Re: gpmc-nand broken since v4.12

[Boris Brezillon <boris.brezillon@free-electrons.com>]

Hi Roger,

On Mon, 16 Oct 2017 13:22:04 +0300
Roger Quadros <rogerq@ti.com> wrote:

> 
> Texas Instruments Finland Oy, Porkkalankatu 22, 00180 Helsinki. Y-tunnus/Business ID: 0615521-4. Kotipaikka/Domicile: Helsinki
> 
> On 13/10/17 23:29, Boris Brezillon wrote:
> > On Fri, 13 Oct 2017 22:24:58 +0200
> > Boris Brezillon <boris.brezillon@free-electrons.com> wrote:
> >   
> >> On Fri, 13 Oct 2017 14:50:33 +0200
> >> Boris Brezillon <boris.brezillon@free-electrons.com> wrote:
> >>  
> >>> On Fri, 13 Oct 2017 14:57:29 +0300
> >>> Roger Quadros <rogerq@ti.com> wrote:
> >>>     
> >>>> Hi Boris,
> >>>>
> >>>> NAND on gpmc-omap breaks for me while doing a unmount of a ubi volume since v4.12
> >>>>
> >>>> Behaviour follows through in v4.13 and v4.14-rc as well.
> >>>>
> >>>> Do you have any idea about this?    
> >>
> >> More info on what has changed in 4.12: we no longer allocate the
> >> nand_buffer struct + its internal buffer using a single big kmalloc
> >> call, which means the nand_buffer struct is now likely to be allocated
> >> in a small object slab (sizeof(nand_buffers) = 12). If you have a
> >> use-after-free bug somewhere in the kernel, it might corrupt the  
> 
> Spot on. Problem starts from commit 4d5dea5725f3aa95b9b64e252540e435dd216dbc
> "mtd: nand: allocate aligned buffers if NAND_OWN_BUFFERS is unset"
> 
> > 
> > I meant buffer-overflow, not use-after-free.  
> 
> Your analysis seems correct.
> 
> >   
> >> nand_buffers object, which might explain the bug you see here.
> >>  
> >>>
> >>> Can you try with this patch [1] applied and paste me the values printed
> >>> just before the crash?
> >>>
> >>> [1]http://code.bulix.org/lc8xk0-209746  
> 
> == unmounting volume
> [   36.308792] nand: nand_write_subpage_hwecc:2575 ecc_calc =   (null) oob_poi = ed096800
> [   36.317319] mtd_ooblayout_set_bytes:1330 dst = ed096802 src =   (null)
> [   36.324162] Unable to handle kernel NULL pointer dereference at virtual address 00000000
> 
> 
> Running the following patch
> https://hastebin.com/ulogurutuz.php
> shows
> 
> [   37.260689] nand: nand_write_subpage_hwecc:2547:A ecc_calc = ed116e40 oob_poi = ed117800
> [   37.260772] nand: nand_write_subpage_hwecc:2556:A1:step 0, ecc_calc = ed116e40
> [   37.260779] nand: nand_write_subpage_hwecc:2562:A2:step 0, ecc_calc = ed116e40
> [   37.260834] nand: nand_write_subpage_hwecc:2556:A1:step 1, ecc_calc = ed116e40
> [   37.260840] nand: nand_write_subpage_hwecc:2567:A3-pre:step 1, ecc_calc = ed116e40
> [   37.260846] omap_calculate_ecc_bch
> [   37.260856] nand: nand_write_subpage_hwecc:2570:A3-post:step 1, ecc_calc =   (null)
> [   37.260912] nand: nand_write_subpage_hwecc:2556:A1:step 2, ecc_calc =   (null)
> [   37.260918] nand: nand_write_subpage_hwecc:2562:A2:step 2, ecc_calc =   (null)
> [   37.260972] nand: nand_write_subpage_hwecc:2556:A1:step 3, ecc_calc =   (null)
> [   37.260978] nand: nand_write_subpage_hwecc:2562:A2:step 3, ecc_calc =   (null)
> [   37.260984] nand: nand_write_subpage_hwecc:2587:B ecc_calc =   (null) oob_poi = ed117800
> [   37.260991] mtd_ooblayout_set_bytes:1330 dst = ed117802 src =   (null)
> 
> which means omap_calculate_ecc_bch() it the culprit.
> 
> Looks like the function calculates and stores ECC for all sectors even if we just want ECC
> for just one sector (sub-page).

Yes, looks like you find the root cause.

> 
> Is my understanding correct
> - We should not be hooking the multi-sector ECC calculator omap_calculate_ecc_bch() to ecc.calculate
> - provide a new one sector ECC calculator function (for BCH4/8/16) for omap and hook it to ecc.calculate
> OR
> - override nand_read_subpage() and nand_write_subpage() using omap specific implementation (for BCH4/8/16).

Second solution sounds simpler because the ECC sector information is
not passed to ecc->calculate() hook, which means you'd have to extract
it from the ecc_calc pointer:

	(uintptr_t)(ecc_calc - chip->buffers->ecccalc) / chip->ecc.size

and doing arithmetic on pointers is usually not a good idea.

Anyway, I'd be fine with both solutions, so pick the one you prefer.

Regards,

Boris