Re: ext3 file system livelock and file system corruption, 4.9.166 stable kernel

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Tue, Apr 02, 2019 at 01:08:45PM +0300, Jari Ruusu wrote:
> To trigger this ext4 file system bug, you need a sparse file with
> correct sparse pattern on old-school ext3 file system. I tried
> more simpler ways to trigger this but those attempts did not
> trigger the bug. I have provided compressed sparse file that
> reliably triggers the bug. Size of compressed sparse file 1667256
> bytes. Size of uncompressed sparse file 7369850880 bytes.
> Following commands will demo the problem.
> 
>   wget http://www.elisanet.fi/jariruusu/123/sparse-demo.data.xz
>   xz -d sparse-demo.data.xz
>   mkfs -t ext3 -b 4096 -e remount-ro -O "^dir_index" /dev/sdc1
>   mount -t ext3 /dev/sdc1 /mnt
>   cp -v --sparse=always sparse-demo.data /mnt/aa
>   cp -v --sparse=always sparse-demo.data /mnt/bb
>   umount /mnt
>   mount -t ext3 /dev/sdc1 /mnt
>   cp -v --sparse=always /mnt/bb /mnt/aa
> 
> That last cp command reliably triggers the bug that livelocks and
> after reset you have file system corruption to deal with. Deeply
> unfunny.
> 
> The bug is caused by
> "ext4: brelse all indirect buffer in ext4_ind_remove_space()"
> upstream commit 674a2b27234d1b7afcb0a9162e81b2e53aeef217, from
> <yi.zhang@huawei.com>, who provided a follow-up patch
> "ext4: cleanup bh release code in ext4_ind_remove_space()"
> upstream commit 5e86bdda41534e17621d5a071b294943cae4376e. The
> problem with that follow-up patch is that it is almost criminally
> mislabeled. It should have said "fixes ext3 livelock and file
> system corrupting bug" or something like that, so that Greg KH &
> Co would have understood that it must be backported to stable
> kernels too. Now the bug appears to be in all/most stable kernels
> already.
> 
> Below is the buggy patch that causes the problem. Look at those
> new while loops. Once the while condition is true once, it is
> ALWAYS true, so it livelocks.
> 
> > --- a/fs/ext4/indirect.c
> > +++ b/fs/ext4/indirect.c
> > @@ -1385,10 +1385,14 @@ end_range:
> >  					   partial->p + 1,
> >  					   partial2->p,
> >  					   (chain+n-1) - partial);
> > -			BUFFER_TRACE(partial->bh, "call brelse");
> > -			brelse(partial->bh);
> > -			BUFFER_TRACE(partial2->bh, "call brelse");
> > -			brelse(partial2->bh);
> > +			while (partial > chain) {
> > +				BUFFER_TRACE(partial->bh, "call brelse");
> > +				brelse(partial->bh);
> > +			}
> > +			while (partial2 > chain2) {
> > +				BUFFER_TRACE(partial2->bh, "call brelse");
> > +				brelse(partial2->bh);
> > +			}
> >  			return 0;
> >  		}
> >
> 
> Greg & Co,
> Please revert that above patch from stable kernels or backport the
> follow-up patch that fixes the problem.

So you need 5e86bdda4153 ("ext4: cleanup bh release code in
ext4_ind_remove_space()") applied to all of the stable and LTS kernels
at the moment (as that patch only showed up in 5.1-rc1)?

If so, I need an ack from the ext4 developers/maintainer to do so.

thanks,

greg k-h