[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>

diff --git a/arch/x86_64/kernel/head.S b/arch/x86_64/kernel/head.S
index 4a3326ce709c86ec076ca86eb5cd1cf4f0dfc1a7..1e6f80870679506482ef7d2714621e8ad4df4d08 100644 (file)
--- 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