CVS commit: src/lib/libc/gen

["Taylor R Campbell" <riastradh%netbsd.org@localhost>]

Module Name:    src
Committed By:   riastradh
Date:           Mon Aug 26 15:19:22 UTC 2024

Modified Files:
        src/lib/libc/gen: arc4random.c

Log Message:
arc4random(3): Reseed if system entropy epoch changes.

This can happen, for example, if the system is a VM instance, and the
VM is cloned.

This incurs the cost of a system call on every arc4random call, which
is unfortunate, but

1. we don't currently have a (machine-independent) mechanism for
   exposing a read-only page to userland shared by the kernel to
   enable a cheaper access path to the entropy epoch; and

2. the algorithm here -- a simple application of ChaCha -- is likely
   also a bottleneck and could be much cheaper by

   (a) using sys/crypto/chacha for machine-dependent vectorized
       ChaCha code, and

   (b) filling a buffer (somewhere between a cipher block and a page)
       in a batch at a time, instead of running ChaCha to generate
       only 32 bytes at a time.

So although this might be a performance hit, the security benefit is
worthwhile and we have a clear path to do better than reversing the
performance hit later.

PR kern/58632: getentropy(2) and arc4random(3) do not reseed on VM
fork


To generate a diff of this commit:
cvs rdiff -u -r1.34 -r1.35 src/lib/libc/gen/arc4random.c

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.