Re: [PATCH v5.5 03/30] KVM: Require total number of memslot pages to fit in an unsigned long
From: Maciej S. Szmigiero
Date: Mon Nov 08 2021 - 19:39:22 EST
On 04.11.2021 01:25, Sean Christopherson wrote:
Explicitly disallow creating more memslot pages than can fit in an
unsigned long, KVM doesn't correctly handle a total number of memslot
pages that doesn't fit in an unsigned long and remedying that would be a
waste of time.
For a 64-bit kernel, this is a nop as memslots are not allowed to overlap
in the gfn address space.
With a 32-bit kernel, userspace can at most address 3gb of virtual memory,
whereas wrapping the total number of pages would require 4tb+ of guest
physical memory. Even with x86's second address space for SMM, userspace
would need to alias all of guest memory more than one _thousand_ times.
And on older x86 hardware with MAXPHYADDR < 43, the guest couldn't
actually access any of those aliases even if userspace lied about
guest.MAXPHYADDR.
On 390 and arm64, this is a nop as they don't support 32-bit hosts.
On x86, practically speaking this is simply acknowledging reality as the
existing kvm_mmu_calculate_default_mmu_pages() assumes the total number
of pages fits in an "unsigned long".
On PPC, this is likely a nop as every flavor of PPC KVM assumes gfns (and
gpas!) fit in unsigned long. arch/powerpc/kvm/book3s_32_mmu_host.c goes
a step further and fails the build if CONFIG_PTE_64BIT=y, which
presumably means that it does't support 64-bit physical addresses.
On MIPS, this is also likely a nop as the core MMU helpers assume gpas
fit in unsigned long, e.g. see kvm_mips_##name##_pte.
And finally, RISC-V is a "don't care" as it doesn't exist in any release,
i.e. there is no established ABI to break.
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Reviewed-by: Maciej S. Szmigiero <maciej.szmigiero@xxxxxxxxxx>