Subject: CVE-2021-47227: x86/fpu: Prevent state corruption in __fpu__restore_sig()

From: Greg Kroah-Hartman <[email-masked]>

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

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

x86/fpu: Prevent state corruption in __fpu__restore_sig()

The non-compacted slowpath uses __copy_from_user() and copies the entire
user buffer into the kernel buffer, verbatim.  This means that the kernel
buffer may now contain entirely invalid state on which XRSTOR will #GP.
validate_user_xstate_header() can detect some of that corruption, but that
leaves the onus on callers to clear the buffer.

Prior to XSAVES support, it was possible just to reinitialize the buffer,
completely, but with supervisor states that is not longer possible as the
buffer clearing code split got it backwards. Fixing that is possible but
not corrupting the state in the first place is more robust.

Avoid corruption of the kernel XSAVE buffer by using copy_user_to_xstate()
which validates the XSAVE header contents before copying the actual states
to the kernel. copy_user_to_xstate() was previously only called for
compacted-format kernel buffers, but it works for both compacted and
non-compacted forms.

Using it for the non-compacted form is slower because of multiple
__copy_from_user() operations, but that cost is less important than robust
code in an already slow path.

[ Changelog polished by Dave Hansen ]

The Linux kernel CVE team has assigned CVE-2021-47227 to this issue.

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

	Issue introduced in 5.8 with commit b860eb8dce59 and fixed in 5.10.46 with commit 076f732b16a5
	Issue introduced in 5.8 with commit b860eb8dce59 and fixed in 5.12.13 with commit ec25ea1f3f05
	Issue introduced in 5.8 with commit b860eb8dce59 and fixed in 5.13 with commit 484cea4f362e

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-2021-47227
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:
	arch/x86/kernel/fpu/signal.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/076f732b16a5bf842686e1b43ab6021a2d98233e
	https://git.kernel.org/stable/c/ec25ea1f3f05d6f8ee51d1277efea986eafd4f2a
	https://git.kernel.org/stable/c/484cea4f362e1eeb5c869abbfb5f90eae6421b38