From 8ac88d375e00c91a3db5d78852048322f88be3c1 Mon Sep 17 00:00:00 2001
From: Alex Crichton <alex@alexcrichton.com>
Date: Thu, 20 Sep 2018 09:31:14 -0700
Subject: [PATCH] std: Check for overflow in `str::repeat`
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This commit fixes a buffer overflow issue in the standard library
discovered by Scott McMurray where if a large number was passed to
`str::repeat` it may cause and out of bounds write to the buffer of a `Vec`.
This bug was accidentally introduced in #48657 when optimizing the
`str::repeat` function. The bug affects stable Rust releases 1.26.0 to
1.29.0. We plan on backporting this fix to create a 1.29.1 release, and
the 1.30.0 release onwards will include this fix.

The fix in this commit is to introduce a deterministic panic in the case of
capacity overflow. When repeating a slice where the resulting length is larger
than the address space, there’s no way it can succeed anyway!

The standard library and surrounding libraries were briefly checked to see if
there were othere instances of preallocating a vector with a calculation that
may overflow. No instances of this bug (out of bounds write due to a calculation
overflow) were found at this time.

Note that this commit is the first steps towards fixing this issue,
we'll be making a formal post to the Rust security list once these
commits have been merged.
---
 src/liballoc/slice.rs | 16 +++++++++++++++-
 src/liballoc/str.rs   | 13 +++++++++++++
 2 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/src/liballoc/slice.rs b/src/liballoc/slice.rs
index 9d442b3e00c..6c0b1c33a1f 100644
--- a/src/liballoc/slice.rs
+++ b/src/liballoc/slice.rs
@@ -392,6 +392,10 @@ pub fn into_vec(self: Box<Self>) -> Vec<T> {
 
     /// Creates a vector by repeating a slice `n` times.
     ///
+    /// # Panics
+    ///
+    /// This function will panic if the capacity would overflow.
+    ///
     /// # Examples
     ///
     /// Basic usage:
@@ -403,6 +407,16 @@ pub fn into_vec(self: Box<Self>) -> Vec<T> {
     ///     assert_eq!([1, 2].repeat(3), vec![1, 2, 1, 2, 1, 2]);
     /// }
     /// ```
+    ///
+    /// A panic upon overflow:
+    ///
+    /// ```should_panic
+    /// #![feature(repeat_generic_slice)]
+    /// fn main() {
+    ///     // this will panic at runtime
+    ///     b"0123456789abcdef".repeat(usize::max_value());
+    /// }
+    /// ```
     #[unstable(feature = "repeat_generic_slice",
                reason = "it's on str, why not on slice?",
                issue = "48784")]
@@ -417,7 +431,7 @@ pub fn repeat(&self, n: usize) -> Vec<T> where T: Copy {
         // and `rem` is the remaining part of `n`.
 
         // Using `Vec` to access `set_len()`.
-        let mut buf = Vec::with_capacity(self.len() * n);
+        let mut buf = Vec::with_capacity(self.len().checked_mul(n).expect("capacity overflow"));
 
         // `2^expn` repetition is done by doubling `buf` `expn`-times.
         buf.extend(self);
diff --git a/src/liballoc/str.rs b/src/liballoc/str.rs
index c451a051c74..2af89562d69 100644
--- a/src/liballoc/str.rs
+++ b/src/liballoc/str.rs
@@ -515,6 +515,10 @@ pub fn into_string(self: Box<str>) -> String {
 
     /// Creates a new [`String`] by repeating a string `n` times.
     ///
+    /// # Panics
+    ///
+    /// This function will panic if the capacity would overflow.
+    ///
     /// [`String`]: string/struct.String.html
     ///
     /// # Examples
@@ -524,6 +528,15 @@ pub fn into_string(self: Box<str>) -> String {
     /// ```
     /// assert_eq!("abc".repeat(4), String::from("abcabcabcabc"));
     /// ```
+    ///
+    /// A panic upon overflow:
+    ///
+    /// ```should_panic
+    /// fn main() {
+    ///     // this will panic at runtime
+    ///     "0123456789abcdef".repeat(usize::max_value());
+    /// }
+    /// ```
     #[stable(feature = "repeat_str", since = "1.16.0")]
     pub fn repeat(&self, n: usize) -> String {
         unsafe { String::from_utf8_unchecked(self.as_bytes().repeat(n)) }
-- 
2.44.0