x86/kexec: Disable global pages before writing to control page

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: 5a82223 ("x86/kexec: Mark relocate_kernel page as ROX instead of RWX")
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219592
Reported-by: Nathan Chancellor <[email protected]>
Reported-by: "Ning, Hongyu" <[email protected]>
Co-developed-by: Dave Hansen <[email protected]>
Signed-off-by: Dave Hansen <[email protected]>
Signed-off-by: David Woodhouse <[email protected]>
Signed-off-by: Borislav Petkov (AMD) <[email protected]>
Tested-by: Nathan Chancellor <[email protected]>
Tested-by: "Ning, Hongyu" <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: dwmw2

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)