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

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Date:   Tue, 25 Jul 2023 10:28:31 +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:

> 
> I looked into this issue. What syzkaller is doing is opening an AF_ALG
> socket, and sending a large message which will eventually end in -EFAULT.
> Looking at the code in crypto/algif_hash.c i see that hash_sendmsg is
> calling extract_iter_to_sg() -> extract_user_to_sg(). In the -EFAULT
> case, this function is calling put_page(), which looks like a leftover
> from the old pinning interface. I think this should be a
> unpin_user_page() call now.
> 
> 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?

Good catch, thanks.  I'll whip up a patch or two for it.

David