Console can allocate too much on the stack with a custom encoding

[vcsjones]

In the unlikely scenario where the console's input is using a custom encoding, and the GetMaxCharCount returns a large value, this can lead to a large stack allocation.

Program to reproduce (works on macOS):

using System.Text;

namespace Foo
{
    class Program
    {
        static void Main()
        {
            Console.InputEncoding = new CustomEncoding();
            Console.ReadKey(true);
        }
    }

    public class CustomEncoding : UTF8Encoding
    {
        public override int GetMaxCharCount(int byteCount) => int.MaxValue;
    }
}

The stackalloc is here:

runtime/src/libraries/System.Console/src/System/IO/StdInReader.cs

Line 53 in 0e18cfd

Span<char> chars = stackalloc char[_encoding.GetMaxCharCount(buffer.Length)];

The value being fed in to the stackalloc's length should be guarded since it is dependent on external factors, in this case the encoding used by stdin.

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Issue Details

---

In the unlikely scenario where the console's input is using a custom encoding, and the GetMaxCharCount returns a large value, this can lead to a large stack allocation.

Program to reproduce (works on macOS):

using System.Text;

namespace Foo
{
    class Program
    {
        static void Main()
        {
            Console.InputEncoding = new CustomEncoding();
            Console.ReadKey(true);
        }
    }

    public class CustomEncoding : UTF8Encoding
    {
        public override int GetMaxCharCount(int byteCount) => int.MaxValue;
    }
}

The stackalloc is here:

runtime/src/libraries/System.Console/src/System/IO/StdInReader.cs

Line 53 in 0e18cfd

Span<char> chars = stackalloc char[_encoding.GetMaxCharCount(buffer.Length)];

The value being fed in to the stackalloc's length should be guarded since it is dependent on external factors, in this case the encoding used by stdin.

Author:	vcsjones
Assignees:	-
Labels:	area-System.Console
Milestone:	-

[stephentoub]

Good catch. Are you planning to put up a PR?

[vcsjones]

@stephentoub Yep.