Date: Sun, 22 Sep 2024 07:36:46 GMT From: Colin Percival <cperciva@FreeBSD.org> To: src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-main@FreeBSD.org Subject: git: c8ebbd28aa91 - main - loader: Expand EFI entropy if < 2048 bytes Message-ID: <202409220736.48M7akhq097269@gitrepo.freebsd.org>
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The branch main has been updated by cperciva: URL: https://cgit.FreeBSD.org/src/commit/?id=c8ebbd28aa91705aea3a67b06018ea6aef5aa6e4 commit c8ebbd28aa91705aea3a67b06018ea6aef5aa6e4 Author: Colin Percival <cperciva@FreeBSD.org> AuthorDate: 2024-09-18 11:02:12 +0000 Commit: Colin Percival <cperciva@FreeBSD.org> CommitDate: 2024-09-22 07:35:48 +0000 loader: Expand EFI entropy if < 2048 bytes The EFI RNG on some platforms takes a long time if we request 2048 bytes of entropy, so we would like to request less; but our kernel Fortuna RNG needs to be fed 2048 bytes in order to consider itself "fully seeded". If we have between 64 bytes (the size of a single Fortuna pool and enough to guarantee cryptographic security) and 2048 bytes (what Fortuna wants) then the boot process will hang waiting for more entropy despite in fact having enough to operate securely. Since 64 bytes of entropy is plenty to be cryptographically secure (an attack of cost ~ 2^128 is infeasible, which implies a mere 16 bytes of entropy), use PBKDF2 (aka pkcs5v2_genkey_raw) to spread the entropy across 2048 bytes. This is secure since PBKDF2 has the property that every subset of output bytes has within O(1) of the maximum possible amount of entropy. Reviewed by: pjd MFC after: 1 week Sponsored by: Amazon Differential Revision: https://reviews.freebsd.org/D46635 --- stand/efi/loader/main.c | 39 ++++++++++++++++++++++++++++++++++++--- 1 file changed, 36 insertions(+), 3 deletions(-) diff --git a/stand/efi/loader/main.c b/stand/efi/loader/main.c index 17676cd9deb8..508e7ad2db36 100644 --- a/stand/efi/loader/main.c +++ b/stand/efi/loader/main.c @@ -57,6 +57,9 @@ #include <bootstrap.h> #include <smbios.h> +#include <dev/random/fortuna.h> +#include <geom/eli/pkcs5v2.h> + #include "efizfs.h" #include "framebuffer.h" @@ -1250,11 +1253,27 @@ command_seed_entropy(int argc, char *argv[]) { EFI_STATUS status; EFI_RNG_PROTOCOL *rng; - unsigned int size = 2048; + unsigned int size_efi = RANDOM_FORTUNA_DEFPOOLSIZE * RANDOM_FORTUNA_NPOOLS; + unsigned int size = RANDOM_FORTUNA_DEFPOOLSIZE * RANDOM_FORTUNA_NPOOLS; + void *buf_efi; void *buf; if (argc > 1) { - size = strtol(argv[1], NULL, 0); + size_efi = strtol(argv[1], NULL, 0); + + /* Don't *compress* the entropy we get from EFI. */ + if (size_efi > size) + size = size_efi; + + /* + * If the amount of entropy we get from EFI is less than the + * size of a single Fortuna pool -- i.e. not enough to ensure + * that Fortuna is safely seeded -- don't expand it since we + * don't want to trick Fortuna into thinking that it has been + * safely seeded when it has not. + */ + if (size_efi < RANDOM_FORTUNA_DEFPOOLSIZE) + size = size_efi; } status = BS->LocateProtocol(&rng_guid, NULL, (VOID **)&rng); @@ -1268,20 +1287,34 @@ command_seed_entropy(int argc, char *argv[]) return (CMD_ERROR); } + if ((buf_efi = malloc(size_efi)) == NULL) { + free(buf); + command_errmsg = "out of memory"; + return (CMD_ERROR); + } + TSENTER2("rng->GetRNG"); - status = rng->GetRNG(rng, NULL, size, (UINT8 *)buf); + status = rng->GetRNG(rng, NULL, size_efi, (UINT8 *)buf_efi); TSEXIT(); if (status != EFI_SUCCESS) { + free(buf_efi); free(buf); command_errmsg = "GetRNG failed"; return (CMD_ERROR); } + if (size_efi < size) + pkcs5v2_genkey_raw(buf, size, "", 0, buf_efi, size_efi, 1); + else + memcpy(buf, buf_efi, size); if (file_addbuf("efi_rng_seed", "boot_entropy_platform", size, buf) != 0) { + free(buf_efi); free(buf); return (CMD_ERROR); } + explicit_bzero(buf_efi, size_efi); + free(buf_efi); free(buf); return (CMD_OK); }
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