CVE-2026-31592 KVM: SEV: Protect *all* of sev_mem_enc_register_region() with kvm->lock
In the Linux kernel, the following vulnerability has been resolved:

KVM: SEV: Protect *all* of sev_mem_enc_register_region() with kvm->lock

Take and hold kvm->lock for before checking sev_guest() in
sev_mem_enc_register_region(), as sev_guest() isn't stable unless kvm->lock
is held (or KVM can guarantee KVM_SEV_INIT{2} has completed and can't
rollack state).  If KVM_SEV_INIT{2} fails, KVM can end up trying to add to
a not-yet-initialized sev->regions_list, e.g. triggering a #GP

  Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP KASAN NOPTI
  KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
  CPU: 110 UID: 0 PID: 72717 Comm: syz.15.11462 Tainted: G     U  W  O        6.16.0-smp-DEV #1 NONE
  Tainted: [U]=USER, [W]=WARN, [O]=OOT_MODULE
  Hardware name: Google, Inc. Arcadia_IT_80/Arcadia_IT_80, BIOS 12.52.0-0 10/28/2024
  RIP: 0010:sev_mem_enc_register_region+0x3f0/0x4f0 ../include/linux/list.h:83
  Code: <41> 80 3c 04 00 74 08 4c 89 ff e8 f1 c7 a2 00 49 39 ed 0f 84 c6 00
  RSP: 0018:ffff88838647fbb8 EFLAGS: 00010256
  RAX: dffffc0000000000 RBX: 1ffff92015cf1e0b RCX: dffffc0000000000
  RDX: 0000000000000000 RSI: 0000000000001000 RDI: ffff888367870000
  RBP: ffffc900ae78f050 R08: ffffea000d9e0007 R09: 1ffffd4001b3c000
  R10: dffffc0000000000 R11: fffff94001b3c001 R12: 0000000000000000
  R13: ffff8982ab0bde00 R14: ffffc900ae78f058 R15: 0000000000000000
  FS:  00007f34e9dc66c0(0000) GS:ffff89ee64d33000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 00007fe180adef98 CR3: 000000047210e000 CR4: 0000000000350ef0
  Call Trace:
   <TASK>
   kvm_arch_vm_ioctl+0xa72/0x1240 ../arch/x86/kvm/x86.c:7371
   kvm_vm_ioctl+0x649/0x990 ../virt/kvm/kvm_main.c:5363
   __se_sys_ioctl+0x101/0x170 ../fs/ioctl.c:51
   do_syscall_x64 ../arch/x86/entry/syscall_64.c:63 [inline]
   do_syscall_64+0x6f/0x1f0 ../arch/x86/entry/syscall_64.c:94
   entry_SYSCALL_64_after_hwframe+0x76/0x7e
  RIP: 0033:0x7f34e9f7e9a9
  Code: <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
  RSP: 002b:00007f34e9dc6038 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
  RAX: ffffffffffffffda RBX: 00007f34ea1a6080 RCX: 00007f34e9f7e9a9
  RDX: 0000200000000280 RSI: 000000008010aebb RDI: 0000000000000007
  RBP: 00007f34ea000d69 R08: 0000000000000000 R09: 0000000000000000
  R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
  R13: 0000000000000000 R14: 00007f34ea1a6080 R15: 00007ffce77197a8
   </TASK>

with a syzlang reproducer that looks like:

  syz_kvm_add_vcpu$x86(0x0, &(0x7f0000000040)={0x0, &(0x7f0000000180)=ANY=[], 0x70}) (async)
  syz_kvm_add_vcpu$x86(0x0, &(0x7f0000000080)={0x0, &(0x7f0000000180)=ANY=[@ANYBLOB="..."], 0x4f}) (async)
  r0 = openat$kvm(0xffffffffffffff9c, &(0x7f0000000200), 0x0, 0x0)
  r1 = ioctl$KVM_CREATE_VM(r0, 0xae01, 0x0)
  r2 = openat$kvm(0xffffffffffffff9c, &(0x7f0000000240), 0x0, 0x0)
  r3 = ioctl$KVM_CREATE_VM(r2, 0xae01, 0x0)
  ioctl$KVM_SET_CLOCK(r3, 0xc008aeba, &(0x7f0000000040)={0x1, 0x8, 0x0, 0x5625e9b0}) (async)
  ioctl$KVM_SET_PIT2(r3, 0x8010aebb, &(0x7f0000000280)={[...], 0x5}) (async)
  ioctl$KVM_SET_PIT2(r1, 0x4070aea0, 0x0) (async)
  r4 = ioctl$KVM_CREATE_VM(0xffffffffffffffff, 0xae01, 0x0)
  openat$kvm(0xffffffffffffff9c, 0x0, 0x0, 0x0) (async)
  ioctl$KVM_SET_USER_MEMORY_REGION(r4, 0x4020ae46, &(0x7f0000000400)={0x0, 0x0, 0x0, 0x2000, &(0x7f0000001000/0x2000)=nil}) (async)
  r5 = ioctl$KVM_CREATE_VCPU(r4, 0xae41, 0x2)
  close(r0) (async)
  openat$kvm(0xffffffffffffff9c, &(0x7f0000000000), 0x8000, 0x0) (async)
  ioctl$KVM_SET_GUEST_DEBUG(r5, 0x4048ae9b, &(0x7f0000000300)={0x4376ea830d46549b, 0x0, [0x46, 0x0, 0x0, 0x0, 0x0, 0x1000]}) (async)
  ioctl$KVM_RUN(r5, 0xae80, 0x0)

Opportunistically use guard() to avoid having to define a new error label
and goto usage.
CVSS v3.1 score: 5.5