crypto: vmac - remove unused VMAC algorithm

crypto: vmac - remove unused VMAC algorithm

Remove the vmac64 template, as it has no known users.  It also continues
to have longstanding bugs such as alignment violations (see
https://lore.kernel.org/r/20241226134847.6690-1-evepolonium@gmail.com/).

This code was added in 2009 by commit f1939f7c5645 ("crypto: vmac - New
hash algorithm for intel_txt support").  Based on the mention of
intel_txt support in the commit title, it seems it was added as a
prerequisite for the contemporaneous patch
"intel_txt: add s3 userspace memory integrity verification"
(https://lore.kernel.org/r/4ABF2B50.6070106@intel.com/).  In the design
proposed by that patch, when an Intel Trusted Execution Technology (TXT)
enabled system resumed from suspend, the "tboot" trusted executable
launched the Linux kernel without verifying userspace memory, and then
the Linux kernel used VMAC to verify userspace memory.

However, that patch was never merged, as reviewers had objected to the
design.  It was later reworked into commit 4bd96a7a8185 ("x86, tboot:
Add support for S3 memory integrity protection") which made tboot verify
the memory instead.  Thus the VMAC support in Linux was never used.

No in-tree user has appeared since then, other than potentially the
usual components that allow specifying arbitrary hash algorithms by
name, namely AF_ALG and dm-integrity.  However there are no indications
that VMAC is being used with these components.  Debian Code Search and
web searches for "vmac64" (the actual algorithm name) do not return any
results other than the kernel itself, suggesting that it does not appear
in any other code or documentation.  Explicitly grepping the source code
of the usual suspects (libell, iwd, cryptsetup) finds no matches either.

Before 2018, the vmac code was also completely broken due to using a
hardcoded nonce and the wrong endianness for the MAC.  It was then fixed
by commit ed331adab35b ("crypto: vmac - add nonced version with big
endian digest") and commit 0917b873127c ("crypto: vmac - remove insecure
version with hardcoded nonce").  These were intentionally breaking
changes that changed all the computed MAC values as well as the
algorithm name ("vmac" to "vmac64").  No complaints were ever received
about these breaking changes, strongly suggesting the absence of users.

The reason I had put some effort into fixing this code in 2018 is
because it was used by an out-of-tree driver.  But if it is still needed
in that particular out-of-tree driver, the code can be carried in that
driver instead.  There is no need to carry it upstream.

Cc: Atharva Tiwari <evepolonium@gmail.com>
Cc: Shane Wang <shane.wang@intel.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org> # m68k
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>