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

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.