CAP(3) CAP(3)
NAME
cap - capability for changing user name
SYNOPSIS
bind #ยค dir
dir/caphash
dir/capuse
DESCRIPTION
Cap allows a process owned by the host owner (see eve(10.2))
to give another process on the same machine a capability to
set its user name to a specified user. The capability is a
string of the form:
[ fromuser@ ] touser@key
where fromuser is a process's current user name, touser is
its new user name, and key is a string of random characters
(eg, produced by security-random(2)).
Caphash is a write-only file that can only be opened by the
host owner. A process enables the use of a capability by
writing the keyed hash of fromuser@touser to caphash. The
hash is computed using Keyring->hmac_sha1 as follows:
kr := load Keyring Keyring->PATH;
IPint: import kr;
users := sys->sprint("%s@%s", fromuser, touser);
cap := sys->sprint("%s@%s", users, key);
digest := array[Keyring->SHA1dlen] of byte;
ausers := array of byte users;
kr->hmac_sha1(ausers, len ausers, array of byte key, digest, nil);
if(sys->write(caphashfd, digest, len digest) < 0)
error();
The capability (eg, cap in the example) can then be passed
to another process.
Capuse is a write-only file that can be opened by any pro-
cess. It can then write a capability string to change its
user name, provided that capability has previously been
enabled by the host owner via caphash, and if the capability
included a fromuser, the writing process currently has that
user name. After a successful write, the writing process
will be owned by touser. Any capability can be used at most
once.
A capability enabled by caphash has a limited lifetime, on
the order of 30 seconds. Caphash can be removed by the host
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CAP(3) CAP(3)
owner to prevent its further use.
SOURCE
/emu/port/devcap.c
/os/port/devcap.c
SEE ALSO
keyring-sha1(2), cons(3), intro(5), eve(10.2)
DIAGNOSTICS
A write to capuse without a previous write to caphash sets
the error string to ``invalid capability''.
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