86def3dcd6
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
134 lines
3.4 KiB
Groff
134 lines
3.4 KiB
Groff
.\" Copyright 2020, 2018 Conrad Meyer <cem@FreeBSD.org>. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHORS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHORS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd January 12, 2020
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.Dt GETRANDOM 2
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.Os
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.Sh NAME
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.Nm getrandom
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.Nd get random data
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In sys/random.h
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.Ft ssize_t
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.Fn getrandom "void *buf" "size_t buflen" "unsigned int flags"
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.Sh DESCRIPTION
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.Fn getrandom
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fills
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.Fa buf
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with up to
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.Fa buflen
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bytes of random data.
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.Pp
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The
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.Fa flags
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argument may include zero or more of the following:
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.Bl -tag -width _GRND_NONBLOCK_
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.It Ql GRND_NONBLOCK
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Return
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.Er EAGAIN
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instead of blocking, if the
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.Xr random 4
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device has not yet been seeded.
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By default,
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.Fn getrandom
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will block until the device is seeded.
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.It Ql GRND_RANDOM
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This flag does nothing on
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.Fx .
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.Pa /dev/random
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and
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.Pa /dev/urandom
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are identical.
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.It Ql GRND_INSECURE
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This flag is treated as an alternative name for
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.Dv GRND_NONBLOCK .
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It is provided solely for API compatibility with Linux.
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.El
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.Pp
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If the
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.Xr random 4
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device has been seeded, reads of up to 256 bytes will always return as many
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bytes as requested and will not be interrupted by signals.
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.Sh RETURN VALUES
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Upon successful completion, the number of bytes which were actually read is
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returned.
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For requests larger than 256 bytes, this can be fewer bytes than were
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requested.
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Otherwise, -1 is returned and the global variable
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.Va errno
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is set to indicate the error.
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.Sh ERRORS
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The
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.Fn getrandom
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operation returns the following errors:
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.Bl -tag -width Er
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.It Bq Er EAGAIN
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The
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.Ql GRND_NONBLOCK
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(or
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.Ql GRND_INSECURE )
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flag was set and the
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.Xr random 4
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device was not yet seeded.
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.It Bq Er EFAULT
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The
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.Fa buf
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parameter points to an invalid address.
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.It Bq Er EINTR
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The sleep was interrupted by a signal.
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.It Bq Er EINVAL
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An invalid
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.Fa flags
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was specified.
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.It Bq Er EINVAL
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The requested
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.Fa buflen
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was larger than
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.Dv IOSIZE_MAX .
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.El
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.Sh SEE ALSO
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.Xr arc4random 3 ,
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.Xr getentropy 3 ,
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.Xr random 4
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.Sh STANDARDS
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.Fn getrandom
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is non-standard.
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It is present in Linux.
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.Sh HISTORY
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The
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.Fn getrandom
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system call first appeared in
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.Fx 12.0 .
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.Sh CAVEATS
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Unlike Linux, the
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.Dv GRND_INSECURE
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flag on
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.Fx
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does not produce any output before the
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.Xr random 4
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device is seeded.
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