Rollup merge of rust-lang#80895 - sfackler:read-to-end-ub, r=m-ou-se

Fix handling of malicious Readers in read_to_end

A malicious `Read` impl could return overly large values from `read`, which would result in the guard's drop impl setting the buffer's length to greater than its capacity! ~~To fix this, the drop impl now uses the safe `truncate` function instead of `set_len` which ensures that this will not happen. The result of calling the function will be nonsensical, but that's fine given the contract violation of the `Read` impl.~~

~~The `Guard` type is also used by `append_to_string` which does not pass untrusted values into the length field, so I've copied the guard type into each function and only modified the one used by `read_to_end`. We could just keep a single one and modify it, but it seems a bit cleaner to keep the guard code close to the functions and related specifically to them.~~

To fix this, we now assert that the returned length is not larger than the buffer passed to the method.

For reference, this bug has been present for ~2.5 years since 1.20: rust-lang@ecbb896.

Closes rust-lang#80894.

library/std/src/io/mod.rs

@@ -366,7 +366,6 @@ where
 {
     let start_len = buf.len();
     let mut g = Guard { len: buf.len(), buf };
-    let ret;
     loop {
         if g.len == g.buf.len() {
             unsafe {
@@ -385,21 +384,20 @@ where
             }
         }
 
-        match r.read(&mut g.buf[g.len..]) {
-            Ok(0) => {
-                ret = Ok(g.len - start_len);
-                break;
+        let buf = &mut g.buf[g.len..];
+        match r.read(buf) {
+            Ok(0) => return Ok(g.len - start_len),
+            Ok(n) => {
+                // We can't allow bogus values from read. If it is too large, the returned vec could have its length
+                // set past its capacity, or if it overflows the vec could be shortened which could create an invalid
+                // string if this is called via read_to_string.
+                assert!(n <= buf.len());
+                g.len += n;
             }
-            Ok(n) => g.len += n,
             Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
-            Err(e) => {
-                ret = Err(e);
-                break;
-            }
+            Err(e) => return Err(e),
         }
     }
-
-    ret
 }
 
 pub(crate) fn default_read_vectored<F>(read: F, bufs: &mut [IoSliceMut<'_>]) -> Result<usize>