[PATCH 1/9] x86/kexec: Disable global pages before writing to control page

[David Woodhouse]

From: David Woodhouse <dwmw at amazon.co.uk>

The kernel switches to a new set of page tables during kexec. The global
mappings (_PAGE_GLOBAL==1) can remain in the TLB after this switch. This
is generally not a problem because the new page tables use a different
portion of the virtual address space than the normal kernel mappings.

The critical exception to that generalisation (and the only mapping
which isn't an identity mapping) is the kexec control page itself —
which was ROX in the original kernel mapping, but should be RWX in the
new page tables. If there is a global TLB entry for that in its prior
read-only state, it definitely needs to be flushed before attempting to
write through that virtual mapping.

It would be possible to just avoid writing to the virtual address of the
page and defer all writes until they can be done through the identity
mapping. But there's no good reason to keep the old TLB entries around,
as they can cause nothing but trouble.

Clear the PGE bit in %cr4 early, before storing data in the control page.

Fixes: 5a82223e0743 ("x86/kexec: Mark relocate_kernel page as ROX instead of RWX")
Co-authored-by: Dave Hansen <dave.hansen at linux.intel.com>
Reported-by: Nathan Chancellor <nathan at kernel.org>
Reported-by: "Ning, Hongyu" <hongyu.ning at linux.intel.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219592
Signed-off-by: Dave Hansen <dave.hansen at linux.intel.com>
Signed-off-by: David Woodhouse <dwmw at amazon.co.uk>
Tested-by: Nathan Chancellor <nathan at kernel.org>
Tested-by: "Ning, Hongyu" <hongyu.ning at linux.intel.com>
---
 arch/x86/kernel/relocate_kernel_64.S | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index 8bc86a1e056a..9bd601dd8659 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -70,14 +70,20 @@ SYM_CODE_START_NOALIGN(relocate_kernel)
 	movq	kexec_pa_table_page(%rip), %r9
 	movq	%r9, %cr3
 
+	/* Leave CR4 in %r13 to enable the right paging mode later. */
+	movq	%cr4, %r13
+
+	/* Disable global pages immediately to ensure this mapping is RWX */
+	movq	%r13, %r12
+	andq	$~(X86_CR4_PGE), %r12
+	movq	%r12, %cr4
+
 	/* Save %rsp and CRs. */
+	movq	%r13, saved_cr4(%rip)
 	movq    %rsp, saved_rsp(%rip)
 	movq	%rax, saved_cr3(%rip)
 	movq	%cr0, %rax
 	movq	%rax, saved_cr0(%rip)
-	/* Leave CR4 in %r13 to enable the right paging mode later. */
-	movq	%cr4, %r13
-	movq	%r13, saved_cr4(%rip)
 
 	/* save indirection list for jumping back */
 	movq	%rdi, pa_backup_pages_map(%rip)

base-commit: 35aafa1d41cee0d3d50164561bca34befc1d9ce3
-- 
2.47.0