Subject: CVE-2021-47277: kvm: avoid speculation-based attacks from out-of-range memslot accesses

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

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

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

kvm: avoid speculation-based attacks from out-of-range memslot accesses

KVM's mechanism for accessing guest memory translates a guest physical
address (gpa) to a host virtual address using the right-shifted gpa
(also known as gfn) and a struct kvm_memory_slot.  The translation is
performed in __gfn_to_hva_memslot using the following formula:

      hva = slot->userspace_addr + (gfn - slot->base_gfn) * PAGE_SIZE

It is expected that gfn falls within the boundaries of the guest's
physical memory.  However, a guest can access invalid physical addresses
in such a way that the gfn is invalid.

__gfn_to_hva_memslot is called from kvm_vcpu_gfn_to_hva_prot, which first
retrieves a memslot through __gfn_to_memslot.  While __gfn_to_memslot
does check that the gfn falls within the boundaries of the guest's
physical memory or not, a CPU can speculate the result of the check and
continue execution speculatively using an illegal gfn. The speculation
can result in calculating an out-of-bounds hva.  If the resulting host
virtual address is used to load another guest physical address, this
is effectively a Spectre gadget consisting of two consecutive reads,
the second of which is data dependent on the first.

Right now it's not clear if there are any cases in which this is
exploitable.  One interesting case was reported by the original author
of this patch, and involves visiting guest page tables on x86.  Right
now these are not vulnerable because the hva read goes through get_user(),
which contains an LFENCE speculation barrier.  However, there are
patches in progress for x86 uaccess.h to mask kernel addresses instead of
using LFENCE; once these land, a guest could use speculation to read
from the VMM's ring 3 address space.  Other architectures such as ARM
already use the address masking method, and would be susceptible to
this same kind of data-dependent access gadgets.  Therefore, this patch
proactively protects from these attacks by masking out-of-bounds gfns
in __gfn_to_hva_memslot, which blocks speculation of invalid hvas.

Sean Christopherson noted that this patch does not cover
kvm_read_guest_offset_cached.  This however is limited to a few bytes
past the end of the cache, and therefore it is unlikely to be useful in
the context of building a chain of data dependent accesses.

The Linux kernel CVE team has assigned CVE-2021-47277 to this issue.

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

	Fixed in 4.4.273 with commit 3098b86390a6
	Fixed in 4.9.273 with commit 740621309b25
	Fixed in 4.14.237 with commit 361ce3b917af
	Fixed in 4.19.195 with commit 22b87fb17a28
	Fixed in 5.4.126 with commit bff1fbf0cf07
	Fixed in 5.10.44 with commit 7af299b97734
	Fixed in 5.12.11 with commit ed0e2a893092
	Fixed in 5.13 with commit da27a83fd6cc

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-2021-47277
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:
	include/linux/kvm_host.h

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/3098b86390a6b9ea52657689f08410baf130ceff
	https://git.kernel.org/stable/c/740621309b25bbf619b8a0ba5fd50a8e58989441
	https://git.kernel.org/stable/c/361ce3b917aff93123e9e966d8608655c967f438
	https://git.kernel.org/stable/c/22b87fb17a28d37331bb9c1110737627b17f6781
	https://git.kernel.org/stable/c/bff1fbf0cf0712686f1df59a83fba6e31d2746a0
	https://git.kernel.org/stable/c/7af299b97734c7e7f465b42a2139ce4d77246975
	https://git.kernel.org/stable/c/ed0e2a893092c7fcb4ff7ba74e5efce53a6f5940
	https://git.kernel.org/stable/c/da27a83fd6cc7780fea190e1f5c19e87019da65c