CVE-2023-52597: KVM: s390: fix setting of fpc register

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

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

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

KVM: s390: fix setting of fpc register

kvm_arch_vcpu_ioctl_set_fpu() allows to set the floating point control
(fpc) register of a guest cpu. The new value is tested for validity by
temporarily loading it into the fpc register.

This may lead to corruption of the fpc register of the host process:
if an interrupt happens while the value is temporarily loaded into the fpc
register, and within interrupt context floating point or vector registers
are used, the current fp/vx registers are saved with save_fpu_regs()
assuming they belong to user space and will be loaded into fp/vx registers
when returning to user space.

test_fp_ctl() restores the original user space / host process fpc register
value, however it will be discarded, when returning to user space.

In result the host process will incorrectly continue to run with the value
that was supposed to be used for a guest cpu.

Fix this by simply removing the test. There is another test right before
the SIE context is entered which will handles invalid values.

This results in a change of behaviour: invalid values will now be accepted
instead of that the ioctl fails with -EINVAL. This seems to be acceptable,
given that this interface is most likely not used anymore, and this is in
addition the same behaviour implemented with the memory mapped interface
(replace invalid values with zero) - see sync_regs() in kvm-s390.c.

The Linux kernel CVE team has assigned CVE-2023-52597 to this issue.


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

	Fixed in 4.19.307 with commit 3a04410b0bc7
	Fixed in 5.4.269 with commit 5e63c9ae8055
	Fixed in 5.10.210 with commit 150a3a387149
	Fixed in 5.15.149 with commit 732a3bea7aba
	Fixed in 6.1.77 with commit 0671f42a9c10
	Fixed in 6.6.16 with commit c87d7d910775
	Fixed in 6.7.4 with commit 2823db0010c4
	Fixed in 6.8-rc1 with commit b988b1bb0053

Please see https://www.kernel.org or 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-2023-52597
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/s390/kvm/kvm-s390.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/3a04410b0bc7e056e0843ac598825dd359246d18
	https://git.kernel.org/stable/c/5e63c9ae8055109d805aacdaf2a4fe2c3b371ba1
	https://git.kernel.org/stable/c/150a3a3871490e8c454ffbac2e60abeafcecff99
	https://git.kernel.org/stable/c/732a3bea7aba5b15026ea42d14953c3425cc7dc2
	https://git.kernel.org/stable/c/0671f42a9c1084db10d68ac347d08dbf6689ecb3
	https://git.kernel.org/stable/c/c87d7d910775a025e230fd6359b60627e392460f
	https://git.kernel.org/stable/c/2823db0010c400e4b2b12d02aa5d0d3ecb15d7c7
	https://git.kernel.org/stable/c/b988b1bb0053c0dcd26187d29ef07566a565cf55