Re: [PATCH] random: allow writes to /dev/urandom to influence fast init

From: "Alex Xu (Hello71)" <alex_y_xu@yahoo.ca>
To: "Jason A. Donenfeld" <Jason@zx2c4.com>,
	linux-crypto@vger.kernel.org, linux-kernel@vger.kernel.org
Cc: Jann Horn <jannh@google.com>,
	Dominik Brodowski <linux@dominikbrodowski.net>,
	Guenter Roeck <linux@roeck-us.net>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Theodore Ts'o <tytso@mit.edu>
Subject: Re: [PATCH] random: allow writes to /dev/urandom to influence fast init
Date: Wed, 23 Mar 2022 00:30:14 -0400	[thread overview]
Message-ID: <1648009787.fah6dos6ya.none@localhost> (raw)
In-Reply-To: <20220322191436.110963-1-Jason@zx2c4.com>

I searched for users of RNDADDTOENTCNT using 
(?s:ioctl.{1,500}RNDADDTOENTCNT) on Debian Code Search and 
"/(?s)ioctl.{1,40},\s*RNDADDTOENTCNT/ -path:incfs_test.c" on GitHub Code 
Search (beta).

Several programs use it for testing purposes, without writing any 
entropy to /dev/random or /dev/urandom, including rauc, wireguard, and 
openSUSE kdump. Several programs use it as intended, after writing 
entropy to /dev/random or /dev/urandom. Of the latter group,

- kata-containers is a lightweight VM implementation. Its guest-side 
  agent offers a gRPC endpoint which will write the provided data to 
  /dev/random, then call RNDADDTOENTCNT with the length of the data, 
  then call RNDRESEEDRNG. As far as I can tell, this endpoint is 
  made available to users on the host, but is not used by 
  kata-containers itself.

- aws-nitro-enclaves-sdk-c is an SDK for building lightweight VMs to be 
  used with AWS Nitro Enclaves. kmstool-enclave is a sample application 
  provided, which writes "up to 256 bytes" (from where?) to /dev/random, 
  then calls RNDADDTOENTCNT, then repeats the process until it reaches 
  1024 bytes.

- sandy-harris/maxwell is a "jitter entropy" daemon, similar to haveged. 
  It writes 4 bytes of "generated entropy" to /dev/random, then calls 
  RNDADDTOENTCNT, then repeats.

- guix is, among other things, a "GNU/"Linux distribution. The provided 
  base services write the seed file to /dev/urandom, then call 
  RNDADDTOENTCNT, then write 512 bytes from /dev/hwrng to /dev/urandom, 
  then call RNDADDTOENTCNT, then "immediately" read 512 bytes from 
  /dev/urandom and write it to the seed file. On shutdown, 512 bytes are 
  read from /dev/urandom and written to the seed file.

I was unable to locate any other public non-archived usages of 
RNDADDTOENTCNT on Debian or GitHub Code Search.

I don't have any particular expertise with the random subsystem or 
conclusions to make from this data, but I hope this helps inform the 
discussion.

Cheers,
Alex.