Re: [PATCH v7 00/12] Direct Map Removal Support for guest_memfd

[Brendan Jackman <jackmanb@google.com>]

On Wed Sep 24, 2025 at 3:10 PM UTC, Patrick Roy wrote:
> From: Patrick Roy <roypat@amazon.co.uk>
>
> [ 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 <f3> 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]  <TASK>
[   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]  </TASK>
[   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 <f3> 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.