From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-wm1-f73.google.com (mail-wm1-f73.google.com [209.85.128.73]) (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 0E2BC32ED5D for ; Fri, 7 Nov 2025 15:54:34 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.128.73 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762530877; cv=none; b=Va0QmehwWwJVBaZEYkwlt22dc/lNuu4w8knMuKCa74KLHqIfa8rtSnFaj+tXdJW0gN1CADSz1/vgSnJ8nSPnkMthGEJziYOEmDIFV3+h8C47Ze/OKZrTcUph8d6WSmgi5M3g86t+vIH6QFnpj4QZeeEgGIHAJdwpC3oZjO7T8oM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762530877; c=relaxed/simple; bh=EGnBT7eq4xDsLWoMBKHX6mL+kag/LZD0AS27uXDXkfo=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=khmDmUj34uwARVpUyBoC/b6o/SXP+cfelfOv6EY8rbp7Dn8iU6EnrX9lK86+SumPThv3kvIPsf7zssevU/pUq9JavrcseGT1luYYZRhirzjJ4KuoA7yo2IMRSxO6NrfD2hQUbpud66DEe226rqzEcS/ZPCmwltUIFKZGXl2eUMU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--jackmanb.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=a9RSXv0+; arc=none smtp.client-ip=209.85.128.73 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--jackmanb.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="a9RSXv0+" Received: by mail-wm1-f73.google.com with SMTP id 5b1f17b1804b1-477171bbf51so7270345e9.3 for ; Fri, 07 Nov 2025 07:54:34 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1762530873; x=1763135673; darn=lists.linux.dev; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=k6QqLCiZ3bKYilApTrBVoLgHK6pPaQj/ENLeWCSsRM0=; b=a9RSXv0+FOxBL/c3sZKouXIJXlw2qjiUVNWertAUBH7LtWUESKMNzeNbwUmCluN/rO 5xidh+Po3BZ/f9lFzi8XK6xF8dT4+7aoe5SoULjaUtUNUOc7ViMhAssDHw/T2IR2hszP qCgYv1q6wV40uXESl3C033kzXABOVDLrXXCyATiB3hVFs9y5VVL8MWCeZ5ULTb6KdcDf 6bjqN2hCMyQIApYnVu/NeSC6clNpSld9TRPGt+5/ZRpA65hC0XEeFEdvqIj6V7EU2Bhs i8QkBGq78n9mWcJXoyn6IVOHF4DshhQ3l8xqsoVeOXfveZ6a1hNrFIZzWN2wewN9In7m yK5Q== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1762530873; x=1763135673; h=cc: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=k6QqLCiZ3bKYilApTrBVoLgHK6pPaQj/ENLeWCSsRM0=; b=r/vDsfI0g/eINCn9XTnsPhVASjZlY0zmUxQgDCn5h55LDAgt3n6jxAnS7uipZKfCv/ WVv/6/WWYTN1qtYw7V9qhpo2Ta4ooeyAGrNhlMX9atzT6NX6uqD0SdmrSVsY87g1brL3 4w3lVRAB5ja1CyBaQdX4JYX3pGauybCEj6lOP+oL0PG+6kc38XOCa8k3Wxj7Vh/+vIO/ 3YvhVI0y17atKfj+4s923sxra5dSfp2sFVLUdcpjusIesjqMDhN2NFiux3YMsMMdAoix Ufkwhu+1OHGBBmezgCZ/a/9eXacykCqgoUKFZ4cYiPDFmWY4jhriztS2QLe1x68btPTm xCeA== X-Forwarded-Encrypted: i=1; AJvYcCVH+A/xanqtrB/nRR/k2pR/Cnp0Alq2CLmyvzPB+mdhmD8Qf3L7OGSvukXPlXiK1pEvv+jNoSc=@lists.linux.dev X-Gm-Message-State: AOJu0YwyILkbfxmgnnenltJaj4v5MAGuKtXVDx0sfiFcOUy2kAoNMNuj fU0iipGrsODMxwb1uMIkJlGI5FN5eo/jisj7mEiHTVAFaPp+rvm0sQyUpDWX9UfnyvUmrxxKkh6 RBeRsCjszcLIdsg== X-Google-Smtp-Source: AGHT+IF10pfYz/aX86VOtk1/4ToFDHxeKDSFR7QzVJHGBwU7n9WXznaipUt0c+mOTXVDyymnjKnmVDeDshscaw== X-Received: from wmbjd18.prod.google.com ([2002:a05:600c:68d2:b0:477:554c:6842]) (user=jackmanb job=prod-delivery.src-stubby-dispatcher) by 2002:a05:600c:4e88:b0:471:14af:c715 with SMTP id 5b1f17b1804b1-4776bc9f963mr29299435e9.3.1762530872213; Fri, 07 Nov 2025 07:54:32 -0800 (PST) Date: Fri, 07 Nov 2025 15:54:31 +0000 In-Reply-To: <20250924151101.2225820-1-patrick.roy@campus.lmu.de> Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20250924151101.2225820-1-patrick.roy@campus.lmu.de> X-Mailer: aerc 0.21.0 Message-ID: Subject: Re: [PATCH v7 00/12] Direct Map Removal Support for guest_memfd From: Brendan Jackman To: Patrick Roy Cc: Patrick Roy , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , Content-Type: text/plain; charset="UTF-8" On Wed Sep 24, 2025 at 3:10 PM UTC, Patrick Roy wrote: > From: Patrick Roy > > [ based on kvm/next ] > > Unmapping virtual machine guest memory from the host kernel's direct map is a > successful mitigation against Spectre-style transient execution issues: If the > kernel page tables do not contain entries pointing to guest memory, then any > attempted speculative read through the direct map will necessarily be blocked > by the MMU before any observable microarchitectural side-effects happen. This > means that Spectre-gadgets and similar cannot be used to target virtual machine > memory. Roughly 60% of speculative execution issues fall into this category [1, > Table 1]. > > This patch series extends guest_memfd with the ability to remove its memory > from the host kernel's direct map, to be able to attain the above protection > for KVM guests running inside guest_memfd. > > Additionally, a Firecracker branch with support for these VMs can be found on > GitHub [2]. > > For more details, please refer to the v5 cover letter [v5]. No > substantial changes in design have taken place since. > > === Changes Since v6 === > > - Drop patch for passing struct address_space to ->free_folio(), due to > possible races with freeing of the address_space. (Hugh) > - Stop using PG_uptodate / gmem preparedness tracking to keep track of > direct map state. Instead, use the lowest bit of folio->private. (Mike, David) > - Do direct map removal when establishing mapping of gmem folio instead > of at allocation time, due to impossibility of handling direct map > removal errors in kvm_gmem_populate(). (Patrick) > - Do TLB flushes after direct map removal, and provide a module > parameter to opt out from them, and a new patch to export > flush_tlb_kernel_range() to KVM. (Will) > > [1]: https://download.vusec.net/papers/quarantine_raid23.pdf > [2]: https://github.com/firecracker-microvm/firecracker/tree/feature/secret-hiding I just got around to trying this out, I checked out this patchset using its base-commit and grabbed the Firecracker branch. Things seem OK until I set the secrets_free flag in the Firecracker config which IIUC makes it set GUEST_MEMFD_FLAG_NO_DIRECT_MAP. If I set it, I find the guest doesn't show anything on the console. Running it in a VM and attaching GDB suggests that it's entering the guest repeatedly, it doesn't seem like the vCPU thread is stuck or anything. I'm a bit clueless about how to debug that (so far, whenever I've broken KVM, things always exploded very dramatically). Anyway, if I then kill the firecracker process, the host sometimes crashes, I think this is the most suggestive splat I've seen: [ 99.673420][ T2] BUG: unable to handle page fault for address: ffff888012804000 [ 99.676216][ T2] #PF: supervisor write access in kernel mode [ 99.678381][ T2] #PF: error_code(0x0002) - not-present page [ 99.680499][ T2] PGD 2e01067 P4D 2e01067 PUD 2e02067 PMD 12801063 PTE 800fffffed7fb020 [ 99.683374][ T2] Oops: Oops: 0002 [#1] SMP [ 99.685004][ T2] CPU: 0 UID: 0 PID: 2 Comm: kthreadd Not tainted 6.17.0-rc7-00366-g473c46a3cb2a #106 NONE [ 99.688514][ T2] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 0.1 11/11/2019 [ 99.691547][ T2] RIP: 0010:clear_page_erms+0x7/0x10 [ 99.693440][ T2] Code: 48 89 47 18 48 89 47 20 48 89 47 28 48 89 47 30 48 89 47 38 48 8d 7f 40 75 d9 90 c3 0f 1f 80 00 00 00 00 b9 00 10 00 00 31 c0 aa c3 66 0f 1f 44 00 00 48 83 f9 40 73 2a 83 f9 08 73 0f 85 c9 [ 99.700188][ T2] RSP: 0018:ffff88800318fc10 EFLAGS: 00010246 [ 99.702321][ T2] RAX: 0000000000000000 RBX: 0000000000400dc0 RCX: 0000000000001000 [ 99.705100][ T2] RDX: ffffea00004a0100 RSI: ffffea00004a0200 RDI: ffff888012804000 [ 99.707861][ T2] RBP: 0000000000000801 R08: 0000000000000000 R09: 0000000000000000 [ 99.710648][ T2] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000002 [ 99.713412][ T2] R13: 0000000000000801 R14: ffffea00004a0100 R15: ffffffff81f4df80 [ 99.716191][ T2] FS: 0000000000000000(0000) GS:ffff8880bbf28000(0000) knlGS:0000000000000000 [ 99.719316][ T2] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 99.721648][ T2] CR2: ffff888012804000 CR3: 0000000007583001 CR4: 0000000000372eb0 [ 99.724421][ T2] Call Trace: [ 99.725608][ T2] [ 99.726646][ T2] get_page_from_freelist+0x6fe/0x14b0 [ 99.728583][ T2] ? fs_reclaim_acquire+0x43/0xe0 [ 99.730325][ T2] ? find_held_lock+0x2b/0x80 [ 99.731965][ T2] __alloc_frozen_pages_noprof+0x147/0x2d0 [ 99.734003][ T2] __alloc_pages_noprof+0x5/0x50 [ 99.735766][ T2] copy_process+0x1b1/0x1b30 [ 99.737398][ T2] ? lock_is_held_type+0x89/0x100 [ 99.739157][ T2] ? kthreadd+0x25/0x190 [ 99.740664][ T2] kernel_clone+0x59/0x390 [ 99.742213][ T2] ? kthreadd+0x25/0x190 [ 99.743728][ T2] kernel_thread+0x55/0x70 [ 99.745310][ T2] ? kthread_complete_and_exit+0x20/0x20 [ 99.747265][ T2] kthreadd+0x117/0x190 [ 99.748748][ T2] ? kthread_is_per_cpu+0x30/0x30 [ 99.750509][ T2] ret_from_fork+0x16b/0x1e0 [ 99.752193][ T2] ? kthread_is_per_cpu+0x30/0x30 [ 99.753992][ T2] ret_from_fork_asm+0x11/0x20 [ 99.755717][ T2] [ 99.756861][ T2] CR2: ffff888012804000 [ 99.758353][ T2] ---[ end trace 0000000000000000 ]--- [ 99.760319][ T2] RIP: 0010:clear_page_erms+0x7/0x10 [ 99.762209][ T2] Code: 48 89 47 18 48 89 47 20 48 89 47 28 48 89 47 30 48 89 47 38 48 8d 7f 40 75 d9 90 c3 0f 1f 80 00 00 00 00 b9 00 10 00 00 31 c0 aa c3 66 0f 1f 44 00 00 48 83 f9 40 73 2a 83 f9 08 73 0f 85 c9 [ 99.769129][ T2] RSP: 0018:ffff88800318fc10 EFLAGS: 00010246 [ 99.771297][ T2] RAX: 0000000000000000 RBX: 0000000000400dc0 RCX: 0000000000001000 [ 99.774126][ T2] RDX: ffffea00004a0100 RSI: ffffea00004a0200 RDI: ffff888012804000 [ 99.777013][ T2] RBP: 0000000000000801 R08: 0000000000000000 R09: 0000000000000000 [ 99.779827][ T2] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000002 [ 99.782641][ T2] R13: 0000000000000801 R14: ffffea00004a0100 R15: ffffffff81f4df80 [ 99.785487][ T2] FS: 0000000000000000(0000) GS:ffff8880bbf28000(0000) knlGS:0000000000000000 [ 99.788671][ T2] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 99.791012][ T2] CR2: ffff888012804000 CR3: 0000000007583001 CR4: 0000000000372eb0 [ 99.793863][ T2] Kernel panic - not syncing: Fatal exception [ 99.796760][ T2] Kernel Offset: disabled [ 99.798296][ T2] ---[ end Kernel panic - not syncing: Fatal exception ]--- This makes me suspect the kvm_gmem_folio_restore_direct_map() path isn't working or isn't getting called. If anyone wants help trying to reproduce this let me know.