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Use GRND_NONBLOCK with getrandom.

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The json-c library is used in cryptsetup for LUKS2 header information.
Since cryptsetup can be called very early during boot, the developers
avoid getrandom() calls in their own code base for now. [1]

Introducing a blocking getrandom() call in json-c therefore introduces
this issue for cryptsetup as well. Even though cryptsetup issues do not
have to be json-c issues, here is my proposal:

Let's use a non-blocking call, falling back to other sources if the call
would block. Since getrandom() accesses urandom, it must mean that we
are in an early boot phase -- otherwise the call would not block
according to its manual page.

As stated in manual page of random(4), accessing /dev/urandom won't
block but return weak random numbers, therefore this fallback would work
for json-c.

While at it, fixed the debug message.

[1] https://gitlab.com/cryptsetup/cryptsetup/-/merge_requests/47
    which references to https://lwn.net/Articles/800509/
This commit is contained in:
Tobias Stoeckmann
2020-08-15 15:41:41 +02:00
parent 104b408ee8
commit f052e42f56
1 changed files with 4 additions and 2 deletions
Download Patch File Download Diff File Expand all files Collapse all files

6
random_seed.c
View File

@@ -164,19 +164,21 @@ retry:
static int get_getrandom_seed(void) static int get_getrandom_seed(void)
{ {
DEBUG_SEED("get_dev_random_seed"); DEBUG_SEED("get_getrandom_seed");
int r; int r;
ssize_t ret; ssize_t ret;
do { do {
ret = getrandom(&r, sizeof(r), 0); ret = getrandom(&r, sizeof(r), GRND_NONBLOCK);
} while ((ret == -1) && (errno == EINTR)); } while ((ret == -1) && (errno == EINTR));
if (ret == -1) if (ret == -1)
{ {
if (errno == ENOSYS) /* syscall not available in kernel */ if (errno == ENOSYS) /* syscall not available in kernel */
return -1; return -1;
if (errno == EAGAIN) /* entropy not yet initialized */
return -1;
fprintf(stderr, "error from getrandom(): %s", strerror(errno)); fprintf(stderr, "error from getrandom(): %s", strerror(errno));
exit(1); exit(1);