Re: [Bugme-new] [Bug 11266] New: unable to handle kernel paging request in ext2_free_blocks

Jan Kara
2008-08-20T10:25:34Z

> On Tue, Aug 19, 2008 at 11:13:39AM +0200, Jan Kara wrote:
> > > Isn't equivalent checking done in ext2_check_descriptors()? It would make
> > > sense to abstract out the "check one group and return error" code and use
> > > it in both places.
> > Actually yes, it is. Good point. Sami, is it the case that you have
> > mounted the filesystem, then intentionally corrupted it and after that
> > the kernel oopsed (as opposed to first corrupting the filesystem image and
> > mounting it after that)? That would explain how corrupted values could get
> > to read_block_bitmap() even though ext2_check_descriptors() checked them.
>
> No, that's not what I do. I corrupt the fs before mounting it, then
> mount it, perform normal filesystem operations on it and unmount it.
OK, thanks. Then we must somehow corrupt group descriptor block during
the operation. Because I'm pretty sure it *is* corrupted - the oops
is: unable to handle kernel paging request at c7e95ffc. If we look into
registers, we see ECX has c7e96000 (which is probably bh->b_data). In
the second oops it's exactly the same - ECX has c11e4000, the oops is at
address c11e3ffc. So in both cases it is ECX-4. So somehow we managed to
pass negative offset into ext2_test_bit(). But as Andreas pointed out,
when we load descriptors into memory, we check that both bitmaps and
inode table is in ext2_check_descriptors()... The other possibility
would be that we managed to corrupts s_first_data_block in the
superblock. Anyway, both possibilities don't look very likely. I'll try
to reproduce the problem and maybe get more insight... How large is your
filesystem BTW?

> Here's the most current script I use (zzuf is the fuzzer):
>
> ------------------------------------------------------------
> #!/bin/sh
>
> if [ "`hostname`" != "fstest" ]; then
> echo "This is a dangerous script."
> echo "Set your hostname to \`fstest\' if you want to use it."
> exit 1
> fi
>
> umount /dev/hdb
> umount /dev/hdc
> /etc/init.d/sysklogd stop
> /etc/init.d/klogd stop
> /etc/init.d/cron stop
> mount /dev/hda / -t ext3 -o remount,ro || exit 1
>
> #ulimit -t 20
>
> for ((s=$1; s<1000000000; s++)); do
> umount /mnt
> echo '***** zzuffing *****' seed $s
> zzuf -r 0:0.03 -s $s </dev/hdc >/dev/hdb || exit
> mount /dev/hdb /mnt -t ext2 -o errors=continue || continue
> cd /mnt || continue
> timeout 30 cp -r doc doc2 >&/dev/null
> timeout 30 find -xdev >&/dev/null
> timeout 30 find -xdev -print0 2>/dev/null |xargs -0 touch -- 2>/dev/null
> timeout 30 mkdir tmp >&/dev/null
> timeout 30 echo whoah >tmp/filu 2>/dev/null
> timeout 30 rm -rf /mnt/* >&/dev/null
> cd /
> done
> ------------------------------------------------------------

Honza
--
Jan Kara <jack@suse.cz>
SuSE CR Labs