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Date: Tue, 6 Feb 2024 16:47:23 -0800
From: Eric Biggers <ebiggers@kernel.org>
To: Mikulas Patocka <mpatocka@redhat.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>,
	"David S. Miller" <davem@davemloft.net>,
	linux-crypto@vger.kernel.org, dm-devel@lists.linux.dev
Subject: Re: A question about modifying the buffer under authenticated
 encryption
Message-ID: <20240207004723.GA35324@sol.localdomain>
References: <f22dae2c-9cac-a63-fff-3b0b7305be6@redhat.com>
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In-Reply-To: <f22dae2c-9cac-a63-fff-3b0b7305be6@redhat.com>

On Tue, Feb 06, 2024 at 10:46:59PM +0100, Mikulas Patocka wrote:
> Hi
> 
> I'm trying to fix some problems in dm-crypt that it may report 
> authentication failures when the user reads data with O_DIRECT and 
> modifies the read buffer while it is being read.
> 
> I'd like to ask you:
> 
> 1. If the authenticated encryption encrypts a message, reading from 
>    buffer1 and writing to buffer2 - and buffer1 changes while reading from 
>    it - is it possible that it generates invalid authentication tag?
> 
> 2. If the authenticated encryption decrypts a message, reading from 
>    buffer1 and writing to buffer2 - and buffer2 changes while writing to 
>    it - is is possible that it reports authentication tag mismatch?
> 

Yes, both scenarios are possible.  But it depends on the AEAD algorithm and how
it happens to be implemented, and on whether the data overlaps or not.

This is very much a "don't do that" sort of thing.

- Eric