From: Sean Christopherson <seanjc@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>,
kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
Alexander Potapenko <glider@google.com>
Subject: Re: [PATCH 0/2] KVM: Fix a guest_memfd memslot UAF
Date: Tue, 2 Dec 2025 07:27:10 -0800 [thread overview]
Message-ID: <aS8FTggpT_7cY3cr@google.com> (raw)
In-Reply-To: <20251202020334.1171351-1-seanjc@google.com>
[-- Attachment #1: Type: text/plain, Size: 904 bytes --]
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...
[-- Attachment #2: reproducer.c --]
[-- Type: text/x-csrc, Size: 5755 bytes --]
// 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;
}
next prev parent reply other threads:[~2025-12-02 15:27 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-12-02 2:03 [PATCH 0/2] KVM: Fix a guest_memfd memslot UAF Sean Christopherson
2025-12-02 2:03 ` [PATCH 1/2] KVM: Disallow toggling KVM_MEM_GUEST_MEMFD on an existing memslot Sean Christopherson
2025-12-02 2:03 ` [PATCH 2/2] KVM: Harden and prepare for modifying existing guest_memfd memslots Sean Christopherson
2025-12-02 15:27 ` Sean Christopherson [this message]
2025-12-05 16:59 ` [PATCH 0/2] KVM: Fix a guest_memfd memslot UAF Sean Christopherson
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