In the unique reclaim path of BytesMut::reserve, the condition

if v_capacity >= new_cap + offset

uses an unchecked addition. When new_cap + offset overflows usize in release builds, this condition may incorrectly pass, causing self.cap to be set to a value that exceeds the actual allocated capacity. Subsequent APIs such as spare_capacity_mut() then trust this corrupted cap value and may create out-of-bounds slices, leading to UB.

This behavior is observable in release builds (integer overflow wraps), whereas debug builds panic due to overflow checks.

PoC

use bytes::*;

fn main() {
    let mut a = BytesMut::from(&b"hello world"[..]);
    let mut b = a.split_off(5);

    // Ensure b becomes the unique owner of the backing storage
    drop(a);

    // Trigger overflow in new_cap + offset inside reserve
    b.reserve(usize::MAX - 6);

    // This call relies on the corrupted cap and may cause UB & HBO
    b.put_u8(b'h');
}

Workarounds

Users of BytesMut::reserve are only affected if integer overflow checks are configured to wrap. When integer overflow is configured to panic, this issue does not apply.

Impact

When user-provided input is provided to any type that parses with the RFC 2822 format, a denial of service attack via stack exhaustion is possible. The attack relies on formally deprecated and rarely-used features that are part of the RFC 2822 format used in a malicious manner. Ordinary, non-malicious input will never encounter this scenario.

Patches

A limit to the depth of recursion was added in v0.3.47. From this version, an error will be returned rather than exhausting the stack.

Workarounds

Limiting the length of user input is the simplest way to avoid stack exhaustion, as the amount of the stack consumed would be at most a factor of the length of the input.

Uint::overflowing_shl/overflowing_shr returned false-negative overflow flags. overflowing_shl missed bits shifted above BITS but within the top limb (non-limb-aligned widths such as U160), and limbs wholly discarded by shifts >= 64; overflowing_shr missed wholly discarded low limbs. Shifted values were correct; only the flag was wrong.

The wrong flag propagates: checked_shl/checked_shr return Some instead of None, strict_* fail to panic, and saturating_* return a wrapped value instead of saturating. The incorrect checked_shl result causes to_base_be (and string formatting) to loop forever on no-alloc builds for non-limb-aligned widths — a denial of service if formatting is reachable from untrusted input.

Separately, wrapping_shl/wrapping_shr on 64/128/256-bit types truncated the shift amount modulo 2^32, so shifts >= 2^32 returned an incorrectly wrapped value instead of zero; on 32-bit targets the generic path also truncated 64-bit shift amounts.

Callers using checked or saturating shift semantics on untrusted shift amounts may compute incorrect results.