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linux/virt/kvm
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Sean Christopherson 89dbbe6ff3 KVM: Disallow toggling KVM_MEM_GUEST_MEMFD on an existing memslot 
commit 9935df5333 upstream.

Reject attempts to disable KVM_MEM_GUEST_MEMFD on a memslot that was
initially created with a guest_memfd binding, as KVM doesn't support
toggling KVM_MEM_GUEST_MEMFD on existing memslots.  KVM prevents enabling
KVM_MEM_GUEST_MEMFD, but doesn't prevent clearing the flag.

Failure to reject the new memslot results in a use-after-free due to KVM
not unbinding from the guest_memfd instance.  Unbinding on a FLAGS_ONLY
change is easy enough, and can/will be done as a hardening measure (in
anticipation of KVM supporting dirty logging on guest_memfd at some point),
but fixing the use-after-free would only address the immediate symptom.

  ==================================================================
  BUG: KASAN: slab-use-after-free in kvm_gmem_release+0x362/0x400 [kvm]
  Write of size 8 at addr ffff8881111ae908 by task repro/745

  CPU: 7 UID: 1000 PID: 745 Comm: repro Not tainted 6.18.0-rc6-115d5de2eef3-next-kasan #3 NONE
  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/0x5c0
   kasan_report+0xb4/0xe0
   kvm_gmem_release+0x362/0x400 [kvm]
   __fput+0x2fa/0x9d0
   task_work_run+0x12c/0x200
   do_exit+0x6ae/0x2100
   do_group_exit+0xa8/0x230
   __x64_sys_exit_group+0x3a/0x50
   x64_sys_call+0x737/0x740
   do_syscall_64+0x5b/0x900
   entry_SYSCALL_64_after_hwframe+0x4b/0x53
  RIP: 0033:0x7f581f2eac31
   </TASK>

  Allocated by task 745 on cpu 6 at 9.746971s:
   kasan_save_stack+0x20/0x40
   kasan_save_track+0x13/0x50
   __kasan_kmalloc+0x77/0x90
   kvm_set_memory_region.part.0+0x652/0x1110 [kvm]
   kvm_vm_ioctl+0x14b0/0x3290 [kvm]
   __x64_sys_ioctl+0x129/0x1a0
   do_syscall_64+0x5b/0x900
   entry_SYSCALL_64_after_hwframe+0x4b/0x53

  Freed by task 745 on cpu 6 at 9.747467s:
   kasan_save_stack+0x20/0x40
   kasan_save_track+0x13/0x50
   __kasan_save_free_info+0x37/0x50
   __kasan_slab_free+0x3b/0x60
   kfree+0xf5/0x440
   kvm_set_memslot+0x3c2/0x1160 [kvm]
   kvm_set_memory_region.part.0+0x86a/0x1110 [kvm]
   kvm_vm_ioctl+0x14b0/0x3290 [kvm]
   __x64_sys_ioctl+0x129/0x1a0
   do_syscall_64+0x5b/0x900
   entry_SYSCALL_64_after_hwframe+0x4b/0x53

Reported-by: Alexander Potapenko <glider@google.com>
Fixes: a7800aa80e ("KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory")
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20251202020334.1171351-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
History 2026-01-08 10:14:21 +01:00
..
async_pf.c Revert "KVM: async_pf: avoid recursive flushing of work items" 2024-06-03 08:55:55 -07:00
async_pf.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 504 2019-06-19 17:09:56 +02:00
binary_stats.c KVM: stats: remove dead stores 2021-08-13 03:35:15 -04:00
coalesced_mmio.c KVM: Clean up coalesced MMIO ring full check 2024-08-29 19:38:33 -07:00
coalesced_mmio.h License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
dirty_ring.c KVM: Discard zero mask with function kvm_dirty_ring_reset 2024-06-20 17:20:11 -04:00
eventfd.c KVM: Allow building irqbypass.ko as as module when kvm.ko is a module 2025-04-20 10:15:54 +02:00
guest_memfd.c KVM: guest_memfd: Remove bindings on memslot deletion when gmem is dying 2025-11-24 10:36:06 +01:00
irqchip.c KVM: Setup empty IRQ routing when creating a VM 2024-06-11 14:18:34 -07:00
Kconfig KVM: Allow building irqbypass.ko as as module when kvm.ko is a module 2025-04-20 10:15:54 +02:00
kvm_main.c KVM: Disallow toggling KVM_MEM_GUEST_MEMFD on an existing memslot 2026-01-08 10:14:21 +01:00
kvm_mm.h KVM: Drop unused @may_block param from gfn_to_pfn_cache_invalidate_start() 2024-04-11 12:58:53 -07:00
Makefile.kvm KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory 2023-11-14 08:01:03 -05:00
pfncache.c KVM: Validate hva in kvm_gpc_activate_hva() to fix __kvm_gpc_refresh() WARN 2024-06-28 08:31:46 -07:00
vfio.c introduce fd_file(), convert all accessors to it. 2024-08-12 22:00:43 -04:00
vfio.h License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00