Re: 3.6rc6 slab corruption.

[Konrad Rzeszutek Wilk]

On Tue, Sep 18, 2012 at 11:38:44AM -0700, Linus Torvalds wrote:
> Quoting the entire email, since I added Greg to the list of people (as
> the documented maintainer of debugfs) along with what I think are the
> guilty parties.
> 
> Dave, is trinity perhaps doing read calls on the same file in parallel?
> 
> Because it looks to me like debugfs is racy for that case. At least
> the file->private_data accesses are, for the case of that "u32_array"
> case.
> 
> In fact it is racy in two different ways:
> 
>  - the whole "file->private_data" access is racy (and this is, I
> think, the bug you are hitting)
> 
>    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.
> 
> It should be easyish to fix by just adding a lock around those things.

Like this:

diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
index 2340f69..309b235 100644
--- a/fs/debugfs/file.c
+++ b/fs/debugfs/file.c
@@ -524,6 +524,7 @@ EXPORT_SYMBOL_GPL(debugfs_create_blob);
 struct array_data {
 	void *array;
 	u32 elements;
+	struct mutex lock;
 };
 
 static int u32_array_open(struct inode *inode, struct file *file)
@@ -580,6 +581,7 @@ static ssize_t u32_array_read(struct file *file, char __user *buf, size_t len,
 	struct array_data *data = inode->i_private;
 	size_t size;
 
+	mutex_lock(&data->lock);
 	if (*ppos == 0) {
 		if (file->private_data) {
 			kfree(file->private_data);
@@ -594,6 +596,7 @@ static ssize_t u32_array_read(struct file *file, char __user *buf, size_t len,
 	if (file->private_data)
 		size = strlen(file->private_data);
 
+	mutex_unlock(&data->lock);
 	return simple_read_from_buffer(buf, len, ppos,
 					file->private_data, size);
 }
@@ -643,6 +646,7 @@ struct dentry *debugfs_create_u32_array(const char *name, umode_t mode,
 
 	data->array = array;
 	data->elements = elements;
+	mutex_init(&data->lock);
 
 	return debugfs_create_file(name, mode, parent, data, &u32_array_fops);
 }
> 
> The other bug is slightly subtler and probably harder to hit (but also
> harder to fix):
> 
>  - the whole format_array_alloc() code is one buggy piece of sh*t,
> since afaik there is nothing that guarantees that the values cannot
> change. So the notion of "let's format the output once to know how big
> it is, and then a second time to actually print things into the array
> we just allocated based on the first time" is pure and utter garbage,
> afaik.

Yikes. The fix could be to allocate a buffer large enough for the maximum
that %u could take * array_size and not bother with the first pass.
> 
> Anyway, this is all fairly recent, and came in through the Xen tree.
> See commit 9fe2a7015393 ("debugfs: Add support to print u32 array in
> debugfs").
> 
> Guys?

Let me include Jeremy on this. He was the original author of it and
perhaps he has some patch stashed away were he re-implemented it..

> 
>               Linus
> 
> On Tue, Sep 18, 2012 at 7:35 AM, Dave Jones <davej@xxxxxxxxxx> wrote:
> > I was chasing a networking bug, and had trinity reduced to just making read & setsockopt calls,
> > and let that run overnight.  I woke up to 800mb of traces from a different bug..
> >
> > The traces look mostly like this..
> >
> >
> > =============================================================================
> > 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
> >         vfs_read+0xac/0x180
> >         sys_read+0x4d/0x90
> >         system_call_fastpath+0x1a/0x1f
> > 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
> >         vfs_read+0xac/0x180
> >         sys_read+0x4d/0x90
> >         system_call_fastpath+0x1a/0x1f
> > INFO: Slab 0xffffea00007d2d00 objects=41 used=14 fp=0xffff88001f4b7410 flags=0x10000000004081
> > INFO: Object 0xffff88001f4b4930 @offset=2352 fp=0xffff88001f4b7410
> >
> > Bytes b4 ffff88001f4b4920: 1b 20 1c 00 01 00 00 00 5a 5a 5a 5a 5a 5a 5a 5a  . ......ZZZZZZZZ
> > Object ffff88001f4b4930: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
> > Object ffff88001f4b4940: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
> > Object ffff88001f4b4950: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
> > Object ffff88001f4b4960: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b a5  kkkkkkkkkkkkkkk.
> > Redzone ffff88001f4b4970: bb bb bb bb bb bb bb bb                          ........
> > Padding ffff88001f4b4ab0: 5a 5a 5a 5a 5a 5a 5a 5a                          ZZZZZZZZ
> > Pid: 32756, comm: trinity-child52 Not tainted 3.6.0-rc6+ #44
> > Call Trace:
> >  [<ffffffff811c10ad>] ? print_section+0x3d/0x40
> >  [<ffffffff811c23fe>] print_trailer+0xfe/0x160
> >  [<ffffffff811c2592>] check_bytes_and_report+0xe2/0x120
> >  [<ffffffff81023b79>] ? native_sched_clock+0x19/0x80
> >  [<ffffffff811c2b5b>] check_object+0x18b/0x250
> >  [<ffffffff8169b7d7>] free_debug_processing+0xc0/0x1fd
> >  [<ffffffff812d2e29>] ? u32_array_read+0x99/0x110
> >  [<ffffffff8169ba5c>] __slab_free+0x3f/0x3bf
> >  [<ffffffff81365a1c>] ? debug_check_no_obj_freed+0x16c/0x210
> >  [<ffffffff810db04f>] ? lock_release_holdtime.part.26+0xf/0x180
> >  [<ffffffff812d2e29>] ? u32_array_read+0x99/0x110
> >  [<ffffffff811c5725>] kfree+0x2d5/0x310
> >  [<ffffffff812d2e29>] u32_array_read+0x99/0x110
> >  [<ffffffff811df88c>] vfs_read+0xac/0x180
> >  [<ffffffff811df9ad>] sys_read+0x4d/0x90
> >  [<ffffffff816aea2d>] system_call_fastpath+0x1a/0x1f
> > FIX kmalloc-64: Restoring 0xffff88001f4b4970-0xffff88001f4b4977=0xcc
> > =============================================================================
> >
> > Which looks like we read some file (probably something in sysfs/procfs) that corrupted some internal state.
> > Any ideas on what I could do to narrow this down ?
> >
> > The full traces are at http://www.codemonkey.org.uk/junk/slab-corrupt.txt
> > They vary a little later, but it looks like it's probably all the same problem to me.
> > Sometimes it flip-flops between "First byte 0xbb instead of 0xcc" and "First byte 0xcc instead of 0xbb"
> >
> >
> > The one outlier being this weird message..
> >
> > Sep 18 02:00:13 bitcrush kernel: [36617.487681] hrtimer: interrupt took 242337 ns
> >
> > Which is weird, but probably unrelated.
> >
> >         Dave
> >
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