linux-kernel - Re: [PATCH] staging: erofs: fix unexpected out-of-bound data access

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Date:   Fri, 12 Apr 2019 08:10:03 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Gao Xiang <gaoxiang25@...wei.com>
Cc:     Chao Yu <yuchao0@...wei.com>, Ming Lei <ming.lei@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        devel@...verdev.osuosl.org, LKML <linux-kernel@...r.kernel.org>,
        linux-erofs@...ts.ozlabs.org, Chao Yu <chao@...nel.org>,
        Miao Xie <miaoxie@...wei.com>, weidu.du@...wei.com,
        Fang Wei <fangwei1@...wei.com>
Subject: Re: [PATCH] staging: erofs: fix unexpected out-of-bound data access

On Fri, Apr 12, 2019 at 08:06:33AM -0700, Christoph Hellwig wrote:
> > +++ b/drivers/staging/erofs/data.c
> > @@ -304,7 +304,7 @@ static inline struct bio *erofs_read_raw_page(struct bio *bio,
> >  	*last_block = current_block;
> >  
> >  	/* shift in advance in case of it followed by too many gaps */
> > -	if (unlikely(bio->bi_vcnt >= bio->bi_max_vecs)) {
> > +	if (bio->bi_iter.bi_size >= bio->bi_max_vecs * PAGE_SIZE) {
> 
> This is still a very odd check.  bi_max_vecs * PAGE_SIZE is rather
> arbitrary… and more importantly bi_max_vecs is not really a public
> field, in fact this is the only place every using it outside the
> core block layer.
> 
> I think the logic in this function should be reworked to what we
> do elsewhere in the kernel, that is just add to the bio until
> bio_add_page fails, in which case you submit the bio and start
> a new one.  Then once you are done with your operation just submit
> the bio.  Which unless I'm missing something is what the code does,
> except for the goto loop obsfucation that is trying to hide it.
> 
> So why not something like:

And looking at this a little more - I think you should drop the
erofs raw ops entirely.  The iomap-based readpage and readpages
from fs/iomap.c should serve your needs just fine without all that
duplication.  The only thing missing is the cleancache calls,
which could easily be added.