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 <[email protected]>
---
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
-
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