linux-ext4 - Re: [PATCH] ext4: catch integer overflow in ext4_cache

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Message-ID: <20200714123122.GG23073@quack2.suse.cz>
Date:   Tue, 14 Jul 2020 14:31:22 +0200
From:   Jan Kara <jack@...e.cz>
To:     Ritesh Harjani <riteshh@...ux.ibm.com>
Cc:     Jan Kara <jack@...e.cz>, Ted Tso <tytso@....edu>,
        linux-ext4@...r.kernel.org,
        Wolfgang Frisch <wolfgang.frisch@...e.com>
Subject: Re: [PATCH] ext4: catch integer overflow in ext4_cache_extents

On Mon 13-07-20 19:14:47, Ritesh Harjani wrote:
> 
> 
> On 7/13/20 6:28 PM, Jan Kara wrote:
> > From: Wolfgang Frisch <wolfgang.frisch@...e.com>
> > 
> > When extent tree is corrupted we can hit BUG_ON in
> > ext4_es_cache_extent(). Check for this and abort caching instead of
> > crashing the machine.
> 
> Was it intentionally made corrupted by crafting a corrupted disk image?

I'm not sure how Wolfgang hit the issue. I'd expect some fs image
fuzzing... Wolfgang?

> Are there more such logic in place which checks for such corruption at other
> places?

That's a good question. But now that I'm looking at it ext4_ext_check()
should actually catch a corruption like this. It is only the path in
ext4_find_extent()->ext4_cache_extents() that can face the issue so
probably instead of a fix in ext4_cache_extents() we should rather add more
careful extent info checks for the extents contained directly in the inode.
I'll look into it.

> Maybe a background over the issue which you saw may help.
> Also how did it recover out of it?

e2fsck I suppose :)

> Do you think it make sense to still emit a WARN_ON() here and then
> return which warns that this could possibly a corrupted extent
> entry? (maybe WARN_ON_ONCE() or via some ratelimiting if multiple extent
> entries are corrupted for that inode).

No, WARN is definitely wrong in this case. We could call ext4_error() if we
wanted. That would make sence although I've decided not to add it to
the original Wolfgang's fix since this is more like a failing readahead.
But OTOH it's metadata corruption that's unlikely to go away so I can be
easily convinced to put ext4_error() there :).

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR