Subject: [Bug 204371] BUG kmalloc-4k (Tainted: G W ): Object padding overwritten

From: [email-masked]
Date: Sat, 17 Aug 2019 08:09:51 +0000

https://bugzilla.kernel.org/show_bug.cgi?id=204371

--- Comment #38 from Christophe Leroy ([email-masked]) ---
Le 30/07/2019 à 20:52, [email-masked] a écrit :
> https://bugzilla.kernel.org/show_bug.cgi?id=204371
> 
> --- Comment #2 from Andrew Morton ([email-masked]) ---
> (switched to email.  Please respond via emailed reply-to-all, not via the
> bugzilla web interface).

Reply all replies to [email-masked] only.

[...]

> 
> cc'ing various people here.

Hum ... only got that email through the bugzilla interface, and CC'ed 
people don't show up.

> 
> I suspect proc_cgroup_show() is innocent and that perhaps
> bpf_prepare_filter() had a memory scribble.  iirc there has been at
> least one recent pretty serious bpf fix applied recently.  Can others
> please take a look?
> 
> (Seriously - please don't modify this report via the bugzilla web interface!)
> 

Haven't got the original CC'ed list, so please reply with missing Cc's 
if any.

We have well progressed on this case.

Erhard made a relation being this "Object padding overwritten" issue 
arising on any driver, and the presence of the BTRFS driver.

Then he was able to bisect the issue to:

commit 69d2480456d1baf027a86e530989d7bedd698d5f
Author: David Sterba <[email-masked]>
Date:   Fri Jun 29 10:56:44 2018 +0200

     btrfs: use copy_page for copying pages instead of memcpy

     Use the helper that's possibly optimized for full page copies.

     Signed-off-by: David Sterba <[email-masked]>

After looking in the code, it has appeared that some of the said "pages" 
were allocated with "kzalloc()".

Using the patch https://patchwork.ozlabs.org/patch/1148033/ Erhard 
confirmed that some btrfs functions were calling copy_page() with 
misaligned destinations.

copy_page(), at least on powerpc, expects cache aligned destination.

The patch https://patchwork.ozlabs.org/patch/1148606/ fixes the issue.

Christophe

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