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char/lrng: add switchable DRNG support
The DRNG switch support allows replacing the DRNG mechanism of the LRNG. The switching support rests on the interface definition of include/linux/lrng.h. A new DRNG is implemented by filling in the interface defined in this header file. In addition to the DRNG, the extension also has to provide a hash implementation that is used to hash the entropy pool for random number extraction. Note: It is permissible to implement a DRNG whose operations may sleep. However, the hash function must not sleep. The switchable DRNG support allows replacing the DRNG at runtime. However, only one DRNG extension is allowed to be loaded at any given time. Before replacing it with another DRNG implementation, the possibly existing DRNG extension must be unloaded. The switchable DRNG extension activates the new DRNG during load time. It is expected, however, that such a DRNG switch would be done only once by an administrator to load the intended DRNG implementation. It is permissible to compile DRNG extensions either as kernel modules or statically. The initialization of the DRNG extension should be performed with a late_initcall to ensure the extension is available when user space starts but after all other initialization completed. The initialization is performed by registering the function call data structure with the lrng_set_drng_cb function. In order to unload the DRNG extension, lrng_set_drng_cb must be invoked with the NULL parameter. The DRNG extension should always provide a security strength that is at least as strong as LRNG_DRNG_SECURITY_STRENGTH_BITS. The hash extension must not sleep and must not maintain a separate state. CC: Torsten Duwe <duwe@lst.de> CC: "Eric W. Biederman" <ebiederm@xmission.com> CC: "Alexander E. Patrakov" <patrakov@gmail.com> CC: "Ahmed S. Darwish" <darwish.07@gmail.com> CC: "Theodore Y. Ts'o" <tytso@mit.edu> CC: Willy Tarreau <w@1wt.eu> CC: Matthew Garrett <mjg59@srcf.ucam.org> CC: Vito Caputo <vcaputo@pengaru.com> CC: Andreas Dilger <adilger.kernel@dilger.ca> CC: Jan Kara <jack@suse.cz> CC: Ray Strode <rstrode@redhat.com> CC: William Jon McCann <mccann@jhu.edu> CC: zhangjs <zachary@baishancloud.com> CC: Andy Lutomirski <luto@kernel.org> CC: Florian Weimer <fweimer@redhat.com> CC: Lennart Poettering <mzxreary@0pointer.de> CC: Nicolai Stange <nstange@suse.de> Reviewed-by: Alexander Lobakin <alobakin@pm.me> Tested-by: Alexander Lobakin <alobakin@pm.me> Reviewed-by: Marcelo Henrique Cerri <marcelo.cerri@canonical.com> Reviewed-by: Roman Drahtmueller <draht@schaltsekun.de> Tested-by: Marcelo Henrique Cerri <marcelo.cerri@canonical.com> Tested-by: Neil Horman <nhorman@redhat.com> Tested-by: Jirka Hladky <jhladky@redhat.com> Reviewed-by: Jirka Hladky <jhladky@redhat.com> Signed-off-by: Stephan Mueller <smueller@chronox.de>
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