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getrandom(2): Add Linux GRND_INSECURE API flag

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Treat it as a synonym for GRND_NONBLOCK.  The reasoning is this:

We have two choices for handling Linux's GRND_INSECURE API flag.

1. We could ignore it completely (like GRND_RANDOM).  However, this might
produce the surprising result of GRND_INSECURE requests blocking, when the
Linux API does not block.

2. Alternatively, we could treat GRND_INSECURE requests as requests for
GRND_NONBLOCk.  Here, the surprising result for Linux programs is that
invocations with unseeded random(4) will produce EAGAIN, rather than
garbage.

Honoring the flag in the way Linux does seems fraught.  If we actually use
the output of a random(4) implementation prior to seeding, we leak some
entropy (in an information theory and also practical sense) from what will
be the initial seed to attackers (or allow attackers to arbitrary DoS
initial seeding, if we don't leak).  This seems unacceptable -- it defeats
the purpose of blocking on initial seeding.

Secondary to that concern, before seeding we may have arbitrarily little
entropy collected; producing output from zero or a handful of entropy bits
does not seem particularly useful to userspace.

If userspace can accept garbage, insecure, non-random bytes, they can create
their own insecure garbage with srandom(time(NULL)) or similar.  Any program
which would be satisfied with a 3-bit key CTR stream has no need for CSPRNG
bytes.  So asking the kernel to produce such an output from the secure
getrandom(2) API seems inane.

For now, we've elected to emulate GRND_INSECURE as an alternative spelling
of GRND_NONBLOCK (2).  Consider this API not-quite stable for now.  We
guarantee it will never block.  But we will attempt to monitor actual port
uptake of this bizarre API and may revise our plans for the unseeded
behavior (prior stable/13 branching).

Approved by:	csprng(markm), manpages(bcr)
See also:	https://lwn.net/ml/linux-kernel/cover.1577088521.git.luto@kernel.org/
See also:	https://lwn.net/ml/linux-kernel/20200107204400.GH3619@mit.edu/
Differential Revision:	https://reviews.freebsd.org/D23130
This commit is contained in:
Conrad Meyer 2020-01-12 20:47:38 +00:00
parent 526473251e
commit 86def3dcd6
3 changed files with 53 additions and 6 deletions

22
lib/libc/sys/getrandom.2

@ -1,5 +1,4 @@
.\" Copyright (c) 2018 Conrad Meyer <cem@FreeBSD.org>
.\" All rights reserved.
.\" Copyright 2020, 2018 Conrad Meyer <cem@FreeBSD.org>. All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
@ -24,7 +23,7 @@
.\"
.\" $FreeBSD$
.\"
.Dd February 24, 2018
.Dd January 12, 2020
.Dt GETRANDOM 2
.Os
.Sh NAME
@ -64,13 +63,16 @@ This flag does nothing on
and
.Pa /dev/urandom
are identical.
.It Ql GRND_INSECURE
This flag is treated as an alternative name for
.Dv GRND_NONBLOCK .
It is provided solely for API compatibility with Linux.
.El
.Pp
If the
.Xr random 4
device has been seeded, reads of up to 256 bytes will always return as many
bytes as requested and will not be interrupted by signals.
.Pp
.Sh RETURN VALUES
Upon successful completion, the number of bytes which were actually read is
returned.
@ -87,6 +89,8 @@ operation returns the following errors:
.It Bq Er EAGAIN
The
.Ql GRND_NONBLOCK
(or
.Ql GRND_INSECURE )
flag was set and the
.Xr random 4
device was not yet seeded.
@ -111,7 +115,7 @@ was larger than
.Xr getentropy 3 ,
.Xr random 4
.Sh STANDARDS
.Fn getentropy
.Fn getrandom
is non-standard.
It is present in Linux.
.Sh HISTORY
@ -119,3 +123,11 @@ The
.Fn getrandom
system call first appeared in
.Fx 12.0 .
.Sh CAVEATS
Unlike Linux, the
.Dv GRND_INSECURE
flag on
.Fx
does not produce any output before the
.Xr random 4
device is seeded.

36
sys/kern/sys_getrandom.c

@ -38,7 +38,7 @@ __FBSDID("$FreeBSD$");
#include <sys/systm.h>
#include <sys/uio.h>
#define GRND_VALIDFLAGS (GRND_NONBLOCK | GRND_RANDOM)
#define GRND_VALIDFLAGS (GRND_NONBLOCK | GRND_RANDOM | GRND_INSECURE)
/*
* random_read_uio(9) returns EWOULDBLOCK if a nonblocking request would block,
@ -60,6 +60,40 @@ kern_getrandom(struct thread *td, void *user_buf, size_t buflen,
if (buflen > IOSIZE_MAX)
return (EINVAL);
/*
* Linux compatibility: We have two choices for handling Linux's
* GRND_INSECURE.
*
* 1. We could ignore it completely (like GRND_RANDOM). However, this
* might produce the surprising result of GRND_INSECURE requests
* blocking, when the Linux API does not block.
*
* 2. Alternatively, we could treat GRND_INSECURE requests as requests
* for GRND_NONBLOCk. Here, the surprising result for Linux programs
* is that invocations with unseeded random(4) will produce EAGAIN,
* rather than garbage.
*
* Honoring the flag in the way Linux does seems fraught. If we
* actually use the output of a random(4) implementation prior to
* seeding, we leak some entropy about the initial seed to attackers.
* This seems unacceptable -- it defeats the purpose of blocking on
* initial seeding.
*
* Secondary to that concern, before seeding we may have arbitrarily
* little entropy collected; producing output from zero or a handful of
* entropy bits does not seem particularly useful to userspace.
*
* If userspace can accept garbage, insecure non-random bytes, they can
* create their own insecure garbage with srandom(time(NULL)) or
* similar. Asking the kernel to produce it from the secure
* getrandom(2) API seems inane.
*
* We elect to emulate GRND_INSECURE as an alternative spelling of
* GRND_NONBLOCK (2).
*/
if ((flags & GRND_INSECURE) != 0)
flags |= GRND_NONBLOCK;
if (buflen == 0) {
td->td_retval[0] = 0;
return (0);

1
sys/sys/random.h

@ -159,6 +159,7 @@ void random_harvest_deregister_source(enum random_entropy_source);
#define GRND_NONBLOCK 0x1
#define GRND_RANDOM 0x2
#define GRND_INSECURE 0x4
__BEGIN_DECLS
ssize_t getrandom(void *buf, size_t buflen, unsigned int flags);