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From:   David Howells <dhowells@redhat.com>
In-Reply-To: <yt9d8rb44cbe.fsf@linux.ibm.com>
References: <yt9d8rb44cbe.fsf@linux.ibm.com> <000000000000273d0105ff97bf56@google.com>
To:     Sven Schnelle <svens@linux.ibm.com>
Cc:     dhowells@redhat.com,
        syzbot <syzbot+9b82859567f2e50c123e@syzkaller.appspotmail.com>,
        akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, syzkaller-bugs@googlegroups.com,
        Herbert Xu <herbert@gondor.apana.org.au>,
        "David S. Miller" <davem@davemloft.net>,
        linux-crypto@vger.kernel.org
Subject: Re: [syzbot] [mm?] WARNING in try_grab_page
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Date:   Wed, 26 Jul 2023 09:25:24 +0100
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Sven Schnelle <svens@linux.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