Re: [PATCH 0/2] KVM: Fix a guest_memfd memslot UAF

[Sean Christopherson]

On Mon, Dec 01, 2025, Sean Christopherson wrote:
> Fix a UAF due to leaving a dangling guest_memfd memslot binding by
> disallowing clearing KVM_MEM_GUEST_MEMFD on a memslot.  The intent was
> that guest_memfd memslots would be immutable (could only be deleted),
> but somewhat ironically we missed the case where KVM_MEM_GUEST_MEMFD
> itself is the only flag that's toggled.
> 
> This is an ABI change, but I can't imagine anyone was relying on
> disappearing a guest_memfd memslot.
> 
> Patch 2 hardens against the UAF, and prepares for allowing FLAGS_ONLY
> changes on guest_memfd memslots.  Sooner or later, we're going to allow
> dirty logging on guest_memfd, so I think it makes sense to guard against
> that so that whoever adds dirty logging support doesn't forget to unbind
> on a FLAGS_ONLY change.
> 
> I'll respond with the syzkaller reproducer (it's comically simple).

And almost forgot...
// autogenerated by syzkaller (https://github.com/google/syzkaller)

#define _GNU_SOURCE

#include <endian.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>

uint64_t r[3] = {0xffffffffffffffff, 0xffffffffffffffff, 0xffffffffffffffff};

int main(void)
{
  syscall(__NR_mmap, /*addr=*/0x1ffffffff000ul, /*len=*/0x1000ul, /*prot=*/0ul,
          /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/ 0x32ul,
          /*fd=*/(intptr_t)-1, /*offset=*/0ul);
  syscall(__NR_mmap, /*addr=*/0x200000000000ul, /*len=*/0x1000000ul,
          /*prot=PROT_WRITE|PROT_READ|PROT_EXEC*/ 7ul,
          /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/ 0x32ul,
          /*fd=*/(intptr_t)-1, /*offset=*/0ul);
  syscall(__NR_mmap, /*addr=*/0x200001000000ul, /*len=*/0x1000ul, /*prot=*/0ul,
          /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/ 0x32ul,
          /*fd=*/(intptr_t)-1, /*offset=*/0ul);
  const char* reason;
  (void)reason;
  intptr_t res = 0;
  if (write(1, "executing program\n", sizeof("executing program\n") - 1)) {
  }
  //  openat$kvm arguments: [
  //    fd: const = 0xffffffffffffff9c (8 bytes)
  //    file: ptr[in, buffer] {
  //      buffer: {2f 64 65 76 2f 6b 76 6d 00} (length 0x9)
  //    }
  //    flags: open_flags = 0x0 (4 bytes)
  //    mode: const = 0x0 (2 bytes)
  //  ]
  //  returns fd_kvm
  memcpy((void*)0x200000000000, "/dev/kvm\000", 9);
  res = syscall(__NR_openat, /*fd=*/0xffffffffffffff9cul,
                /*file=*/0x200000000000ul, /*flags=*/0, /*mode=*/0);
  if (res != -1)
    r[0] = res;
  //  ioctl$KVM_CREATE_VM arguments: [
  //    fd: fd_kvm (resource)
  //    cmd: const = 0xae01 (4 bytes)
  //    type: intptr = 0x0 (8 bytes)
  //  ]
  //  returns fd_kvmvm
  res = syscall(__NR_ioctl, /*fd=*/r[0], /*cmd=*/0xae01, /*type=*/0ul);
  if (res != -1)
    r[1] = res;
  //  ioctl$KVM_CREATE_GUEST_MEMFD arguments: [
  //    fd: fd_kvmvm (resource)
  //    cmd: const = 0xc040aed4 (4 bytes)
  //    arg: ptr[in, kvm_create_guest_memfd] {
  //      kvm_create_guest_memfd {
  //        size: int64 = 0x200001fe0000 (8 bytes)
  //        flags: int64 = 0x0 (8 bytes)
  //        reserved: buffer: {00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  //        00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  //        00 00 00 00 00 00 00 00 00} (length 0x30)
  //      }
  //    }
  //  ]
  //  returns fd_kvm_guest_memfd
  *(uint64_t*)0x2000000001c0 = 0x200001fe0000;
  *(uint64_t*)0x2000000001c8 = 0;
  memset((void*)0x2000000001d0, 0, 48);
  res = syscall(__NR_ioctl, /*fd=*/r[1], /*cmd=*/0xc040aed4,
                /*arg=*/0x2000000001c0ul);
  if (res != -1)
    r[2] = res;
  //  ioctl$KVM_SET_USER_MEMORY_REGION2 arguments: [
  //    fd: fd_kvmvm (resource)
  //    cmd: const = 0x40a0ae49 (4 bytes)
  //    arg: ptr[in, kvm_userspace_memory_region2] {
  //      kvm_userspace_memory_region2 {
  //        slot: kvm_mem_slots = 0x4 (4 bytes)
  //        flags: kvm_mem_region_flags = 0x4 (4 bytes)
  //        paddr: kvm_guest_addrs = 0x80a0000 (8 bytes)
  //        size: len = 0x2000 (8 bytes)
  //        addr: VMA[0x2000]
  //        guest_memfd_offset: int64 = 0x4000 (8 bytes)
  //        guest_memfd: fd_kvm_guest_memfd (resource)
  //        pad1: const = 0x0 (4 bytes)
  //        pad2: buffer: {00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  //        00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  //        00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  //        00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  //        00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  //        00 00} (length 0x70)
  //      }
  //    }
  //  ]
  *(uint32_t*)0x200000000180 = 4;
  *(uint32_t*)0x200000000184 = 4;
  *(uint64_t*)0x200000000188 = 0x80a0000;
  *(uint64_t*)0x200000000190 = 0x2000;
  *(uint64_t*)0x200000000198 = 0x200000ffc000;
  *(uint64_t*)0x2000000001a0 = 0x4000;
  *(uint32_t*)0x2000000001a8 = r[2];
  *(uint32_t*)0x2000000001ac = 0;
  memset((void*)0x2000000001b0, 0, 112);
  syscall(__NR_ioctl, /*fd=*/r[1], /*cmd=*/0x40a0ae49,
          /*arg=*/0x200000000180ul);
  //  ioctl$KVM_SET_USER_MEMORY_REGION2 arguments: [
  //    fd: fd_kvmvm (resource)
  //    cmd: const = 0x40a0ae49 (4 bytes)
  //    arg: ptr[in, kvm_userspace_memory_region2] {
  //      kvm_userspace_memory_region2 {
  //        slot: kvm_mem_slots = 0x4 (4 bytes)
  //        flags: kvm_mem_region_flags = 0x1 (4 bytes)
  //        paddr: kvm_guest_addrs = 0xffff1000 (8 bytes)
  //        size: len = 0x2000 (8 bytes)
  //        addr: VMA[0x2000]
  //        guest_memfd_offset: int64 = 0x8 (8 bytes)
  //        guest_memfd: fd_kvm_guest_memfd (resource)
  //        pad1: const = 0x0 (4 bytes)
  //        pad2: buffer: {00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  //        00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  //        00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  //        00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  //        00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  //        00 00} (length 0x70)
  //      }
  //    }
  //  ]
  *(uint32_t*)0x200000000240 = 4;
  *(uint32_t*)0x200000000244 = 1;
  *(uint64_t*)0x200000000248 = 0xffff1000;
  *(uint64_t*)0x200000000250 = 0x2000;
  *(uint64_t*)0x200000000258 = 0x200000ffc000;
  *(uint64_t*)0x200000000260 = 8;
  *(uint32_t*)0x200000000268 = r[2];
  *(uint32_t*)0x20000000026c = 0;
  memset((void*)0x200000000270, 0, 112);
  syscall(__NR_ioctl, /*fd=*/r[1], /*cmd=*/0x40a0ae49,
          /*arg=*/0x200000000240ul);
  return 0;
}