diff --git a/tokio/src/io/blocking.rs b/tokio/src/io/blocking.rs
index f189136b52e..ebc32c5c6ef 100644
--- a/tokio/src/io/blocking.rs
+++ b/tokio/src/io/blocking.rs
@@ -21,6 +21,7 @@ pub(crate) struct Blocking<T> {
 pub(crate) struct Buf {
     buf: Vec<u8>,
     pos: usize,
+    init_len: usize,
 }
 
 pub(crate) const DEFAULT_MAX_BUF_SIZE: usize = 2 * 1024 * 1024;
@@ -190,6 +191,7 @@ impl Buf {
         Buf {
             buf: Vec::with_capacity(n),
             pos: 0,
+            init_len: 0,
         }
     }
 
@@ -220,6 +222,7 @@ impl Buf {
         let n = cmp::min(src.len(), max_buf_size);
 
         self.buf.extend_from_slice(&src[..n]);
+        self.init_len = cmp::max(self.init_len, self.buf.len());
         n
     }
 
@@ -236,6 +239,27 @@ impl Buf {
             self.buf.reserve(len - self.buf.len());
         }
 
+        if self.init_len < len {
+            debug_assert!(
+                self.init_len < self.buf.capacity(),
+                "init_len of Vec is bigger than the capacity"
+            );
+            debug_assert!(
+                len <= self.buf.capacity(),
+                "uninit area of Vec is bigger than the capacity"
+            );
+
+            let uninit_len = len - self.init_len;
+            // SAFETY: the area is within the allocation of the Vec
+            unsafe {
+                self.buf
+                    .as_mut_ptr()
+                    .add(self.init_len)
+                    .write_bytes(0, uninit_len);
+            }
+        }
+
+        // SAFETY: `len` is within the capacity and is init
         unsafe {
             self.buf.set_len(len);
         }
@@ -287,6 +311,7 @@ cfg_fs! {
                 self.buf.extend_from_slice(&buf[..len]);
                 rem -= len;
             }
+            self.init_len = cmp::max(self.init_len, self.buf.len());
 
             max_buf_size - rem
         }
diff --git a/tokio/src/io/mod.rs b/tokio/src/io/mod.rs
index dc2c4309e66..7a5b4c6b346 100644
--- a/tokio/src/io/mod.rs
+++ b/tokio/src/io/mod.rs
@@ -293,3 +293,6 @@ cfg_io_blocking! {
         pub(crate) use crate::blocking::JoinHandle as Blocking;
     }
 }
+
+#[cfg(test)]
+mod tests;
diff --git a/tokio/src/io/tests.rs b/tokio/src/io/tests.rs
new file mode 100644
index 00000000000..81813f5cc2f
--- /dev/null
+++ b/tokio/src/io/tests.rs
@@ -0,0 +1,39 @@
+cfg_io_blocking! {
+    use std::{
+        io::{self, Read},
+        mem::MaybeUninit,
+    };
+
+    use crate::io::ReadBuf;
+    use super::blocking::Buf;
+
+    const PAYLOAD: &[u8] = b"hello test!";
+
+    // Have miri check that `Buf::ensure_capacity_for` initializes its length.
+    #[test]
+    fn buf_ensure_capacity_for_len_is_init() {
+        const MAX_BUF_SIZE: usize = 128;
+
+        let mut buf = Buf::with_capacity(0);
+
+        let mut dst = [MaybeUninit::uninit(); 64];
+        buf.ensure_capacity_for(&ReadBuf::uninit(&mut dst), MAX_BUF_SIZE);
+        miri_assert_init(buf.bytes());
+        let res = buf.read_from(&mut EnsureInitReader(PAYLOAD)).unwrap();
+        assert_eq!(res, PAYLOAD.len());
+        assert_eq!(buf.bytes(), PAYLOAD);
+    }
+
+    struct EnsureInitReader<R>(R);
+
+    impl<R: Read> Read for EnsureInitReader<R> {
+        fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+            miri_assert_init(buf);
+            self.0.read(buf)
+        }
+    }
+
+    fn miri_assert_init(buf: &[u8]) {
+        for &_b in buf {}
+    }
+}