linux-kernel - Re: [PATCH 1/5] clear_refs: Sanitize accepted commands declaration

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Date:	Thu, 11 Apr 2013 14:17:35 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Pavel Emelyanov <xemul@...allels.com>
Cc:	Linux MM <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/5] clear_refs: Sanitize accepted commands declaration

On Thu, 11 Apr 2013 15:28:51 +0400 Pavel Emelyanov <xemul@...allels.com> wrote:

> A new clear-refs type will be added in the next patch, so prepare
> code for that.
> 
> @@ -730,7 +733,7 @@ static ssize_t clear_refs_write(struct file *file, const char __user *buf,
>  	char buffer[PROC_NUMBUF];
>  	struct mm_struct *mm;
>  	struct vm_area_struct *vma;
> -	int type;
> +	enum clear_refs_types type;
>  	int rv;
>  
>  	memset(buffer, 0, sizeof(buffer));
> @@ -738,10 +741,10 @@ static ssize_t clear_refs_write(struct file *file, const char __user *buf,
>  		count = sizeof(buffer) - 1;
>  	if (copy_from_user(buffer, buf, count))
>  		return -EFAULT;
> -	rv = kstrtoint(strstrip(buffer), 10, &type);
> +	rv = kstrtoint(strstrip(buffer), 10, (int *)&type);

This is naughty.  The compiler is allowed to put the enum into storage
which is smaller (or, I guess, larger) than sizeof(int).  I've seen one
compiler which puts such an enum into a 16-bit word.

--- a/fs/proc/task_mmu.c~clear_refs-sanitize-accepted-commands-declaration-fix
+++ a/fs/proc/task_mmu.c
@@ -734,6 +734,7 @@ static ssize_t clear_refs_write(struct f
 	struct mm_struct *mm;
 	struct vm_area_struct *vma;
 	enum clear_refs_types type;
+	int itype;
 	int rv;
 
 	memset(buffer, 0, sizeof(buffer));
@@ -741,9 +742,10 @@ static ssize_t clear_refs_write(struct f
 		count = sizeof(buffer) - 1;
 	if (copy_from_user(buffer, buf, count))
 		return -EFAULT;
-	rv = kstrtoint(strstrip(buffer), 10, (int *)&type);
+	rv = kstrtoint(strstrip(buffer), 10, &itype);
 	if (rv < 0)
 		return rv;
+	type = (enum clear_refs_types)itype;
 	if (type < CLEAR_REFS_ALL || type >= CLEAR_REFS_LAST)
 		return -EINVAL;
 	task = get_proc_task(file_inode(file));
_

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