linux-kernel - Re: [devkmsg_write] BUG: KASAN: slab-out-of-bounds in copyin+0xea/0x170

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Date:   Tue, 7 Nov 2017 21:05:25 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Fengguang Wu <fengguang.wu@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Aleksey Makarov <aleksey.makarov@...aro.org>,
        Ingo Molnar <mingo@...nel.org>,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>
Subject: Re: [devkmsg_write] BUG: KASAN: slab-out-of-bounds in
 copyin+0xea/0x170

On (11/07/17 12:09), Dmitry Vyukov wrote:
> > On (11/07/17 17:39), Fengguang Wu wrote:
[..]
> > devkmsg_write() does
> >
> >         buf = kmalloc(len+1, GFP_KERNEL);
> >         ...
> >         kfree(buf);
> >
> > kasan reports that this kfree() is actually happening in unpack_to_rootfs(),
> > before we do copy_from_iter_full().
> 
> 
> Please ignore the free stack. For slab-out-of-bound bugs the object is
> not actually freed and KASAN prints the free stack where it was freed
> before it was re-allocated as new object.

ah, ok.

> Can that len+1 overflow? Is it checked?

hm, I don't think it overflows there.

	__kernel_write()
	 __vfs_write()
	  new_sync_write()

__kernel_write() makes sure that if count > MAX_RW_COUNT then
count = MAX_RW_COUNT.

	-ss