[PATCH 4.15 036/163] x86/kexec: Make kexec (mostly) work in 5-level paging mode

Greg Kroah-Hartman
2018-02-21T12:47:45Z

From: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

commit 5bf30316991d5bcda046343ee77d823cf16fdd03 upstream.

Currently kexec() will crash when switching into a 5-level paging
enabled kernel.

I missed that we need to change relocate_kernel() to set CR4.LA57
flag if the kernel has 5-level paging enabled.

I avoided using #ifdef CONFIG_X86_5LEVEL here and inferred if we need to
enable 5-level paging from previous CR4 value. This way the code is
ready for boot-time switching between paging modes.

With this patch applied, in addition to kexec 4-to-4 which always worked,
we can kexec 4-to-5 and 5-to-5 - while 5-to-4 will need more work.

Reported-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Tested-by: Baoquan He <bhe@redhat.com>
Cc: <stable@vger.kernel.org> # v4.14+
Cc: Borislav Petkov <bp@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mm@kvack.org
Fixes: 77ef56e4f0fb ("x86: Enable 5-level paging support via CONFIG_X86_5LEVEL=y")
Link: http://lkml.kernel.org/r/20180129110845.26633-1-kirill.shutemov@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/x86/kernel/relocate_kernel_64.S | 8 ++++++++
1 file changed, 8 insertions(+)

--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -68,6 +68,9 @@ relocate_kernel:
movq %cr4, %rax
movq %rax, CR4(%r11)

+ /* Save CR4. Required to enable the right paging mode later. */
+ movq %rax, %r13
+
/* zero out flags, and disable interrupts */
pushq $0
popfq
@@ -126,8 +129,13 @@ identity_mapped:
/*
* Set cr4 to a known state:
* - physical address extension enabled
+ * - 5-level paging, if it was enabled before
*/
movl $X86_CR4_PAE, %eax
+ testq $X86_CR4_LA57, %r13
+ jz 1f
+ orl $X86_CR4_LA57, %eax
+1:
movq %rax, %cr4

jmp 1f