x86 mmiotrace: fix save/restore page table state

From baa99e2b32449ec7bf147c234adfa444caecac8a Mon Sep 17 00:00:00 2001
From: Pekka Paalanen <pq@iki.fi>
Date: Sun, 22 Feb 2009 20:02:43 +0200

Blindly setting _PAGE_PRESENT in disarm_kmmio_fault_page() overlooks the
possibility, that the page was not present when it was armed.

Make arm_kmmio_fault_page() store the previous page presence in struct
kmmio_fault_page and use it on disarm.

This patch was originally written by Stuart Bennett, but Pekka Paalanen
rewrote it a little different.

Signed-off-by: Pekka Paalanen <pq@iki.fi>
Cc: Stuart Bennett <stuart@freedesktop.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 44 insertions, 22 deletions

diff --git a/arch/x86/mm/kmmio.c b/arch/x86/mm/kmmio.c
index fb1f11546fcd..be361eb828c8 100644
--- a/arch/x86/mm/kmmio.c
+++ b/arch/x86/mm/kmmio.c
@@ -32,6 +32,8 @@ struct kmmio_fault_page {
         struct list_head list;
         struct kmmio_fault_page *release_next;
         unsigned long page; /* location of the fault page */
+        bool old_presence; /* page presence prior to arming */
+        bool armed;
 
         /*
          * Number of times this page has been registered as a part
@@ -105,8 +107,7 @@ static struct kmmio_fault_page *get_kmmio_fault_page(unsigned long page)
         return NULL;
 }
 
-static int set_page_present(unsigned long addr, bool present,
-                                                        unsigned int *pglevel)
+static int set_page_presence(unsigned long addr, bool present, bool *old)
 {
         pteval_t pteval;
         pmdval_t pmdval;
@@ -119,20 +120,21 @@ static int set_page_present(unsigned long addr, bool present,
                 return -1;
         }
 
-        if (pglevel)
-                *pglevel = level;
-
         switch (level) {
         case PG_LEVEL_2M:
                 pmd = (pmd_t *)pte;
-                pmdval = pmd_val(*pmd) & ~_PAGE_PRESENT;
+                pmdval = pmd_val(*pmd);
+                *old = !!(pmdval & _PAGE_PRESENT);
+                pmdval &= ~_PAGE_PRESENT;
                 if (present)
                         pmdval |= _PAGE_PRESENT;
                 set_pmd(pmd, __pmd(pmdval));
                 break;
 
         case PG_LEVEL_4K:
-                pteval = pte_val(*pte) & ~_PAGE_PRESENT;
+                pteval = pte_val(*pte);
+                *old = !!(pteval & _PAGE_PRESENT);
+                pteval &= ~_PAGE_PRESENT;
                 if (present)
                         pteval |= _PAGE_PRESENT;
                 set_pte_atomic(pte, __pte(pteval));
@@ -148,19 +150,39 @@ static int set_page_present(unsigned long addr, bool present,
         return 0;
 }
 
-/** Mark the given page as not present. Access to it will trigger a fault. */
-static int arm_kmmio_fault_page(unsigned long page, unsigned int *pglevel)
+/*
+ * Mark the given page as not present. Access to it will trigger a fault.
+ *
+ * Struct kmmio_fault_page is protected by RCU and kmmio_lock, but the
+ * protection is ignored here. RCU read lock is assumed held, so the struct
+ * will not disappear unexpectedly. Furthermore, the caller must guarantee,
+ * that double arming the same virtual address (page) cannot occur.
+ *
+ * Double disarming on the other hand is allowed, and may occur when a fault
+ * and mmiotrace shutdown happen simultaneously.
+ */
+static int arm_kmmio_fault_page(struct kmmio_fault_page *f)
 {
-        int ret = set_page_present(page & PAGE_MASK, false, pglevel);
-        WARN_ONCE(ret < 0, KERN_ERR "kmmio arming 0x%08lx failed.\n", page);
+        int ret;
+        WARN_ONCE(f->armed, KERN_ERR "kmmio page already armed.\n");
+        if (f->armed) {
+                pr_warning("kmmio double-arm: page 0x%08lx, ref %d, old %d\n",
+                                        f->page, f->count, f->old_presence);
+        }
+        ret = set_page_presence(f->page, false, &f->old_presence);
+        WARN_ONCE(ret < 0, KERN_ERR "kmmio arming 0x%08lx failed.\n", f->page);
+        f->armed = true;
         return ret;
 }
 
-/** Mark the given page as present. */
-static void disarm_kmmio_fault_page(unsigned long page, unsigned int *pglevel)
+/** Restore the given page to saved presence state. */
+static void disarm_kmmio_fault_page(struct kmmio_fault_page *f)
 {
-        int ret = set_page_present(page & PAGE_MASK, true, pglevel);
-        WARN_ONCE(ret < 0, KERN_ERR "kmmio disarming 0x%08lx failed.\n", page);
+        bool tmp;
+        int ret = set_page_presence(f->page, f->old_presence, &tmp);
+        WARN_ONCE(ret < 0,
+                        KERN_ERR "kmmio disarming 0x%08lx failed.\n", f->page);
+        f->armed = false;
 }
 
 /*
@@ -207,7 +229,7 @@ int kmmio_handler(struct pt_regs *regs, unsigned long addr)
 
         ctx = &get_cpu_var(kmmio_ctx);
         if (ctx->active) {
-                disarm_kmmio_fault_page(faultpage->page, NULL);
+                disarm_kmmio_fault_page(faultpage);
                 if (addr == ctx->addr) {
                         /*
                          * On SMP we sometimes get recursive probe hits on the
@@ -249,7 +271,7 @@ int kmmio_handler(struct pt_regs *regs, unsigned long addr)
         regs->flags &= ~X86_EFLAGS_IF;
 
         /* Now we set present bit in PTE and single step. */
-        disarm_kmmio_fault_page(ctx->fpage->page, NULL);
+        disarm_kmmio_fault_page(ctx->fpage);
 
         /*
          * If another cpu accesses the same page while we are stepping,
@@ -288,7 +310,7 @@ static int post_kmmio_handler(unsigned long condition, struct pt_regs *regs)
         if (ctx->probe && ctx->probe->post_handler)
                 ctx->probe->post_handler(ctx->probe, condition, regs);
 
-        arm_kmmio_fault_page(ctx->fpage->page, NULL);
+        arm_kmmio_fault_page(ctx->fpage);
 
         regs->flags &= ~X86_EFLAGS_TF;
         regs->flags |= ctx->saved_flags;
@@ -320,19 +342,19 @@ static int add_kmmio_fault_page(unsigned long page)
         f = get_kmmio_fault_page(page);
         if (f) {
                 if (!f->count)
-                        arm_kmmio_fault_page(f->page, NULL);
+                        arm_kmmio_fault_page(f);
                 f->count++;
                 return 0;
         }
 
-        f = kmalloc(sizeof(*f), GFP_ATOMIC);
+        f = kzalloc(sizeof(*f), GFP_ATOMIC);
         if (!f)
                 return -1;
 
         f->count = 1;
         f->page = page;
 
-        if (arm_kmmio_fault_page(f->page, NULL)) {
+        if (arm_kmmio_fault_page(f)) {
                 kfree(f);
                 return -1;
         }
@@ -356,7 +378,7 @@ static void release_kmmio_fault_page(unsigned long page,
         f->count--;
         BUG_ON(f->count < 0);
         if (!f->count) {
-                disarm_kmmio_fault_page(f->page, NULL);
+                disarm_kmmio_fault_page(f);
                 f->release_next = *release_list;
                 *release_list = f;
         }