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[PATCH] Reload CS when startup_64 is used.

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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>
This commit is contained in:
Eric W. Biederman 2006-09-26 10:52:38 +02:00 committed by Andi Kleen
parent 0136611c62
commit 26374c7b7d
1 changed files with 7 additions and 4 deletions
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11
arch/x86_64/kernel/head.S
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@ -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