CVE-2021-47213: NFSD: Fix exposure in nfsd4_decode_bitmap()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

NFSD: Fix exposure in nfsd4_decode_bitmap()

rtm@csail.mit.edu reports:
> nfsd4_decode_bitmap4() will write beyond bmval[bmlen-1] if the RPC
> directs it to do so. This can cause nfsd4_decode_state_protect4_a()
> to write client-supplied data beyond the end of
> nfsd4_exchange_id.spo_must_allow[] when called by
> nfsd4_decode_exchange_id().

Rewrite the loops so nfsd4_decode_bitmap() cannot iterate beyond
@bmlen.

Reported by: rtm@csail.mit.edu

The Linux kernel CVE team has assigned CVE-2021-47213 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 5.11 with commit d1c263a031e8 and fixed in 5.15.5 with commit 10c22d9519f3
	Issue introduced in 5.11 with commit d1c263a031e8 and fixed in 5.16 with commit c0019b7db1d7

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2021-47213
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	fs/nfsd/nfs4xdr.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/10c22d9519f3f5939de61a1500aa3a926b778d3a
	https://git.kernel.org/stable/c/c0019b7db1d7ac62c711cda6b357a659d46428fe