netdev - Re: Regression bisected to "crypto: af_alg: Convert af_alg_sendpage() to use MSG_SPLICE

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Date: Tue, 04 Jul 2023 09:50:37 +0100
From: David Howells <dhowells@...hat.com>
To: Ondrej Mosnacek <omosnacek@...il.com>
Cc: dhowells@...hat.com,
    Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
    Herbert Xu <herbert@...dor.apana.org.au>,
    Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org
Subject: Re: Regression bisected to "crypto: af_alg: Convert af_alg_sendpage() to use MSG_SPLICE_PAGES"

One problem with libkcapi is that it's abusing vmsplice().  It must not use
vmsplice(SPLICE_F_GIFT) on a buffer that's in the heap.  To quote the manual
page:

	      The user pages are a gift to the kernel.   The  application  may
              not  modify  this  memory ever, otherwise the page cache and on-
              disk data may differ.  Gifting pages to the kernel means that  a
              subsequent  splice(2)  SPLICE_F_MOVE  can  successfully move the
              pages;  if  this  flag  is  not  specified,  then  a  subsequent
              splice(2)  SPLICE_F_MOVE must copy the pages.  Data must also be
              properly page aligned, both in memory and length.

Basically, this can destroy the integrity of the process's heap as the
allocator may have metadata there that then gets excised.

If I remove the flag, it still crashes, so that's not the only problem.

David