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

Thu Jul 4 10:18:55 CEST 2024

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'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?

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
> > 
> 
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