x86/mm: Flush global TLB when switching to trampoline page-table

Move the switching code into a function so that it can be re-used and
add a global TLB flush. This makes sure that usage of memory which is
not mapped in the trampoline page-table is reliably caught.

Also move the clearing of CR4.PCIDE before the CR3 switch because the
cr4_clear_bits() function will access data not mapped into the
trampoline page-table.
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20211202153226.22946-4-joro@8bytes.org

arch/x86/include/asm/realmode.h

@@ -89,6 +89,7 @@ static inline void set_real_mode_mem(phys_addr_t mem)
 }
 
 void reserve_real_mode(void);
+void load_trampoline_pgtable(void);
 
 #endif /* __ASSEMBLY__ */
 

arch/x86/kernel/reboot.c

@@ -113,17 +113,9 @@ void __noreturn machine_real_restart(unsigned int type)
 	spin_unlock(&rtc_lock);
 
 	/*
-	 * Switch back to the initial page table.
+	 * Switch to the trampoline page table.
 	 */
-#ifdef CONFIG_X86_32
-	load_cr3(initial_page_table);
-#else
-	write_cr3(real_mode_header->trampoline_pgd);
-
-	/* Exiting long mode will fail if CR4.PCIDE is set. */
-	if (boot_cpu_has(X86_FEATURE_PCID))
-		cr4_clear_bits(X86_CR4_PCIDE);
-#endif
+	load_trampoline_pgtable();
 
 	/* Jump to the identity-mapped low memory code */
 #ifdef CONFIG_X86_32

arch/x86/realmode/init.c

@@ -17,6 +17,32 @@ u32 *trampoline_cr4_features;
 /* Hold the pgd entry used on booting additional CPUs */
 pgd_t trampoline_pgd_entry;
 
+void load_trampoline_pgtable(void)
+{
+#ifdef CONFIG_X86_32
+	load_cr3(initial_page_table);
+#else
+	/*
+	 * This function is called before exiting to real-mode and that will
+	 * fail with CR4.PCIDE still set.
+	 */
+	if (boot_cpu_has(X86_FEATURE_PCID))
+		cr4_clear_bits(X86_CR4_PCIDE);
+
+	write_cr3(real_mode_header->trampoline_pgd);
+#endif
+
+	/*
+	 * The CR3 write above will not flush global TLB entries.
+	 * Stale, global entries from previous page tables may still be
+	 * present.  Flush those stale entries.
+	 *
+	 * This ensures that memory accessed while running with
+	 * trampoline_pgd is *actually* mapped into trampoline_pgd.
+	 */
+	__flush_tlb_all();
+}
+
 void __init reserve_real_mode(void)
 {
 	phys_addr_t mem;