From mboxrd@z Thu Jan 1 00:00:00 1970 From: Shreyansh Chouhan Subject: Re: Verify the items that we read from blocks Date: Mon, 12 Jul 2021 09:14:22 +0530 Message-ID: References: <20210705113329.GE15373@quack2.suse.cz> Mime-Version: 1.0 Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=FNaAgJi+fq8ys+9B4sOf2Hdg4MkZwkLSB0lQ2Ck2TCQ=; b=MlypPXS4oN/FOvVKZJObV515kSpjtrHQAyEhmUy6GubTrZv9qB52SgcVTeO+nUkfSk wSaw1+dFmS5ay8WcsyjgOeTLfBnYfXPtSnqZOYnJuJNHUvq2ip7Ph4++4LkMLxqBnLR+ R5kQEQexTWga7ky6Z8Q1AZc+sB6yx5On1mqOcgOPrSuB211PMdQZ+r1hKCdyOxXTF9E6 8Hfudo3501rVSMUgNpq9CeifYmj70lhyS4HlLexxI3FmL80RuJMDn2GNKXkgIMOm0TV1 XKqLZY84+FjId+jzErP9mQeWTjA2wcoAb/IC27dmOSwcvUpR1ELQaYfY4+vbIKLIH6oJ drdg== Content-Disposition: inline In-Reply-To: <20210705113329.GE15373@quack2.suse.cz> List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Jan Kara Cc: rkovhaev@gmail.com, reiserfs-devel@vger.kernel.org, linux-kernel@vger.kernel.org Hi, 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 > SUSE Labs, CR Thanks, Shreyansh Chouhan