[lkml]   [2006]   [Aug]   [22]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
Patch in this message
Subject[PATCH] x86_64: Reload CS when startup_64 is used.

In long mode the %cs is largely a relic. However there are a few cases
like lret 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 <>
arch/x86_64/kernel/head.S | 7 +++++--
1 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86_64/kernel/head.S b/arch/x86_64/kernel/head.S
index 923f080..6c89f6a 100644
--- a/arch/x86_64/kernel/head.S
+++ b/arch/x86_64/kernel/head.S
@@ -186,10 +186,13 @@ #define CR0_PAGING (1<<31)
/* 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
+ * jump. In addition we need to ensure %cs is set so we make this
+ * a far return.
movq initial_code(%rip),%rax
- jmp *%rax
+ pushq $__KERNEL_CS
+ pushq %rax
+ lretq

/* SMP bootup changes these two */
.align 8
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at
 \ /
  Last update: 2006-08-22 10:41    [W:0.036 / U:22.124 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site