CVE-2026-64247: KVM: x86: hyper-v: Bound the bank index when querying sparse banks

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

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

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

KVM: x86: hyper-v: Bound the bank index when querying sparse banks

When checking if a VP ID is included in a sparse bank set, explicitly check
that the ID can actually be contained in a sparse bank (the TLFS allows for
a maximum of 64 banks of 64 vCPUs each).  When handling a paravirtual TLB
flush for L2, the VP ID is copied verbatim from the enlightened VMCS,
without any bounds check, i.e. isn't guaranteed to be under the limit of
4096.

Failure to check the bounds of the VP ID leads to an out-of-bounds read
when testing the sparse bank, and super strictly speaking could lead to KVM
performing an unnecessary TLB flush for an L2 vCPU.

  ==================================================================
  BUG: KASAN: use-after-free in hv_is_vp_in_sparse_set+0x85/0x100 [kvm]
  Read of size 8 at addr ffff88811ba5f598 by task hyperv_evmcs/2802

  CPU: 12 UID: 1000 PID: 2802 Comm: hyperv_evmcs Not tainted 7.1.0-rc2 #7 PREEMPT
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  Call Trace:
   <TASK>
   dump_stack_lvl+0x51/0x60
   print_report+0xcb/0x5d0
   kasan_report+0xb4/0xe0
   kasan_check_range+0x35/0x1b0
   hv_is_vp_in_sparse_set+0x85/0x100 [kvm]
   kvm_hv_flush_tlb+0xe9e/0x16c0 [kvm]
   kvm_hv_hypercall+0xe6b/0x1e60 [kvm]
   vmx_handle_exit+0x485/0x1b60 [kvm_intel]
   kvm_arch_vcpu_ioctl_run+0x22e3/0x5070 [kvm]
   kvm_vcpu_ioctl+0x5d0/0x10c0 [kvm]
   __x64_sys_ioctl+0x129/0x1a0
   do_syscall_64+0xb9/0xcf0
   entry_SYSCALL_64_after_hwframe+0x4b/0x53
  RIP: 0033:0x7f0e62d1a9bf
   </TASK>

  The buggy address belongs to the physical page:
  page: refcount:0 mapcount:0 mapping:0000000000000000 index:0xffffffffffffffff pfn:0x11ba5f
  flags: 0x4000000000000000(zone=1)
  raw: 4000000000000000 0000000000000000 00000000ffffffff 0000000000000000
  raw: ffffffffffffffff 0000000000000000 00000000ffffffff 0000000000000000
  page dumped because: kasan: bad access detected

  Memory state around the buggy address:
   ffff88811ba5f480: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
   ffff88811ba5f500: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
  >ffff88811ba5f580: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
                              ^
   ffff88811ba5f600: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
   ffff88811ba5f680: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
  ==================================================================
  Disabling lock debugging due to kernel taint

Opportunistically add a compile time assertion to ensure the maximum number
of sparse banks exactly matches the number of possible bits in the passed
in mask.

[sean: add KASAN splat, drop comment, add assert, massage changelog]

The Linux kernel CVE team has assigned CVE-2026-64247 to this issue.


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

	Issue introduced in 6.2 with commit c58a318f6090efe06e6702b8882e2026f44f620e and fixed in 6.6.144 with commit d18756b12aab30d07794446445c93112e5c69a2e
	Issue introduced in 6.2 with commit c58a318f6090efe06e6702b8882e2026f44f620e and fixed in 6.12.95 with commit 83c2f52c6a78b1590034e955cff3fe0b052fe4ae
	Issue introduced in 6.2 with commit c58a318f6090efe06e6702b8882e2026f44f620e and fixed in 6.18.38 with commit e36095d8d922bb26ce860231aacf0cd14edea07c
	Issue introduced in 6.2 with commit c58a318f6090efe06e6702b8882e2026f44f620e and fixed in 7.1.3 with commit f636cf6a1e7b7f40d48d8d08bd5f152aa61dd130
	Issue introduced in 6.2 with commit c58a318f6090efe06e6702b8882e2026f44f620e and fixed in 7.2-rc1 with commit 4721f8160f17554b003e8928bb61e6c9b2fe92a3

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-2026-64247
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/kvm/hyperv.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/d18756b12aab30d07794446445c93112e5c69a2e
	https://git.kernel.org/stable/c/83c2f52c6a78b1590034e955cff3fe0b052fe4ae
	https://git.kernel.org/stable/c/e36095d8d922bb26ce860231aacf0cd14edea07c
	https://git.kernel.org/stable/c/f636cf6a1e7b7f40d48d8d08bd5f152aa61dd130
	https://git.kernel.org/stable/c/4721f8160f17554b003e8928bb61e6c9b2fe92a3