
 		break;