diff --git a/Lib/test/test_logging.py b/Lib/test/test_logging.py
index 9a4543e6195..ef52f147764 100644
--- a/Lib/test/test_logging.py
+++ b/Lib/test/test_logging.py
@@ -4119,8 +4119,6 @@ def test_90195(self):
         # Logger should be enabled, since explicitly mentioned
         self.assertFalse(logger.disabled)
 
-    # TODO: RUSTPYTHON - SemLock not implemented on Windows
-    @unittest.expectedFailureIfWindows("TODO: RUSTPYTHON")
     def test_111615(self):
         # See gh-111615
         import_helper.import_module('_multiprocessing')  # see gh-113692
diff --git a/Lib/test/test_socket.py b/Lib/test/test_socket.py
index e792d4f30a9..fe1fc94b69e 100644
--- a/Lib/test/test_socket.py
+++ b/Lib/test/test_socket.py
@@ -6620,7 +6620,6 @@ def remoteProcessServer(cls, q):
         s2.close()
         s.close()
 
-    @unittest.expectedFailure  # TODO: RUSTPYTHON; multiprocessing.SemLock not implemented
     def testShare(self):
         # Transfer the listening server socket to another process
         # and service it from there.
diff --git a/crates/stdlib/src/mmap.rs b/crates/stdlib/src/mmap.rs
index 20c1fc32c10..c9a6be3b392 100644
--- a/crates/stdlib/src/mmap.rs
+++ b/crates/stdlib/src/mmap.rs
@@ -557,9 +557,15 @@ mod mmap {
             // Parse tagname: None or a string
             let tag_str: Option<String> = match tagname {
                 Some(ref obj) if !vm.is_none(obj) => {
-                    Some(obj.try_to_value::<String>(vm).map_err(|_| {
+                    let s = obj.try_to_value::<String>(vm).map_err(|_| {
                         vm.new_type_error("tagname must be a string or None".to_owned())
-                    })?)
+                    })?;
+                    if s.contains('\0') {
+                        return Err(vm.new_value_error(
+                            "tagname must not contain null characters".to_owned(),
+                        ));
+                    }
+                    Some(s)
                 }
                 _ => None,
             };
diff --git a/crates/stdlib/src/multiprocessing.rs b/crates/stdlib/src/multiprocessing.rs
index b18cbbb24d9..cad20972ac1 100644
--- a/crates/stdlib/src/multiprocessing.rs
+++ b/crates/stdlib/src/multiprocessing.rs
@@ -3,8 +3,373 @@ pub(crate) use _multiprocessing::module_def;
 #[cfg(windows)]
 #[pymodule]
 mod _multiprocessing {
-    use crate::vm::{PyResult, VirtualMachine, function::ArgBytesLike};
+    use crate::vm::{
+        Context, FromArgs, Py, PyPayload, PyRef, PyResult, VirtualMachine,
+        builtins::{PyDict, PyType, PyTypeRef},
+        function::{ArgBytesLike, FuncArgs, KwArgs},
+        types::Constructor,
+    };
+    use core::sync::atomic::{AtomicI32, AtomicU32, Ordering};
+    use windows_sys::Win32::Foundation::{
+        CloseHandle, ERROR_TOO_MANY_POSTS, HANDLE, INVALID_HANDLE_VALUE, WAIT_FAILED,
+        WAIT_OBJECT_0, WAIT_TIMEOUT,
+    };
     use windows_sys::Win32::Networking::WinSock::{self, SOCKET};
+    use windows_sys::Win32::System::Threading::{
+        CreateSemaphoreW, GetCurrentThreadId, INFINITE, ReleaseSemaphore, WaitForSingleObjectEx,
+    };
+
+    // These match the values in Lib/multiprocessing/synchronize.py
+    const RECURSIVE_MUTEX: i32 = 0;
+    const SEMAPHORE: i32 = 1;
+
+    macro_rules! ismine {
+        ($self:expr) => {
+            $self.count.load(Ordering::Acquire) > 0
+                && $self.last_tid.load(Ordering::Acquire) == unsafe { GetCurrentThreadId() }
+        };
+    }
+
+    #[derive(FromArgs)]
+    struct SemLockNewArgs {
+        #[pyarg(positional)]
+        kind: i32,
+        #[pyarg(positional)]
+        value: i32,
+        #[pyarg(positional)]
+        maxvalue: i32,
+        #[pyarg(positional)]
+        name: String,
+        #[pyarg(positional)]
+        unlink: bool,
+    }
+
+    #[pyattr]
+    #[pyclass(name = "SemLock", module = "_multiprocessing")]
+    #[derive(Debug, PyPayload)]
+    struct SemLock {
+        handle: SemHandle,
+        kind: i32,
+        maxvalue: i32,
+        name: Option<String>,
+        last_tid: AtomicU32,
+        count: AtomicI32,
+    }
+
+    #[derive(Debug)]
+    struct SemHandle {
+        raw: HANDLE,
+    }
+
+    unsafe impl Send for SemHandle {}
+    unsafe impl Sync for SemHandle {}
+
+    impl SemHandle {
+        fn create(value: i32, maxvalue: i32, vm: &VirtualMachine) -> PyResult<Self> {
+            let handle =
+                unsafe { CreateSemaphoreW(core::ptr::null(), value, maxvalue, core::ptr::null()) };
+            if handle == 0 as HANDLE {
+                return Err(vm.new_last_os_error());
+            }
+            Ok(SemHandle { raw: handle })
+        }
+
+        #[inline]
+        fn as_raw(&self) -> HANDLE {
+            self.raw
+        }
+    }
+
+    impl Drop for SemHandle {
+        fn drop(&mut self) {
+            if self.raw != 0 as HANDLE && self.raw != INVALID_HANDLE_VALUE {
+                unsafe {
+                    CloseHandle(self.raw);
+                }
+            }
+        }
+    }
+
+    /// _GetSemaphoreValue - get value of semaphore by briefly acquiring and releasing
+    fn get_semaphore_value(handle: HANDLE) -> Result<i32, ()> {
+        match unsafe { WaitForSingleObjectEx(handle, 0, 0) } {
+            WAIT_OBJECT_0 => {
+                let mut previous: i32 = 0;
+                if unsafe { ReleaseSemaphore(handle, 1, &mut previous) } == 0 {
+                    return Err(());
+                }
+                Ok(previous + 1)
+            }
+            WAIT_TIMEOUT => Ok(0),
+            _ => Err(()),
+        }
+    }
+
+    #[pyclass(with(Constructor), flags(BASETYPE))]
+    impl SemLock {
+        #[pygetset]
+        fn handle(&self) -> isize {
+            self.handle.as_raw() as isize
+        }
+
+        #[pygetset]
+        fn kind(&self) -> i32 {
+            self.kind
+        }
+
+        #[pygetset]
+        fn maxvalue(&self) -> i32 {
+            self.maxvalue
+        }
+
+        #[pygetset]
+        fn name(&self) -> Option<String> {
+            self.name.clone()
+        }
+
+        #[pymethod]
+        fn acquire(&self, args: FuncArgs, vm: &VirtualMachine) -> PyResult<bool> {
+            let blocking: bool = args
+                .kwargs
+                .get("block")
+                .or_else(|| args.args.first())
+                .map(|o| o.clone().try_to_bool(vm))
+                .transpose()?
+                .unwrap_or(true);
+
+            let timeout_obj = args
+                .kwargs
+                .get("timeout")
+                .or_else(|| args.args.get(1))
+                .cloned();
+
+            // Calculate timeout in milliseconds
+            let full_msecs: u32 = if !blocking {
+                0
+            } else if timeout_obj.as_ref().is_none_or(|o| vm.is_none(o)) {
+                INFINITE
+            } else {
+                let timeout: f64 = timeout_obj.unwrap().try_float(vm)?.to_f64();
+                let timeout = timeout * 1000.0; // convert to ms
+                if timeout < 0.0 {
+                    0
+                } else if timeout >= 0.5 * INFINITE as f64 {
+                    return Err(vm.new_overflow_error("timeout is too large".to_owned()));
+                } else {
+                    (timeout + 0.5) as u32
+                }
+            };
+
+            // Check whether we already own the lock
+            if self.kind == RECURSIVE_MUTEX && ismine!(self) {
+                self.count.fetch_add(1, Ordering::Release);
+                return Ok(true);
+            }
+
+            // Check whether we can acquire without blocking
+            match unsafe { WaitForSingleObjectEx(self.handle.as_raw(), 0, 0) } {
+                WAIT_OBJECT_0 => {
+                    self.last_tid
+                        .store(unsafe { GetCurrentThreadId() }, Ordering::Release);
+                    self.count.fetch_add(1, Ordering::Release);
+                    return Ok(true);
+                }
+                WAIT_FAILED => return Err(vm.new_last_os_error()),
+                _ => {}
+            }
+
+            // Poll with signal checking (CPython uses WaitForMultipleObjectsEx
+            // with sigint_event; we poll since RustPython has no sigint event)
+            let poll_ms: u32 = 100;
+            let mut elapsed: u32 = 0;
+            loop {
+                let wait_ms = if full_msecs == INFINITE {
+                    poll_ms
+                } else {
+                    let remaining = full_msecs.saturating_sub(elapsed);
+                    if remaining == 0 {
+                        return Ok(false);
+                    }
+                    remaining.min(poll_ms)
+                };
+
+                let res = unsafe { WaitForSingleObjectEx(self.handle.as_raw(), wait_ms, 0) };
+
+                match res {
+                    WAIT_OBJECT_0 => {
+                        self.last_tid
+                            .store(unsafe { GetCurrentThreadId() }, Ordering::Release);
+                        self.count.fetch_add(1, Ordering::Release);
+                        return Ok(true);
+                    }
+                    WAIT_TIMEOUT => {
+                        vm.check_signals()?;
+                        if full_msecs != INFINITE {
+                            elapsed = elapsed.saturating_add(wait_ms);
+                        }
+                    }
+                    WAIT_FAILED => return Err(vm.new_last_os_error()),
+                    _ => {
+                        return Err(vm.new_runtime_error(format!(
+                            "WaitForSingleObject() gave unrecognized value {res}"
+                        )));
+                    }
+                }
+            }
+        }
+
+        #[pymethod]
+        fn release(&self, vm: &VirtualMachine) -> PyResult<()> {
+            if self.kind == RECURSIVE_MUTEX {
+                if !ismine!(self) {
+                    return Err(vm.new_exception_msg(
+                        vm.ctx.exceptions.assertion_error.to_owned(),
+                        "attempt to release recursive lock not owned by thread".to_owned(),
+                    ));
+                }
+                if self.count.load(Ordering::Acquire) > 1 {
+                    self.count.fetch_sub(1, Ordering::Release);
+                    return Ok(());
+                }
+            }
+
+            if unsafe { ReleaseSemaphore(self.handle.as_raw(), 1, core::ptr::null_mut()) } == 0 {
+                let err = unsafe { windows_sys::Win32::Foundation::GetLastError() };
+                if err == ERROR_TOO_MANY_POSTS {
+                    return Err(
+                        vm.new_value_error("semaphore or lock released too many times".to_owned())
+                    );
+                }
+                return Err(vm.new_last_os_error());
+            }
+
+            self.count.fetch_sub(1, Ordering::Release);
+            Ok(())
+        }
+
+        #[pymethod(name = "__enter__")]
+        fn enter(&self, vm: &VirtualMachine) -> PyResult<bool> {
+            self.acquire(
+                FuncArgs::new::<Vec<_>, KwArgs>(
+                    vec![vm.ctx.new_bool(true).into()],
+                    KwArgs::default(),
+                ),
+                vm,
+            )
+        }
+
+        #[pymethod]
+        fn __exit__(&self, _args: FuncArgs, vm: &VirtualMachine) -> PyResult<()> {
+            self.release(vm)
+        }
+
+        #[pyclassmethod(name = "_rebuild")]
+        fn rebuild(
+            cls: PyTypeRef,
+            handle: isize,
+            kind: i32,
+            maxvalue: i32,
+            name: Option<String>,
+            vm: &VirtualMachine,
+        ) -> PyResult {
+            // On Windows, _rebuild receives the handle directly (no sem_open)
+            let zelf = SemLock {
+                handle: SemHandle {
+                    raw: handle as HANDLE,
+                },
+                kind,
+                maxvalue,
+                name,
+                last_tid: AtomicU32::new(0),
+                count: AtomicI32::new(0),
+            };
+            zelf.into_ref_with_type(vm, cls).map(Into::into)
+        }
+
+        #[pymethod]
+        fn _after_fork(&self) {
+            self.count.store(0, Ordering::Release);
+            self.last_tid.store(0, Ordering::Release);
+        }
+
+        #[pymethod]
+        fn __reduce__(&self, vm: &VirtualMachine) -> PyResult {
+            Err(vm.new_type_error("cannot pickle 'SemLock' object".to_owned()))
+        }
+
+        #[pymethod]
+        fn _count(&self) -> i32 {
+            self.count.load(Ordering::Acquire)
+        }
+
+        #[pymethod]
+        fn _is_mine(&self) -> bool {
+            ismine!(self)
+        }
+
+        #[pymethod]
+        fn _get_value(&self, vm: &VirtualMachine) -> PyResult<i32> {
+            get_semaphore_value(self.handle.as_raw()).map_err(|_| vm.new_last_os_error())
+        }
+
+        #[pymethod]
+        fn _is_zero(&self, vm: &VirtualMachine) -> PyResult<bool> {
+            let val =
+                get_semaphore_value(self.handle.as_raw()).map_err(|_| vm.new_last_os_error())?;
+            Ok(val == 0)
+        }
+
+        #[extend_class]
+        fn extend_class(ctx: &Context, class: &Py<PyType>) {
+            class.set_attr(
+                ctx.intern_str("RECURSIVE_MUTEX"),
+                ctx.new_int(RECURSIVE_MUTEX).into(),
+            );
+            class.set_attr(ctx.intern_str("SEMAPHORE"), ctx.new_int(SEMAPHORE).into());
+            class.set_attr(
+                ctx.intern_str("SEM_VALUE_MAX"),
+                ctx.new_int(i32::MAX).into(),
+            );
+        }
+    }
+
+    impl Constructor for SemLock {
+        type Args = SemLockNewArgs;
+
+        fn py_new(_cls: &Py<PyType>, args: Self::Args, vm: &VirtualMachine) -> PyResult<Self> {
+            if args.kind != RECURSIVE_MUTEX && args.kind != SEMAPHORE {
+                return Err(vm.new_value_error("unrecognized kind".to_owned()));
+            }
+            if args.maxvalue <= 0 {
+                return Err(vm.new_value_error("maxvalue must be positive".to_owned()));
+            }
+            if args.value < 0 || args.value > args.maxvalue {
+                return Err(vm.new_value_error("invalid value".to_owned()));
+            }
+
+            let handle = SemHandle::create(args.value, args.maxvalue, vm)?;
+            let name = if args.unlink { None } else { Some(args.name) };
+
+            Ok(SemLock {
+                handle,
+                kind: args.kind,
+                maxvalue: args.maxvalue,
+                name,
+                last_tid: AtomicU32::new(0),
+                count: AtomicI32::new(0),
+            })
+        }
+    }
+
+    // On Windows, sem_unlink is a no-op
+    #[pyfunction]
+    fn sem_unlink(_name: String) {}
+
+    #[pyattr]
+    fn flags(vm: &VirtualMachine) -> PyRef<PyDict> {
+        // On Windows, no HAVE_SEM_OPEN / HAVE_SEM_TIMEDWAIT / HAVE_BROKEN_SEM_GETVALUE
+        vm.ctx.new_dict()
+    }
 
     #[pyfunction]
     fn closesocket(socket: usize, vm: &VirtualMachine) -> PyResult<()> {
@@ -17,14 +382,15 @@ mod _multiprocessing {
     }
 
     #[pyfunction]
-    fn recv(socket: usize, size: usize, vm: &VirtualMachine) -> PyResult<libc::c_int> {
-        let mut buf = vec![0; size];
+    fn recv(socket: usize, size: usize, vm: &VirtualMachine) -> PyResult<Vec<u8>> {
+        let mut buf = vec![0u8; size];
         let n_read =
             unsafe { WinSock::recv(socket as SOCKET, buf.as_mut_ptr() as *mut _, size as i32, 0) };
         if n_read < 0 {
             Err(vm.new_last_os_error())
         } else {
-            Ok(n_read)
+            buf.truncate(n_read as usize);
+            Ok(buf)
         }
     }
 
diff --git a/crates/vm/src/exceptions.rs b/crates/vm/src/exceptions.rs
index 192549d096b..027dbec6964 100644
--- a/crates/vm/src/exceptions.rs
+++ b/crates/vm/src/exceptions.rs
@@ -2061,17 +2061,37 @@ pub(super) mod types {
                     if !vm.is_none(&filename) {
                         let mut args_reduced: Vec<PyObjectRef> = vec![errno, msg, filename];
 
-                        if let Ok(filename2) = obj.get_attr("filename2", vm)
-                            && !vm.is_none(&filename2)
-                        {
+                        let filename2 = obj
+                            .get_attr("filename2", vm)
+                            .ok()
+                            .filter(|f| !vm.is_none(f));
+                        #[cfg(windows)]
+                        let winerror = obj.get_attr("winerror", vm).ok().filter(|w| !vm.is_none(w));
+
+                        if let Some(filename2) = filename2 {
+                            #[cfg(windows)]
+                            {
+                                args_reduced.push(winerror.unwrap_or_else(|| vm.ctx.none()));
+                            }
+                            #[cfg(not(windows))]
+                            args_reduced.push(vm.ctx.none());
                             args_reduced.push(filename2);
+                        } else {
+                            // Diverges from CPython: include winerror even without
+                            // filename2 so it survives pickle round-trips.
+                            #[cfg(windows)]
+                            if let Some(winerror) = winerror {
+                                args_reduced.push(winerror);
+                            }
                         }
                         result.push(args_reduced.into_pytuple(vm).into());
                     } else {
-                        result.push(vm.new_tuple((errno, msg)).into());
+                        // filename is None - use original args as-is
+                        // (may contain winerror at position 3)
+                        result.push(args.into());
                     }
                 } else {
-                    result.push(vm.new_tuple((errno, msg)).into());
+                    result.push(args.into());
                 }
             } else {
                 result.push(args.into());
diff --git a/crates/vm/src/stdlib/_winapi.rs b/crates/vm/src/stdlib/_winapi.rs
index 580300b2b7b..6a78b36f869 100644
--- a/crates/vm/src/stdlib/_winapi.rs
+++ b/crates/vm/src/stdlib/_winapi.rs
@@ -1807,6 +1807,13 @@ mod _winapi {
     ) -> PyResult<WinHandle> {
         use windows_sys::Win32::System::Memory::CreateFileMappingW;
 
+        if let Some(ref n) = name
+            && n.as_str().contains('\0')
+        {
+            return Err(vm.new_value_error(
+                "CreateFileMapping: name must not contain null characters".to_owned(),
+            ));
+        }
         let name_wide = name.as_ref().map(|n| n.as_wtf8().to_wide_with_nul());
         let name_ptr = name_wide.as_ref().map_or(null(), |n| n.as_ptr());
 
@@ -1837,6 +1844,11 @@ mod _winapi {
     ) -> PyResult<WinHandle> {
         use windows_sys::Win32::System::Memory::OpenFileMappingW;
 
+        if name.as_str().contains('\0') {
+            return Err(vm.new_value_error(
+                "OpenFileMapping: name must not contain null characters".to_owned(),
+            ));
+        }
         let name_wide = name.as_wtf8().to_wide_with_nul();
         let handle = unsafe {
             OpenFileMappingW(