[PATCH 0/4] fs: ubifs: Fix crash and add safeguards

Thu Aug 1 08:53:39 CEST 2024

Hi all

On Thu, Aug 1, 2024 at 8:50 AM Alexander Dahl <ada at thorsis.com> wrote:
>
> Hei,
>
> Am Thu, Jul 04, 2024 at 10:18:55AM +0200 schrieb Alexander Dahl:
> > Hello Heiko,
> >
> > Am Thu, Jul 04, 2024 at 06:28:31AM +0200 schrieb Heiko Schocher:
> > > Hello Alexander,
> > >
> > > On 03.07.24 12:12, Alexander Dahl wrote:
> > > > Hei hei,
> > > >
> > > > filesystem handling is different in U-Boot and beyond that UBI/UBIFS is
> > > > different from other filesystems in U-Boot.  There's UBI and UBIFS code
> > > > ported from Linux (quite old already now, maybe someone wants to update
> > > > that?), and there's "glue code" or "wrapper code" to interface with
> > > > U-Boot scripts, commands, and filesystem handling.  The fixes and
> > > > improvements in this patch series are for this U-Boot specific glue
> > > > code.
> > >
> > > Yes, the linux base is very old ... patches are welcome!
> >
> > The last sync was back in 2015 from linux v4.2, there were 800+
> > changes to ubi/ubifs in Linux since then. :-/
> >
> > > And for me it is not that easy, as I do not have a hardware with
> > > current mainline U-Boot running on it... I want to update a hardware
> > > I have to current mainline, but I had no time yet for it...
> >
> > Besides the custom hardware here, I used Microchip SAM9X60-Curiosity
> > lately, which is coming with a raw NAND flash and can boot from it.
> >

I read it now, I have one mainline board with ubifs running on it. If the patch
are not get applied I will take a look today of the thread. Sync to
newest version
of the kernel it's a good idea. I will check if someone in the company
can start this task

Michael

> > >
> > > > I'm no filesystem expert, but after days of debugging I'm quite sure the
> > > > bug is in U-Boot since UBIFS support was added in 2009, and it was
> > > > repeated in 2015 when generic filesystem support for UBIFS was added.
> > > > So please review carefully!
> > >
> > > Which bug?
> >
> > The memory leak and double free fixed with patch 1 of the series.
> >
> > >
> > > > The crashes were not easily reproducible, only with boards using the old
> > > > distroboot _and_ a boot script inspired by (but not equal to) the one
> > > > proposed by RAUC [1], which basically boils down to:
> > > >
> > > >    ubifsmount ubi0:boot (from distroboot)
> > > >    test -e (from distroboot)
> > > >    ubifsmount ubi0:rootfs1 (this time from the boot script,
> > > >                             triggering a ubifs_umount)
> > >
> > > So, you have a special sequence you execute to trigger the bug, good!
> > >
> > > In special 2 ubifsmount in a row... may not often needed for booting!
> > > (I ask me, why that is needed? Boottime is not good than...)
> >
> > Using distroboot with a script here.  The script is in a separate UBI
> > volume ubi0:boot, kernel is loaded from ubi0:rootfs1 or ubi0:rootfs2
> > however.  So there is 'ubifsmount ubi0:boot' from distroboot and in the
> > script found, loaded, and run there is 'ubifsmount ubi0:rootfs1' (or
> > rootfs2) later.  ubifsmount calls ubifsumount internally if some
> > volume is mounted already.
> >
> > >
> > > BTW: Is this really a good bootcmd in [1] as on *every* boot your
> > >      Environment is saved? This is not good for lifetime of your
> > >      storage device ... why not using bootcounter?
> >
> > Well, I was not aware of bootcounter, but I had a look and the actual
> > counter must be stored somewhere.  Possible are:
> >
> > - pmic → has no storage possibility on my board
> > - rtc → soc internal only, volatile in the end (if battery is empty)
> > - i2c eeprom → missing
> > - spi flash → missing
> > - filesystem → ends up on the flash
> > - nvmem → no other nvmems present
> > - syscon or some cpu register or sram → volatile
> >
> > So none of these are possible in my case, I only have a raw NAND as
> > storage and thus I have to use U-Boot env, which is stored in UBI here
> > btw to not stress the flash too much.
> >
> > I could investigate if it would be possible to let RAUC use the
> > U-Boot bootcounter infrastructure, but currently RAUC depends on
> > U-Boot environment variables for tracking boot attempts.
> >
> > btw: documentation of bootcount is sparse, I only found the very short
> > 'doc/README.bootcount' and it's not even migrated to recent U-Boot
> > sphinx based docs. ;-)
> >
> > But from what I understood the concept is the same, U-Boot counts
> > something and Linux userspace has to reset it.  The counter must be
> > stored somewhere, for example in U-Boot env if no other storage is
> > possible.
> >
> > >
> > > > Crashes can be triggered more easily, if patch order is changed and
> > > > patch 2 (resetting pointers to NULL after free) comes first, or if patch
> > > > 2 is applied on its own only.
> > >
> > > Hmm...
> > >
> > > > The fix is in the first patch, and on my boards I see no crashes
> > > > anymore.  I also tested all kinds of combinations of calling `ubi part`,
> > > > `ubi detach`, `ubifsmount`, `ubifsumount`, `ubifsls`, `ubifsload`, `ls`,
> > > > `load`, `size`, and `test -e` and got no crashes anymore after the fix.
> > >
> > > That sounds good! Hmm.. test -e has nothing to do with ubi/ubifs I think.
> >
> > Oh it has, 'test -e' calls file_exists() which calls fs_exists() which
> > ends up calling ubifs_exists() which is one of the functions causing
> > an immediate memory leak, see patch 1.
> >
> > > On what hardware do you test? Is it in mainline?
> >
> > Tested on custom hardware, but I'm confident it should be reproducible
> > on any board using ubifs, especially after applying patch 2 resetting
> > pointers of freed memory to NULL.  This should trigger the bug with
> > the simple sequence already described:
> >
> >   > ubifsmount ubi0:anyvolume
> >   > ls ubi ubi0:anyvolume / # (or load, or test -e, or size)
> >   > ubifsumount
> >
> > ubifsumount will call ubifs_umount() which calls
> > ubi_close_volume(c->ubi), that pointer is either invalid leading to a
> > double free inside of ubi_close_volume() and it will crash only in
> > certain conditions or the pointer is NULL after applying patch 2 of
> > the series, then ubi_close_volume() crashes right away with a NULL
> > pointer exception.
> >
> > Note: without patch 2 it very much depends on the sequence of
> > commands, but after the first ubi_close_volume() triggered by
> > ls/load/size/exists the pointer in ubifs_sb is invalid, but accessed later
> > by the second ubi_close_volume() triggered by ubifs_umount().  If you
> > do something in between those using the freed memory by something else
> > again, the second ubi_close_volume() access might get corrupted data
> > or access things outside of RAM.  Patch 2 redirects this on a clean
> > NULL pointer exception you can easily trigger.
> >
> > In my case I got a pointer variable actually containing a string
> > "ng.." aka 0x2e2e676e which looked suspiciously similar to a valid
> > pointer on the platform somewhere in RAM between 0x20000000 and
> > 0x28000000 so it took me two days to realize it's not a pointer. ;-)
> >
> > >
> > > > The three additional patches (2 to 4) are more or less safeguards and
> > > > improvements for the future, and come from me trying and my debugging
> > > > code done on the way, more or less optional, but I think nice to have.
> > >
> > > I will look at them .. but give me some time, as I am in holidays the
> > > next 2 weeks ... Hmm.. and it would be good to get some Tested-by
> > > from people with hardware...
> >
> > Take your time, no need to work in holidays.  Would appreciate a
> > Tested-by by anyone else though, maybe some of the raw NAND folks?
>
> Well, apparently nobody had a look in the month of July, I add the raw
> NAND maintainers in Cc, maybe I should have done in the first place.
>
> Would be happy if someone could have a look at the fix, maybe read the
> patches first before the discussion? ;-)
>
> Greets
> Alex
>
> >
> > Greets
> > Alex
> >
> > >
> > > bye,
> > > Heiko
> > > >
> > > > Greets
> > > > Alex
> > > >
> > > > [1] https://github.com/rauc/rauc/blob/master/contrib/uboot.sh
> > > >
> > > > Alexander Dahl (4):
> > > >    fs: ubifs: Fix memleak and double free in u-boot wrapper functions
> > > >    fs: ubifs: Set pointers to NULL after free
> > > >    fs: ubifs: Make k(z)alloc/kfree symmetric
> > > >    fs: ubifs: Add volume mounted check
> > > >
> > > >   fs/ubifs/super.c |  8 ++++++--
> > > >   fs/ubifs/ubifs.c | 31 +++++++++++++++++++------------
> > > >   2 files changed, 25 insertions(+), 14 deletions(-)
> > > >
> > > >
> > > > base-commit: 65fbdab27224ee3943a89496b21862db83c34da2
> > > >
> > >
> > > --
> > > DENX Software Engineering GmbH,      Managing Director: Erika Unter
> > > HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
> > > Phone: +49-8142-66989-52   Fax: +49-8142-66989-80   Email: hs at denx.de

-- 
Michael Nazzareno Trimarchi
Co-Founder & Chief Executive Officer
M. +39 347 913 2170
michael at amarulasolutions.com
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