Re: Verify the items that we read from blocks

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I thought that my last email wasn't an appropriate response, since to me
it looked as if I hadn't read your suggestions before sending a
response. (Couldn't quote anything because I wasn't able to find the email on
mutt (messed up filters,) and had to write a quick email with the
in-reply-to option.) So I thought I'd resend the response after I've
fixed my inbox.

On Mon, Jul 05, 2021 at 01:33:29PM +0200, Jan Kara wrote:
> Hello!
> On Fri 02-07-21 20:35:41, Shreyansh Chouhan wrote:
> > I was trying to work on this[1] bug. After a lot of reading the code and
> > running it under gdb, I found out that the error happens because
> > syzkaller creates a segment with raw binary data in the reproducer[2],
> > that has the wrong deh_location for the `..` directory item. (The value
> > is 0x5d (93), where as it should have been 0x20 (32).)
> First, I'd like to note that reiserfs is a legacy filesystem which gets
> little maintenance and I think distributions are close to disabling it in
> their default kernels if they didn't do it already. So I'm not sure how
> much is it worth it to do any larger fixes to it. But if you have a
> personal passion for reiserfs feel free to go ahead and try to fix these
> issues.

I had already spent a considerable amount of time on the debugging
portion, (to find an obvious mistake, now that I look back at it in
hindsight,) so I thought I'd just send in a patch.

> > I think that the solution would involve checking the items that we read,
> > and verify that they are actually valid. But this check could actually
> > happen in two places:
> > 
> > - First idea would be to check as soon as we read a
> >   block, and one way of doing that would be adding a wrapper around
> >   ll_rw_block that validates the leaf node blocks that we read. The
> >   benifits to this would be that since we're solving the problem at it's
> >   root, very few functions would have to be changed. But I don't know
> >   how much of a performance hit would it be.
> It depends on how heavy the checks are going to be but generally checking
> when loading from the disk is the way how most filesystems handle this.

The checks would be an O(n) traversal of directory headers, which
themselves check if the deh_location is greater than item length. The
item header checks were already present in the `is_leaf`(?) function.

> > - Second idea would be to do these validation checks lazily. This should
> >   be faster than the first idea, but this would involve changing the
> >   code at more places than in the first idea.
> > 
> > For how the validation happens, the first idea that comes to mind is
> > reading the item headers from the block that we read and verifying if
> > the header is valid, and if the items themselves are valid according to
> > the header.
> Looks sound.

I have added the implementation for the above idea to the `is_leaf`
function. Thanks a lot for your suggestions.

> 								Honza
> -- 
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR

Shreyansh Chouhan

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