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Auto merge of #54397 - alexcrichton:fix-bug-stable, r=steveklabnik

[stable] std: Check for overflow in `str::repeat`

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.
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bors committed Sep 20, 2018
2 parents aa3ca19 + 1b94b84 commit b801ae66425cf7c3c71052b19ef8f145b0d0513d
Showing with 42 additions and 2 deletions.
  1. +13 −0 RELEASES.md
  2. +1 −1 src/bootstrap/channel.rs
  3. +15 −1 src/liballoc/slice.rs
  4. +13 −0 src/liballoc/str.rs
View
@@ -1,3 +1,16 @@
Version 1.29.1 (2018-09-25)
===========================
Security Notes
--------------
- The standard library's `str::repeat` function contained an out of bounds write
caused by an integer overflow. This has been fixed by deterministically
panicking when an overflow happens.
Thank you to Scott McMurray for responsibily disclosing this vulnerability to
us.
Version 1.29.0 (2018-09-13)
==========================
View
@@ -24,7 +24,7 @@ use Build;
use config::Config;
// The version number
pub const CFG_RELEASE_NUM: &str = "1.29.0";
pub const CFG_RELEASE_NUM: &str = "1.29.1";
pub struct GitInfo {
inner: Option<Info>,
View
@@ -392,6 +392,10 @@ impl<T> [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 @@ impl<T> [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 @@ impl<T> [T] {
// 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);
View
@@ -515,6 +515,10 @@ impl str {
/// Create a [`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 @@ impl str {
/// ```
/// 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)) }

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