Fix unsoundness in msys_tty_on (#183)

src/windows_term/mod.rs

@@ -8,15 +8,12 @@ use std::mem;
 use std::os::raw::c_void;
 use std::os::windows::ffi::OsStrExt;
 use std::os::windows::io::AsRawHandle;
-use std::slice;
 use std::{char, mem::MaybeUninit};
 
 use encode_unicode::error::InvalidUtf16Tuple;
 use encode_unicode::CharExt;
 use windows_sys::Win32::Foundation::{HANDLE, INVALID_HANDLE_VALUE, MAX_PATH};
-use windows_sys::Win32::Storage::FileSystem::{
-    FileNameInfo, GetFileInformationByHandleEx, FILE_NAME_INFO,
-};
+use windows_sys::Win32::Storage::FileSystem::{FileNameInfo, GetFileInformationByHandleEx};
 use windows_sys::Win32::System::Console::{
     FillConsoleOutputAttribute, FillConsoleOutputCharacterA, GetConsoleCursorInfo, GetConsoleMode,
     GetConsoleScreenBufferInfo, GetNumberOfConsoleInputEvents, GetStdHandle, ReadConsoleInputW,
@@ -527,22 +524,37 @@ pub fn msys_tty_on(term: &Term) -> bool {
             }
         }
 
-        let size = mem::size_of::<FILE_NAME_INFO>();
-        let mut name_info_bytes = vec![0u8; size + MAX_PATH as usize * mem::size_of::<u16>()];
+        /// Mirrors windows_sys::Win32::Storage::FileSystem::FILE_NAME_INFO, giving
+        /// it a fixed length that we can stack allocate
+        #[repr(C)]
+        #[allow(non_snake_case)]
+        struct FILE_NAME_INFO {
+            FileNameLength: u32,
+            FileName: [u16; MAX_PATH as usize],
+        }
+
+        let mut name_info = FILE_NAME_INFO {
+            FileNameLength: 0,
+            FileName: [0; MAX_PATH as usize],
+        };
         let res = GetFileInformationByHandleEx(
             handle as HANDLE,
             FileNameInfo,
-            &mut *name_info_bytes as *mut _ as *mut c_void,
-            name_info_bytes.len() as u32,
+            &mut name_info as *mut _ as *mut c_void,
+            std::mem::size_of::<FILE_NAME_INFO>() as u32,
         );
         if res == 0 {
             return false;
         }
-        let name_info: &FILE_NAME_INFO = &*(name_info_bytes.as_ptr() as *const FILE_NAME_INFO);
-        let s = slice::from_raw_parts(
-            name_info.FileName.as_ptr(),
-            name_info.FileNameLength as usize / 2,
-        );
+
+        // Use `get` because `FileNameLength` can be out of range.
+        let s = match name_info
+            .FileName
+            .get(..name_info.FileNameLength as usize / 2)
+        {
+            Some(s) => s,
+            None => return false,
+        };
         let name = String::from_utf16_lossy(s);
         // This checks whether 'pty' exists in the file name, which indicates that
         // a pseudo-terminal is attached. To mitigate against false positives