From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pj1-f74.google.com (mail-pj1-f74.google.com [209.85.216.74]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E514F252292 for ; Tue, 2 Dec 2025 15:27:12 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.216.74 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764689234; cv=none; b=RWWYTjnIfjP9Gy7JOJmxQZ99ij8W8U6tF7V6ExtYjkCxxXQzXGwp8RNnSTJs2KefilwPP2xvPWHL5AIoF29+EAbUNg1/jvBbAUeLA5FON3hu3FQwuJyi9lpdJDFDrctUqE2/GLFHrbdkGEtQqTbhzLe0bEr1aYpVpnfhE9rIpyg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764689234; c=relaxed/simple; bh=H+rBOGQRY8iwXaLnmYSjB3wNuaemUT2VNQocG5svCK0=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Content-Type; b=U77GY5whCpGh4ItDkWWJBajRYRCIde09+j9spcz17hi+LeZB8eWuv37VRn1HkzPZLIlXTzyDLvrTMYRYFhVx7kn94Xye3AIWJePFbavX92pV6MsAL6tLl4He7RdD4dLiJCTFzfEKAo7/XKI1f/rrgC9Su+QeTMgEcNVNdfTG1GM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=K3wuuAXp; arc=none smtp.client-ip=209.85.216.74 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="K3wuuAXp" Received: by mail-pj1-f74.google.com with SMTP id 98e67ed59e1d1-341aec498fdso6554850a91.2 for ; Tue, 02 Dec 2025 07:27:12 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1764689232; x=1765294032; darn=vger.kernel.org; h=to:from:subject:message-id:references:mime-version:in-reply-to:date :from:to:cc:subject:date:message-id:reply-to; bh=Orny64yCul+MV1Pms4wehlYly0YZEji2U1uJ3NC+LQ8=; b=K3wuuAXpZ/s04O1Q56EncQcVmX+v1mnED0CECG9mg9VRJlbCeQZQwq4yn61T0rbTnf ChQTnGLwgQ5FwJW5ZeUpMnIZ02KgBuyNaQo0ZmwvxhzoOkBoXgyZ0Dtn0HBHAjhMC1Po stSoMrrDKGRwg289RdroeETmRSqZTKtIumL4If6kIlbS+7ipUcc9J3/MFndZFybpxcFB mOxGS7Wserq0J3qn655HhH690Q5AmWrxpoNi5WURrK3dYNZuvYifZmDdepdAOijvoEmn yRjqlV/NMUI46VB0jFr2YQIm2SvOsOIvr1bh8owpcyNCfH3kjZDgrRMqeRqYm2ht6fTA GmfQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1764689232; x=1765294032; h=to:from:subject:message-id:references:mime-version:in-reply-to:date :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=Orny64yCul+MV1Pms4wehlYly0YZEji2U1uJ3NC+LQ8=; b=NRTA/ZO8S6Z7fY/z8gaOJ0b43vReUJGYkRZ+v7OnVR3Qt3HqUl2FPeQomDj9uSjMqS 7X/I99RfEjz0eleNhY0mz8opIH2YNLjhS24jllCmH9STEy7vSMGpflPOBHTwa7iFqRp0 gWBIQxSKJjzBWaFNEvRGnA0iJvG/8W5sOewA4CZAZ60g5jWYEPYH4rSu+YSX8SBnp31e SgmYGvWe67Vf0cj4de6aTzecpMn/R7DSwnOh3iooS4h4dhP+yjrGg3faSVYC9MFosd0L RXa6X6z8X/REp0BEMFmAV9xCa89aCTlvjYVQDLNdDtG4Lvg3PGJ1eDWVmOLCgSVGAzLl v11g== X-Forwarded-Encrypted: i=1; AJvYcCUDJC9u/QkeQfS/k6pYBfZHuY7l6nPL5GBNiEmoFrjo5GnEsCxkLdudjMGna0uqviiqgSg=@vger.kernel.org X-Gm-Message-State: AOJu0Yxqi9eTjUsxHRkZdBUsKvpvsxnSQ97l6HmUAcsLUKx4I/XtLrdv nC6PEGfuwyoTZZcFc4GqiHrrhrqezrX9Ik4x0sbGyz+nxucPg32dru7clZFFTtQPFE4u78qARgr cFo4ENw== X-Google-Smtp-Source: AGHT+IGQgBm2f7MBgeNPNiOST5+5uXlbW4GbuouSx+idpH0+WuLL2SiEYxWcT+xKLnx1WXLgUzDyjncR9+A= X-Received: from pjbgk19.prod.google.com ([2002:a17:90b:1193:b0:347:76e2:5ff6]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90b:4f88:b0:341:8c15:959e with SMTP id 98e67ed59e1d1-34733f305bfmr44774155a91.17.1764689232200; Tue, 02 Dec 2025 07:27:12 -0800 (PST) Date: Tue, 2 Dec 2025 07:27:10 -0800 In-Reply-To: <20251202020334.1171351-1-seanjc@google.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20251202020334.1171351-1-seanjc@google.com> Message-ID: Subject: Re: [PATCH 0/2] KVM: Fix a guest_memfd memslot UAF From: Sean Christopherson To: Paolo Bonzini , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Alexander Potapenko Content-Type: multipart/mixed; charset="UTF-8"; boundary="2/MWZxQFC6AlmTv/" --2/MWZxQFC6AlmTv/ Content-Type: text/plain; charset=us-ascii Content-Disposition: inline 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... --2/MWZxQFC6AlmTv/ Content-Type: text/x-csrc; charset=us-ascii Content-Disposition: attachment; filename="reproducer.c" // autogenerated by syzkaller (https://github.com/google/syzkaller) #define _GNU_SOURCE #include #include #include #include #include #include #include #include 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; } --2/MWZxQFC6AlmTv/--