linux-kernel - Re: 3.6rc6 slab corruption.

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Date:	Wed, 19 Sep 2012 14:49:46 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Konrad Rzeszutek Wilk <konrad@...nel.org>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Suzuki Poulose <suzuki@...ibm.com>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
Subject: Re: 3.6rc6 slab corruption.

On Wed, 19 Sep 2012, David Rientjes wrote:

> > From 0806b133b5b28081adf23d0d04a99636ed3b861b Mon Sep 17 00:00:00 2001
> > From: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> > Date: Wed, 19 Sep 2012 11:23:01 -0400
> > Subject: [PATCH 1/2] debugfs: Add lock for u32_array_read
> > 
> > Dave Jones spotted that the u32_array_read was doing something funny:
> > 
> > =============================================================================
> > BUG kmalloc-64 (Not tainted): Redzone overwritten
> > -----------------------------------------------------------------------------
> > 
> > INFO: 0xffff88001f4b4970-0xffff88001f4b4977. First byte 0xbb instead of 0xcc
> > INFO: Allocated in u32_array_read+0xd1/0x110 age=0 cpu=6 pid=32767
> >         __slab_alloc+0x516/0x5a5
> >         __kmalloc+0x213/0x2c0
> >         u32_array_read+0xd1/0x110
> > .. snip..
> > INFO: Freed in u32_array_read+0x99/0x110 age=0 cpu=0 pid=32749
> >         __slab_free+0x3f/0x3bf
> >         kfree+0x2d5/0x310
> >         u32_array_read+0x99/0x110
> > 
> > Linus tracked it down and found out that "debugfs is racy for that case
> > [read calls in parallel on the debugfs]. At least the file->private_data
> > accesses are, for the case of that "u32_array" case.
> > 
> > In fact it is racy in ...  the whole "file->private_data" access ..
> > If you have multiple readers on the same file, the whole
> > 
> > 	if (file->private_data) {
> > 		kfree(file->private_data);
> > 		file->private_data = NULL;
> > 	}
> > 
> > 	file->private_data = format_array_alloc("%u", data->array,
> >                                                               data->elements);
> > 
> > thing is just a disaster waiting to happen." He suggested
> > putting a lock which this patch does.
> > 
> 
> Since these are non-seekable files, it must also race to find *ppos == 0.
> 
> > The consequence of this is that it will trigger more spinlock usage,
> > as this particular debugfs is used to provide a histogram of spinlock
> > contention. But memory corruption is a worst offender then that.
> > 
> > Reported-by: Dave Jones <davej@...hat.com>
> > Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> 
> Tested-by: David Rientjes <rientjes@...gle.com>
> 

An alternative to this, though, might be to never test for *ppos == 0 in 
u32_array_read() and do the format_array_alloc() in u32_array_open() to 
initialize file->private_data.  If that allocation fails, just return 
-ENOMEM.  Then you never need to add a mutex in the read path.

Any reason we can't do this?
---
 fs/debugfs/file.c |   33 +++++++++++----------------------
 1 file changed, 11 insertions(+), 22 deletions(-)

diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
--- a/fs/debugfs/file.c
+++ b/fs/debugfs/file.c
@@ -526,12 +526,6 @@ struct array_data {
 	u32 elements;
 };
 
-static int u32_array_open(struct inode *inode, struct file *file)
-{
-	file->private_data = NULL;
-	return nonseekable_open(inode, file);
-}
-
 static size_t format_array(char *buf, size_t bufsize, const char *fmt,
 			   u32 *array, u32 array_size)
 {
@@ -573,26 +567,21 @@ static char *format_array_alloc(const char *fmt, u32 *array,
 	return ret;
 }
 
-static ssize_t u32_array_read(struct file *file, char __user *buf, size_t len,
-			      loff_t *ppos)
+static int u32_array_open(struct inode *inode, struct file *file)
 {
-	struct inode *inode = file->f_path.dentry->d_inode;
 	struct array_data *data = inode->i_private;
-	size_t size;
 
-	if (*ppos == 0) {
-		if (file->private_data) {
-			kfree(file->private_data);
-			file->private_data = NULL;
-		}
-
-		file->private_data = format_array_alloc("%u", data->array,
-							      data->elements);
-	}
+	file->private_data = format_array_alloc("%u", data->array,
+						      data->elements);
+	if (!file->private_data)
+		return -ENOMEM;
+	return nonseekable_open(inode, file);
+}
 
-	size = 0;
-	if (file->private_data)
-		size = strlen(file->private_data);
+static ssize_t u32_array_read(struct file *file, char __user *buf, size_t len,
+			      loff_t *ppos)
+{
+	size_t size = strlen(file->private_data);
 
 	return simple_read_from_buffer(buf, len, ppos,
 					file->private_data, size);
--
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