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 <jack@xxxxxxxx> > SUSE Labs, CR Thanks, Shreyansh Chouhan
