x86/mm: Fix PTI for i386 some more

So it turns out that we have to do two passes of
pti_clone_entry_text(), once before initcalls, such that device and
late initcalls can use user-mode-helper / modprobe and once after
free_initmem() / mark_readonly().

Now obviously mark_readonly() can cause PMD splits, and
pti_clone_pgtable() doesn't like that much.

Allow the late clone to split PMDs so that pagetables stay in sync.

[peterz: Changelog and comments]
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lkml.kernel.org/r/20240806184843.GX37996@noisy.programming.kicks-ass.net

diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
index bfdf5f45b13704a4a213a9bb8177a98fe3628c05..851ec8f1363a8b389ea4579cc68bf3300a4df27c 100644 (file)
--- a/arch/x86/mm/pti.c
+++ b/arch/x86/mm/pti.c
@@ -241,7 +241,7 @@ static pmd_t *pti_user_pagetable_walk_pmd(unsigned long address)
  *
  * Returns a pointer to a PTE on success, or NULL on failure.
  */
-static pte_t *pti_user_pagetable_walk_pte(unsigned long address)
+static pte_t *pti_user_pagetable_walk_pte(unsigned long address, bool late_text)
 {
        gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
        pmd_t *pmd;
@@ -251,10 +251,15 @@ static pte_t *pti_user_pagetable_walk_pte(unsigned long address)
        if (!pmd)
                return NULL;
 
-       /* We can't do anything sensible if we hit a large mapping. */
+       /* Large PMD mapping found */
        if (pmd_leaf(*pmd)) {
-               WARN_ON(1);
-               return NULL;
+               /* Clear the PMD if we hit a large mapping from the first round */
+               if (late_text) {
+                       set_pmd(pmd, __pmd(0));
+               } else {
+                       WARN_ON_ONCE(1);
+                       return NULL;
+               }
        }
 
        if (pmd_none(*pmd)) {
@@ -283,7 +288,7 @@ static void __init pti_setup_vsyscall(void)
        if (!pte || WARN_ON(level != PG_LEVEL_4K) || pte_none(*pte))
                return;
 
-       target_pte = pti_user_pagetable_walk_pte(VSYSCALL_ADDR);
+       target_pte = pti_user_pagetable_walk_pte(VSYSCALL_ADDR, false);
        if (WARN_ON(!target_pte))
                return;
 
@@ -301,7 +306,7 @@ enum pti_clone_level {
 
 static void
 pti_clone_pgtable(unsigned long start, unsigned long end,
-                 enum pti_clone_level level)
+                 enum pti_clone_level level, bool late_text)
 {
        unsigned long addr;
 
@@ -390,7 +395,7 @@ pti_clone_pgtable(unsigned long start, unsigned long end,
                                return;
 
                        /* Allocate PTE in the user page-table */
-                       target_pte = pti_user_pagetable_walk_pte(addr);
+                       target_pte = pti_user_pagetable_walk_pte(addr, late_text);
                        if (WARN_ON(!target_pte))
                                return;
 
@@ -452,7 +457,7 @@ static void __init pti_clone_user_shared(void)
                phys_addr_t pa = per_cpu_ptr_to_phys((void *)va);
                pte_t *target_pte;
 
-               target_pte = pti_user_pagetable_walk_pte(va);
+               target_pte = pti_user_pagetable_walk_pte(va, false);
                if (WARN_ON(!target_pte))
                        return;
 
@@ -475,7 +480,7 @@ static void __init pti_clone_user_shared(void)
        start = CPU_ENTRY_AREA_BASE;
        end   = start + (PAGE_SIZE * CPU_ENTRY_AREA_PAGES);
 
-       pti_clone_pgtable(start, end, PTI_CLONE_PMD);
+       pti_clone_pgtable(start, end, PTI_CLONE_PMD, false);
 }
 #endif /* CONFIG_X86_64 */
 
@@ -492,11 +497,11 @@ static void __init pti_setup_espfix64(void)
 /*
  * Clone the populated PMDs of the entry text and force it RO.
  */
-static void pti_clone_entry_text(void)
+static void pti_clone_entry_text(bool late)
 {
        pti_clone_pgtable((unsigned long) __entry_text_start,
                          (unsigned long) __entry_text_end,
-                         PTI_LEVEL_KERNEL_IMAGE);
+                         PTI_LEVEL_KERNEL_IMAGE, late);
 }
 
 /*
@@ -571,7 +576,7 @@ static void pti_clone_kernel_text(void)
         * pti_set_kernel_image_nonglobal() did to clear the
         * global bit.
         */
-       pti_clone_pgtable(start, end_clone, PTI_LEVEL_KERNEL_IMAGE);
+       pti_clone_pgtable(start, end_clone, PTI_LEVEL_KERNEL_IMAGE, false);
 
        /*
         * pti_clone_pgtable() will set the global bit in any PMDs
@@ -638,8 +643,15 @@ void __init pti_init(void)
 
        /* Undo all global bits from the init pagetables in head_64.S: */
        pti_set_kernel_image_nonglobal();
+
        /* Replace some of the global bits just for shared entry text: */
-       pti_clone_entry_text();
+       /*
+        * This is very early in boot. Device and Late initcalls can do
+        * modprobe before free_initmem() and mark_readonly(). This
+        * pti_clone_entry_text() allows those user-mode-helpers to function,
+        * but notably the text is still RW.
+        */
+       pti_clone_entry_text(false);
        pti_setup_espfix64();
        pti_setup_vsyscall();
 }
@@ -656,10 +668,11 @@ void pti_finalize(void)
        if (!boot_cpu_has(X86_FEATURE_PTI))
                return;
        /*
-        * We need to clone everything (again) that maps parts of the
-        * kernel image.
+        * This is after free_initmem() (all initcalls are done) and we've done
+        * mark_readonly(). Text is now NX which might've split some PMDs
+        * relative to the early clone.
         */
-       pti_clone_entry_text();
+       pti_clone_entry_text(true);
        pti_clone_kernel_text();
 
        debug_checkwx_user();