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fix(uffd): register guest memory with WRITE_PROTECT in addition to MISSING #12

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ValentaTomas wants to merge 1 commit into from
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fix(uffd): register guest memory with WRITE_PROTECT in addition to MISSING #12

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2 changes: 1 addition & 1 deletion src/vmm/Cargo.toml
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Original file line number Diff line number Diff line change
Expand Up @@ -46,7 +46,7 @@ serde = { version = "1.0.228", features = ["derive", "rc"] }
serde_json = "1.0.149"
slab = "0.4.12"
thiserror = "2.0.18"
userfaultfd = "0.9.0"
userfaultfd = { version = "0.9.0", features = ["linux5_7"] }
utils = { path = "../utils" }
uuid = "1.23.0"
vhost = { version = "0.15.0", features = ["vhost-user-frontend"] }
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20 changes: 17 additions & 3 deletions src/vmm/src/persist.rs
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Expand Up @@ -14,7 +14,7 @@ use std::sync::{Arc, Mutex};

use semver::Version;
use serde::{Deserialize, Serialize};
use userfaultfd::{FeatureFlags, Uffd, UffdBuilder};
use userfaultfd::{FeatureFlags, RegisterMode, Uffd, UffdBuilder};
use vmm_sys_util::sock_ctrl_msg::ScmSocket;

#[cfg(target_arch = "aarch64")]
Expand Down Expand Up @@ -553,9 +553,23 @@ fn guest_memory_from_uffd(
.create()
.map_err(GuestMemoryFromUffdError::Create)?;

// Register every region for both MISSING and WRITE_PROTECT faults.
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@cursor cursor Bot Apr 18, 2026

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Missing WP feature negotiation during UFFDIO_API handshake

Medium Severity

The code registers regions with RegisterMode::WRITE_PROTECT but never negotiates FeatureFlags::PAGEFAULT_FLAG_WP during the UFFDIO_API handshake. The Linux man page states the user needs to check availability of UFFD_FEATURE_PAGEFAULT_FLAG_WP via UFFDIO_API before using write-protect mode. Adding this to require_features would also provide an early, clear failure on platforms that lack WP support instead of a less informative error at registration time.

Fix in Cursor Fix in Web

Reviewed by Cursor Bugbot for commit 32330a4. Configure here.

//
// MISSING is needed so the orchestrator's UFFD handler is woken up the first time the guest
// touches a page that has not yet been populated from the snapshot's memory file.
//
// WRITE_PROTECT is needed so the handler can keep pages it serves in a write-protected state
// (via UFFDIO_COPY_MODE_WP) and observe subsequent writes as new faults — the standard CoW
// tracking pattern that lets the orchestrator know which pages got dirtied after restore.
// Without WRITE_PROTECT registration, UFFDIO_COPY with MODE_WP fails synchronously with
// EINVAL on the very first read fault, breaking the snapshot resume path.
for mem_region in guest_memory.iter() {
uffd.register(mem_region.as_ptr().cast(), mem_region.size() as _)
.map_err(GuestMemoryFromUffdError::Register)?;
uffd.register_with_mode(
mem_region.as_ptr().cast(),
mem_region.size() as _,
RegisterMode::MISSING | RegisterMode::WRITE_PROTECT,
)
.map_err(GuestMemoryFromUffdError::Register)?;
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@cursor cursor Bot Apr 18, 2026

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UFFD WRITE_PROTECT registration breaks aarch64 on kernels below 6.10

High Severity

Unconditionally registering with RegisterMode::WRITE_PROTECT will cause UFFDIO_REGISTER to fail with EINVAL on aarch64 with kernels before 6.10, because arm64 userfaultfd write-protect page table support (pgtable_uffd_wp_supported()) was only added in Linux 6.10. Firecracker supports aarch64 on kernels 5.10 and 6.1, both of which lack this support. This is a regression that breaks all UFFD-based snapshot restores on aarch64, not just WP-tracking ones, since the registration itself fails.

Fix in Cursor Fix in Web

Reviewed by Cursor Bugbot for commit 32330a4. Configure here.

}

send_uffd_handshake(mem_uds_path, &backend_mappings, &uffd)?;
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