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Date:   Fri, 22 Mar 2019 22:07:54 +0100
From:   Martin Blumenstingl <martin.blumenstingl@...glemail.com>
To:     Matthew Wilcox <willy@...radead.org>, liang.yang@...ogic.com
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, akpm@...ux-foundation.org,
        mhocko@...e.com, rppt@...ux.ibm.com,
        linux-amlogic@...ts.infradead.org, linux@...linux.org.uk,
        linux-mtd@...ts.infradead.org
Subject: Re: 32-bit Amlogic (ARM) SoC: kernel BUG in kfree()

Hi Matthew,

On Thu, Mar 21, 2019 at 10:44 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Thu, Mar 21, 2019 at 09:17:34PM +0100, Martin Blumenstingl wrote:
> > Hello,
> >
> > I am experiencing the following crash:
> >   ------------[ cut here ]------------
> >   kernel BUG at mm/slub.c:3950!
>
>         if (unlikely(!PageSlab(page))) {
>                 BUG_ON(!PageCompound(page));
>
> You called kfree() on the address of a page which wasn't allocated by slab.
>
> > I have traced this crash to the kfree() in meson_nfc_read_buf().
> > my observation is as follows:
> > - meson_nfc_read_buf() is called 7 times without any crash, the
> > kzalloc() call returns 0xe9e6c600 (virtual address) / 0x29e6c600
> > (physical address)
> > - the eight time meson_nfc_read_buf() is called kzalloc() call returns
> > 0xee39a38b (virtual address) / 0x2e39a38b (physical address) and the
> > final kfree() crashes
> > - changing the size in the kzalloc() call from PER_INFO_BYTE (= 8) to
> > PAGE_SIZE works around that crash
>
> I suspect you're doing something which corrupts memory.  Overrunning
> the end of your allocation or something similar.  Have you tried KASAN
> or even the various slab debugging (eg redzones)?
KASAN is not available on 32-bit ARM. there was some progress last
year [0] but it didn't make it into mainline. I tried to make the
patches apply again and got it to compile (and my kernel is still
booting) but I have no idea if it's still working. for anyone
interested, my patches are here: [1] (I consider this a HACK because I
don't know anything about the code which is being touched in the
patches, I only made it compile)

SLAB debugging (redzones) were a great hint, thank you very much for
that Matthew! I enabled:
  CONFIG_SLUB_DEBUG=y
  CONFIG_SLUB_DEBUG_ON=y
and with that I now get "BUG kmalloc-64 (Not tainted): Redzone
overwritten" (a larger kernel log extract is attached).

I'm starting to wonder if the NAND controller (hardware) writes more
than 8 bytes.
some context: the "info" buffer allocated in meson_nfc_read_buf is
then passed to the NAND controller IP (after using dma_map_single).

Liang, how does the NAND controller know that it only has to send
PER_INFO_BYTE (= 8) bytes when called from meson_nfc_read_buf? all
other callers of meson_nfc_dma_buffer_setup (which passes the info
buffer to the hardware) are using (nand->ecc.steps * PER_INFO_BYTE)
bytes?


Regards
Martin


[0] https://lore.kernel.org/patchwork/cover/913212/
[1] https://github.com/xdarklight/linux/tree/arm-kasan-hack-v5.1-rc1

View attachment "slub-redzones.txt" of type "text/plain" (23501 bytes)

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