netdev - Re: [PATCH net-next] tls: cap the output scatter list to something reasonable

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Date:   Mon, 7 Feb 2022 17:15:30 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     davem@...emloft.net, netdev@...r.kernel.org, borisp@...dia.com,
        john.fastabend@...il.com, daniel@...earbox.net,
        vfedorenko@...ek.ru, kernel-team@...com, axboe@...nel.dk
Subject: Re: [PATCH net-next] tls: cap the output scatter list to something
 reasonable

On Wed, Feb 02, 2022 at 02:20:31PM -0800, Jakub Kicinski wrote:
> TLS recvmsg() passes user pages as destination for decrypt.
> The decrypt operation is repeated record by record, each
> record being 16kB, max. TLS allocates an sg_table and uses
> iov_iter_get_pages() to populate it with enough pages to
> fit the decrypted record.
> 
> Even though we decrypt a single message at a time we size
> the sg_table based on the entire length of the iovec.
> This leads to unnecessarily large allocations, risking
> triggering OOM conditions.
> 
> Use iov_iter_truncate() / iov_iter_reexpand() to construct
> a "capped" version of iov_iter_npages(). Alternatively we
> could parametrize iov_iter_npages() to take the size as
> arg instead of using i->count, or do something else..

Er...  Would simply passing 16384/PAGE_SIZE instead of MAX_INT work
for your purposes?