Fix pagemap_read() use of struct mm_walk

Fix some issues in pagemap_read noted by Alexey:

- initialize pagemap_walk.mm to "mm" , so the code starts working as
  advertised

- initialize ->private to "&pm" so it wouldn't immediately oops in
  pagemap_pte_hole()

- unstatic struct pagemap_walk, so two threads won't fsckup each other
  (including those started by root, including flipping ->mm when you don't
  have permissions)

- pagemap_read() contains two calls to ptrace_may_attach(), second one
  looks unneeded.

- avoid possible kmalloc(0) and integer wraparound.

Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Matt Mackall <mpm@selenic.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
[ Personally, I'd just remove the functionality entirely  - Linus ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 38 insertions, 34 deletions

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 05053d701ac5..c492449f3b45 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -602,11 +602,6 @@ static int pagemap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
         return err;
 }
 
-static struct mm_walk pagemap_walk = {
-        .pmd_entry = pagemap_pte_range,
-        .pte_hole = pagemap_pte_hole
-};
-
 /*
  * /proc/pid/pagemap - an array mapping virtual pages to pfns
  *
@@ -641,6 +636,11 @@ static ssize_t pagemap_read(struct file *file, char __user *buf,
         struct pagemapread pm;
         int pagecount;
         int ret = -ESRCH;
+        struct mm_walk pagemap_walk;
+        unsigned long src;
+        unsigned long svpfn;
+        unsigned long start_vaddr;
+        unsigned long end_vaddr;
 
         if (!task)
                 goto out;
@@ -659,11 +659,15 @@ static ssize_t pagemap_read(struct file *file, char __user *buf,
         if (!mm)
                 goto out_task;
 
-        ret = -ENOMEM;
+
         uaddr = (unsigned long)buf & PAGE_MASK;
         uend = (unsigned long)(buf + count);
         pagecount = (PAGE_ALIGN(uend) - uaddr) / PAGE_SIZE;
-        pages = kmalloc(pagecount * sizeof(struct page *), GFP_KERNEL);
+        ret = 0;
+        if (pagecount == 0)
+                goto out_mm;
+        pages = kcalloc(pagecount, sizeof(struct page *), GFP_KERNEL);
+        ret = -ENOMEM;
         if (!pages)
                 goto out_mm;
 
@@ -684,33 +688,33 @@ static ssize_t pagemap_read(struct file *file, char __user *buf,
         pm.out = (u64 *)buf;
         pm.end = (u64 *)(buf + count);
 
-        if (!ptrace_may_attach(task)) {
-                ret = -EIO;
-        } else {
-                unsigned long src = *ppos;
-                unsigned long svpfn = src / PM_ENTRY_BYTES;
-                unsigned long start_vaddr = svpfn << PAGE_SHIFT;
-                unsigned long end_vaddr = TASK_SIZE_OF(task);
-
-                /* watch out for wraparound */
-                if (svpfn > TASK_SIZE_OF(task) >> PAGE_SHIFT)
-                        start_vaddr = end_vaddr;
-
-                /*
-                 * The odds are that this will stop walking way
-                 * before end_vaddr, because the length of the
-                 * user buffer is tracked in "pm", and the walk
-                 * will stop when we hit the end of the buffer.
-                 */
-                ret = walk_page_range(start_vaddr, end_vaddr,
-                                        &pagemap_walk);
-                if (ret == PM_END_OF_BUFFER)
-                        ret = 0;
-                /* don't need mmap_sem for these, but this looks cleaner */
-                *ppos += (char *)pm.out - buf;
-                if (!ret)
-                        ret = (char *)pm.out - buf;
-        }
+        pagemap_walk.pmd_entry = pagemap_pte_range;
+        pagemap_walk.pte_hole = pagemap_pte_hole;
+        pagemap_walk.mm = mm;
+        pagemap_walk.private = &pm;
+
+        src = *ppos;
+        svpfn = src / PM_ENTRY_BYTES;
+        start_vaddr = svpfn << PAGE_SHIFT;
+        end_vaddr = TASK_SIZE_OF(task);
+
+        /* watch out for wraparound */
+        if (svpfn > TASK_SIZE_OF(task) >> PAGE_SHIFT)
+                start_vaddr = end_vaddr;
+
+        /*
+         * The odds are that this will stop walking way
+         * before end_vaddr, because the length of the
+         * user buffer is tracked in "pm", and the walk
+         * will stop when we hit the end of the buffer.
+         */
+        ret = walk_page_range(start_vaddr, end_vaddr, &pagemap_walk);
+        if (ret == PM_END_OF_BUFFER)
+                ret = 0;
+        /* don't need mmap_sem for these, but this looks cleaner */
+        *ppos += (char *)pm.out - buf;
+        if (!ret)
+                ret = (char *)pm.out - buf;
 
 out_pages:
         for (; pagecount; pagecount--) {