Fix incorrect user space access locking in mincore() (CVE-2006-4814)

Doug Chapman noticed that mincore() will doa "copy_to_user()" of the
result while holding the mmap semaphore for reading, which is a big
no-no.  While a recursive read-lock on a semaphore in the case of a page
fault happens to work, we don't actually allow them due to deadlock
schenarios with writers due to fairness issues.

Doug and Marcel sent in a patch to fix it, but I decided to just rewrite
the mess instead - not just fixing the locking problem, but making the
code smaller and (imho) much easier to understand.

Also included are two fixes for the original patch including one
by Oleg Nesterov.
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Adrian Bunk <bunk@stusta.de>

mm/mincore.c

 /*
  *	linux/mm/mincore.c
  *
- * Copyright (C) 1994-1999  Linus Torvalds
+ * Copyright (C) 1994-2006  Linus Torvalds
  */
 
 /*
@@ -38,46 +38,51 @@ static unsigned char mincore_page(struct vm_area_struct * vma,
 	return present;
 }
 
-static long mincore_vma(struct vm_area_struct * vma,
-	unsigned long start, unsigned long end, unsigned char __user * vec)
+/*
+ * Do a chunk of "sys_mincore()". We've already checked
+ * all the arguments, we hold the mmap semaphore: we should
+ * just return the amount of info we're asked for.
+ */
+static long do_mincore(unsigned long addr, unsigned char *vec, unsigned long pages)
 {
-	long error, i, remaining;
-	unsigned char * tmp;
-
-	error = -ENOMEM;
-	if (!vma->vm_file)
-		return error;
-
-	start = ((start - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
-	if (end > vma->vm_end)
-		end = vma->vm_end;
-	end = ((end - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
+	unsigned long i, nr, pgoff;
+	struct vm_area_struct *vma = find_vma(current->mm, addr);
 
-	error = -EAGAIN;
-	tmp = (unsigned char *) __get_free_page(GFP_KERNEL);
-	if (!tmp)
-		return error;
+	/*
+	 * find_vma() didn't find anything above us, or we're
+	 * in an unmapped hole in the address space: ENOMEM.
+	 */
+	if (!vma || addr < vma->vm_start)
+		return -ENOMEM;
 
-	/* (end - start) is # of pages, and also # of bytes in "vec */
-	remaining = (end - start),
+	/*
+	 * Ok, got it. But check whether it's a segment we support
+	 * mincore() on. Right now, we don't do any anonymous mappings.
+	 *
+	 * FIXME: This is just stupid. And returning ENOMEM is 
+	 * stupid too. We should just look at the page tables. But
+	 * this is what we've traditionally done, so we'll just
+	 * continue doing it.
+	 */
+	if (!vma->vm_file)
+		return -ENOMEM;
 
-	error = 0;
-	for (i = 0; remaining > 0; remaining -= PAGE_SIZE, i++) {
-		int j = 0;
-		long thispiece = (remaining < PAGE_SIZE) ?
-						remaining : PAGE_SIZE;
+	/*
+	 * Calculate how many pages there are left in the vma, and
+	 * what the pgoff is for our address.
+	 */
+	nr = (vma->vm_end - addr) >> PAGE_SHIFT;
+	if (nr > pages)
+		nr = pages;
 
-		while (j < thispiece)
-			tmp[j++] = mincore_page(vma, start++);
+	pgoff = (addr - vma->vm_start) >> PAGE_SHIFT;
+	pgoff += vma->vm_pgoff;
 
-		if (copy_to_user(vec + PAGE_SIZE * i, tmp, thispiece)) {
-			error = -EFAULT;
-			break;
-		}
-	}
+	/* And then we just fill the sucker in.. */
+	for (i = 0 ; i < nr; i++, pgoff++)
+		vec[i] = mincore_page(vma, pgoff);
 
-	free_page((unsigned long) tmp);
-	return error;
+	return nr;
 }
 
 /*
@@ -107,82 +112,50 @@ static long mincore_vma(struct vm_area_struct * vma,
 asmlinkage long sys_mincore(unsigned long start, size_t len,
 	unsigned char __user * vec)
 {
-	int index = 0;
-	unsigned long end, limit;
-	struct vm_area_struct * vma;
-	size_t max;
-	int unmapped_error = 0;
-	long error;
-
-	/* check the arguments */
- 	if (start & ~PAGE_CACHE_MASK)
-		goto einval;
+	long retval;
+	unsigned long pages;
+	unsigned char *tmp;
 
-	limit = TASK_SIZE;
-	if (start >= limit)
-		goto enomem;
-
-	if (!len)
-		return 0;
+	/* Check the start address: needs to be page-aligned.. */
+ 	if (start & ~PAGE_CACHE_MASK)
+		return -EINVAL;
 
-	max = limit - start;
-	len = PAGE_CACHE_ALIGN(len);
-	if (len > max || !len)
-		goto enomem;
+	/* ..and we need to be passed a valid user-space range */
+	if (!access_ok(VERIFY_READ, (void __user *) start, len))
+		return -ENOMEM;
 
-	end = start + len;
+	/* This also avoids any overflows on PAGE_CACHE_ALIGN */
+	pages = len >> PAGE_SHIFT;
+	pages += (len & ~PAGE_MASK) != 0;
 
-	/* check the output buffer whilst holding the lock */
-	error = -EFAULT;
-	down_read(&current->mm->mmap_sem);
+	if (!access_ok(VERIFY_WRITE, vec, pages))
+		return -EFAULT;
 
-	if (!access_ok(VERIFY_WRITE, vec, len >> PAGE_SHIFT))
-		goto out;
+	tmp = (void *) __get_free_page(GFP_USER);
+	if (!tmp)
+		return -EAGAIN;
 
+	retval = 0;
+	while (pages) {
 		/*
-	 * If the interval [start,end) covers some unmapped address
-	 * ranges, just ignore them, but return -ENOMEM at the end.
+		 * Do at most PAGE_SIZE entries per iteration, due to
+		 * the temporary buffer size.
 		 */
-	error = 0;
-
-	vma = find_vma(current->mm, start);
-	while (vma) {
-		/* Here start < vma->vm_end. */
-		if (start < vma->vm_start) {
-			unmapped_error = -ENOMEM;
-			start = vma->vm_start;
-		}
+		down_read(&current->mm->mmap_sem);
+		retval = do_mincore(start, tmp, min(pages, PAGE_SIZE));
+		up_read(&current->mm->mmap_sem);
 
-		/* Here vma->vm_start <= start < vma->vm_end. */
-		if (end <= vma->vm_end) {
-			if (start < end) {
-				error = mincore_vma(vma, start, end,
-							&vec[index]);
-				if (error)
-					goto out;
-			}
-			error = unmapped_error;
-			goto out;
+		if (retval <= 0)
+			break;
+		if (copy_to_user(vec, tmp, retval)) {
+			retval = -EFAULT;
+			break;
 		}
-
-		/* Here vma->vm_start <= start < vma->vm_end < end. */
-		error = mincore_vma(vma, start, vma->vm_end, &vec[index]);
-		if (error)
-			goto out;
-		index += (vma->vm_end - start) >> PAGE_CACHE_SHIFT;
-		start = vma->vm_end;
-		vma = vma->vm_next;
+		pages -= retval;
+		vec += retval;
+		start += retval << PAGE_SHIFT;
+		retval = 0;
 	}
-
-	/* we found a hole in the area queried if we arrive here */
-	error = -ENOMEM;
-
-out:
-	up_read(&current->mm->mmap_sem);
-	return error;
-
-einval:
-	return -EINVAL;
-enomem:
-	return -ENOMEM;
+	free_page((unsigned long) tmp);
+	return retval;
 }