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?
Mikulas
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
On Tue, 6 Feb 2024, Eric Biggers wrote:
> 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
I see. So I will copy the data to a kernel buffer before encryption or
decryption.
I assume that authenticated encryption or decryption using the same buffer
as a source and as a destination should be ok. Right?
Mikulas
On Wed, Feb 07, 2024 at 01:51:51PM +0100, Mikulas Patocka wrote:
>
>
> On Tue, 6 Feb 2024, Eric Biggers wrote:
>
> > 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
>
> I see. So I will copy the data to a kernel buffer before encryption or
> decryption.
>
> I assume that authenticated encryption or decryption using the same buffer
> as a source and as a destination should be ok. Right?
>
The crypto_aead API allows the source and destination to overlap, yes.
- Eric