[PATCH] Reload CS when startup_64 is used.

In long mode the %cs is largely a relic.  However there are a few cases
like iret where it matters that we have a valid value.  Without this
patch it is possible to enter the kernel in startup_64 without setting
%cs to a valid value.  With this patch we don't care what %cs value
we enter the kernel with, so long as the cs shadow register indicates
it is a privileged code segment.

Thanks to Magnus Damm for finding this problem and posting the
first workable patch.  I have moved the jump to set %cs down a
few instructions so we don't need to take an extra jump.  Which
keeps the code simpler.

Signed-of-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Andi Kleen <ak@suse.de>

1 files changed, 7 insertions, 4 deletions

diff --git a/arch/x86_64/kernel/head.S b/arch/x86_64/kernel/head.S
index 4a3326ce709c..1e6f80870679 100644
--- a/arch/x86_64/kernel/head.S
+++ b/arch/x86_64/kernel/head.S
@@ -185,12 +185,15 @@ startup_64:
         
         /* Finally jump to run C code and to be on real kernel address
          * Since we are running on identity-mapped space we have to jump
-         * to the full 64bit address , this is only possible as indirect
-         * jump
+         * to the full 64bit address, this is only possible as indirect
+         * jump.  In addition we need to ensure %cs is set so we make this
+         * a far return.
          */
         movq    initial_code(%rip),%rax
-        pushq   $0              # fake return address
-        jmp     *%rax
+        pushq   $0              # fake return address to stop unwinder
+        pushq   $__KERNEL_CS    # set correct cs
+        pushq   %rax            # target address in negative space
+        lretq
 
         /* SMP bootup changes these two */
         .align  8