Commit 26374c7b authored Sep 26, 2006 by Eric W. Biederman Committed by Andi Kleen Sep 26, 2006

[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>

parent 0136611c

arch/x86_64/kernel/head.S

+7 −4

Original line number	Diff line number	Diff line
		@@ -186,11 +186,14 @@ 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
		* 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

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