linux-kernel - Re: [syzbot] [mm?] WARNING in try_grab

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Date:   Wed, 26 Jul 2023 09:25:24 +0100
From:   David Howells <dhowells@...hat.com>
To:     Sven Schnelle <svens@...ux.ibm.com>
Cc:     dhowells@...hat.com,
        syzbot <syzbot+9b82859567f2e50c123e@...kaller.appspotmail.com>,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, syzkaller-bugs@...glegroups.com,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        linux-crypto@...r.kernel.org
Subject: Re: [syzbot] [mm?] WARNING in try_grab_page

Sven Schnelle <svens@...ux.ibm.com> wrote:

> However, hash_sendmsg() also unpins via af_alg_free_sg() in the error
> path. From an API perspective, i would prefer if extract_user_to_sg()
> does the unpinning on error. Any thoughts?

In the event of an error, extract_user_to_sg() unpins only the pages it added
to the scatterlist; it doesn't unpin any pages that were already there and
doesn't free the scatterlist; it also decrements sgtable->nents so that the
caller doesn't see the pages it attempted to add, so calling af_alg_free_sg()
shouldn't hurt.

Note that the error path also frees the result, so we do need to do at least
part of it.

David