[PM-18100] Add mlock and memfd_secret implementations

crates/bitwarden-crypto/Cargo.toml

@@ -51,6 +51,9 @@ wasm-bindgen = { workspace = true, optional = true }
 zeroize = { version = ">=1.7.0, <2.0", features = ["derive", "aarch64"] }
 zeroizing-alloc = ">=0.1.0, <0.2"
 
+[target.'cfg(all(not(target_arch = "wasm32"), not(windows)))'.dependencies]
+memsec = { version = "0.7.0", features = ["alloc_ext"] }
+
 [dev-dependencies]
 criterion = "0.5.1"
 rand_chacha = "0.3.1"
@@ -68,3 +71,7 @@ required-features = ["no-memory-hardening"]
 
 [lints]
 workspace = true
+
+[package.metadata.cargo-udeps.ignore]
+# This is unused when using --all-features, as that disables memory-hardening
+normal = ["memsec"]

crates/bitwarden-crypto/src/store/backend/implementation/custom_slice/linux_memfd_secret.rs

@@ -0,0 +1,111 @@
+use std::{mem::MaybeUninit, ptr::NonNull, sync::OnceLock};
+
+use super::{KeyId, SliceBackend, SliceLike};
+
+// This is an in-memory key store that is protected by memfd_secret on Linux 5.14+.
+// This should be secure against memory dumps from anything except a malicious kernel driver.
+// Note that not all 5.14+ systems have support for memfd_secret enabled, so
+// LinuxMemfdSecretKeyStore::new returns an Option.
+pub(crate) type LinuxMemfdSecretBackend<Key> = SliceBackend<Key, MemfdSecretImplKeyData>;
+
+pub(crate) struct MemfdSecretImplKeyData {
+    ptr: std::ptr::NonNull<[u8]>,
+    capacity: usize,
+}
+
+// For Send+Sync to be safe, we need to ensure that the memory is only accessed mutably from one
+// thread. To do this, we have to make sure that any funcion in `MemfdSecretImplKeyData` that
+// accesses the pointer mutably is defined as &mut self, and that the pointer is never copied or
+// moved outside the struct.
+unsafe impl Send for MemfdSecretImplKeyData {}
+unsafe impl Sync for MemfdSecretImplKeyData {}
+
+impl Drop for MemfdSecretImplKeyData {
+    fn drop(&mut self) {
+        unsafe {
+            memsec::free_memfd_secret(self.ptr);
+        }
+    }
+}
+
+impl<Key: KeyId> SliceLike<Key> for MemfdSecretImplKeyData {
+    fn is_available() -> bool {
+        static IS_SUPPORTED: OnceLock<bool> = OnceLock::new();
+
+        *IS_SUPPORTED.get_or_init(|| unsafe {
+            let Some(ptr) = memsec::memfd_secret_sized(1) else {
+                return false;
+            };
+            memsec::free_memfd_secret(ptr);
+            true
+        })
+    }
+
+    fn with_capacity(capacity: usize) -> Self {
+        let entry_size = std::mem::size_of::<Option<(Key, Key::KeyValue)>>();
+
+        unsafe {
+            let ptr: NonNull<[u8]> = memsec::memfd_secret_sized(capacity * entry_size)
+                .expect("memfd_secret_sized failed");
+
+            // Initialize the array with Nones using MaybeUninit
+            let uninit_slice: &mut [MaybeUninit<_>] = std::slice::from_raw_parts_mut(
+                ptr.as_ptr() as *mut MaybeUninit<Option<(Key, Key::KeyValue)>>,
+                capacity,
+            );
+            for elem in uninit_slice {
+                elem.write(None);
+            }
+
+            MemfdSecretImplKeyData { ptr, capacity }
+        }
+    }
+
+    fn get_key_data(&self) -> &[Option<(Key, Key::KeyValue)>] {
+        let ptr = self.ptr.as_ptr() as *const Option<(Key, Key::KeyValue)>;
+        // SAFETY: The pointer is valid and points to a valid slice of the correct size.
+        // This function is &self so it only takes a immutable *const pointer.
+        unsafe { std::slice::from_raw_parts(ptr, self.capacity) }
+    }
+
+    fn get_key_data_mut(&mut self) -> &mut [Option<(Key, Key::KeyValue)>] {
+        let ptr = self.ptr.as_ptr() as *mut Option<(Key, Key::KeyValue)>;
+        // SAFETY: The pointer is valid and points to a valid slice of the correct size.
+        // This function is &mut self so it can take a mutable *mut pointer.
+        unsafe { std::slice::from_raw_parts_mut(ptr, self.capacity) }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::store::backend::{
+        implementation::custom_slice::tests::{TestKey, TestKeyValue},
+        StoreBackend as _,
+    };
+
+    #[test]
+    fn test_resize() {
+        let mut store = LinuxMemfdSecretBackend::<TestKey>::with_capacity(1).unwrap();
+
+        for (idx, key) in [
+            TestKey::A,
+            TestKey::B(10),
+            TestKey::C,
+            TestKey::B(7),
+            TestKey::A,
+            TestKey::C,
+        ]
+        .into_iter()
+        .enumerate()
+        {
+            store.upsert(key, TestKeyValue::new(idx));
+        }
+
+        assert_eq!(store.get(TestKey::A), Some(&TestKeyValue::new(4)));
+        assert_eq!(store.get(TestKey::B(10)), Some(&TestKeyValue::new(1)));
+        assert_eq!(store.get(TestKey::C), Some(&TestKeyValue::new(5)));
+        assert_eq!(store.get(TestKey::B(7)), Some(&TestKeyValue::new(3)));
+        assert_eq!(store.get(TestKey::B(20)), None);
+    }
+}