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>

diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 6cbab31ac23a20fb3980f06f88becddff135411b..c671b9b015c0e24283062a867e89f6cb3591d591 100644 (file)
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -566,10 +566,10 @@ 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);
@@ -577,16 +577,11 @@ long do_arch_prctl(struct task_struct *task, int code, unsigned long addr)
        }
        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;