[PATCH 09/16] x86/mm/pti: Clone CPU_ENTRY_AREA on PMD level on x86_32

[Joerg Roedel]

From: Joerg Roedel <jroedel@xxxxxxx>

Cloning on the P4D level would clone the complete kernel
address space into the user-space page-tables for PAE
kernels. Cloning on PMD level is fine for PAE and legacy
paging.

Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
---
 arch/x86/mm/pti.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
index ce38f165489b..20be21301a59 100644
--- a/arch/x86/mm/pti.c
+++ b/arch/x86/mm/pti.c
@@ -308,6 +308,7 @@ pti_clone_pmds(unsigned long start, unsigned long end, pmdval_t clear)
 	}
 }
 
+#ifdef CONFIG_X86_64
 /*
  * Clone a single p4d (i.e. a top-level entry on 4-level systems and a
  * next-level entry on 5-level systems.
@@ -322,13 +323,29 @@ static void __init pti_clone_p4d(unsigned long addr)
 	kernel_p4d = p4d_offset(kernel_pgd, addr);
 	*user_p4d = *kernel_p4d;
 }
+#endif
 
 /*
  * Clone the CPU_ENTRY_AREA into the user space visible page table.
  */
 static void __init pti_clone_user_shared(void)
 {
+#ifdef CONFIG_X86_32
+	/*
+	 * On 32 bit PAE systems with 1GB of Kernel address space there is only
+	 * one pgd/p4d for the whole kernel. Cloning that would map the whole
+	 * address space into the user page-tables, making PTI useless. So clone
+	 * the page-table on the PMD level to prevent that.
+	 */
+	unsigned long start, end;
+
+	start = CPU_ENTRY_AREA_BASE;
+	end   = start + (PAGE_SIZE * CPU_ENTRY_AREA_PAGES);
+
+	pti_clone_pmds(start, end, _PAGE_GLOBAL);
+#else
 	pti_clone_p4d(CPU_ENTRY_AREA_BASE);
+#endif
 }
 
 /*
-- 
2.13.6