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lib/libcrypt: use explicit_bzero() to clear sensitive buffers

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Prevent a potentially sufficiently smart compiler from optimising
away our attempts to clear sensitive buffers.

A related change was discussed and rejected in D16059, but I don't
believe the reasoning there applies: the code clearly documents its
intent that the `memset` calls clear sensitive buffers so they don't
hang around.  `explicit_bzero` is the appropriate function for this
purpose.  A potential performance disadvantage seems less important:
the functions in crypt are specifically designed to be slow, so a
few extra calls to guarantee that sensitive buffers are cleared does
not significantly affect runtime.

See also:	D16059
Reviewed by:	delphij, kevans
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D47037

(cherry picked from commit a2c0d2026fb422ade2171da4bc6d5d2773b268a6)
This commit is contained in:
Robert Clausecker 2024-10-10 11:08:35 +02:00
parent 1d271ba05f
commit 1af027e583
3 changed files with 11 additions and 8 deletions
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5
lib/libcrypt/crypt-md5.c
View file

@ -33,6 +33,7 @@
#include <md5.h>
#include <stdio.h>
#include <string.h>
#include <strings.h>
#include <unistd.h>
#include "crypt.h"
@ -85,7 +86,7 @@ crypt_md5(const char *pw, const char *salt, char *buffer)
(u_int)(pl > MD5_SIZE ? MD5_SIZE : pl));
/* Don't leave anything around in vm they could use. */
memset(final, 0, sizeof(final));
explicit_bzero(final, sizeof(final));
/* Then something really weird... */
for (i = strlen(pw); i; i >>= 1)
@ -141,7 +142,7 @@ crypt_md5(const char *pw, const char *salt, char *buffer)
*buffer = '\0';
/* Don't leave anything around in vm they could use. */
memset(final, 0, sizeof(final));
explicit_bzero(final, sizeof(final));
return (0);
}

7
lib/libcrypt/crypt-sha256.c
View file

@ -41,6 +41,7 @@
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <strings.h>
#include "crypt.h"
@ -234,9 +235,9 @@ crypt_sha256(const char *key, const char *salt, char *buffer)
* the SHA256 implementation as well. */
SHA256_Init(&ctx);
SHA256_Final(alt_result, &ctx);
memset(temp_result, '\0', sizeof(temp_result));
memset(p_bytes, '\0', key_len);
memset(s_bytes, '\0', salt_len);
explicit_bzero(temp_result, sizeof(temp_result));
explicit_bzero(p_bytes, key_len);
explicit_bzero(s_bytes, salt_len);
return (0);
}

7
lib/libcrypt/crypt-sha512.c
View file

@ -41,6 +41,7 @@
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <strings.h>
#include "crypt.h"
@ -246,9 +247,9 @@ crypt_sha512(const char *key, const char *salt, char *buffer)
* the SHA512 implementation as well. */
SHA512_Init(&ctx);
SHA512_Final(alt_result, &ctx);
memset(temp_result, '\0', sizeof(temp_result));
memset(p_bytes, '\0', key_len);
memset(s_bytes, '\0', salt_len);
explicit_bzero(temp_result, sizeof(temp_result));
explicit_bzero(p_bytes, key_len);
explicit_bzero(s_bytes, salt_len);
return (0);
}