openssh unix dev - Jan 2001 - ssh drops privs when it can't find ~/.ssh/prng_seed

I'm using OpenSSH 2.3.0p1. When my users use ssh for the first
time, using rhosts authentication, entropy.c drops the privs in
prng_write_seedfile() at the setuid(original_uid) line (line 550,
approx):

void
prng_write_seedfile(void) {
	int fd;
	char seed[1024];
	char filename[1024];
	struct passwd *pw;

	/* Don't bother if we have already saved a seed */
	if (prng_seed_saved)
		return;
	
	setuid(original_uid);
/*      ^^^^^^^^^^^^^^^^^^^^     ***HERE*** */

	prng_seed_saved = 1;
	
	pw = getpwuid(original_uid);
	if (pw == NULL)
		fatal("Couldn't get password entry for current user (%i): %s", 
			original_uid, strerror(errno));
				
	/* Try to ensure that the parent directory is there */
	snprintf(filename, sizeof(filename), "%.512s/%s", pw->pw_dir, 
		SSH_USER_DIR);
	mkdir(filename, 0700);

	snprintf(filename, sizeof(filename), "%.512s/%s", pw->pw_dir, 
		SSH_PRNG_SEED_FILE);

	debug("writing PRNG seed to file %.100s", filename);

	RAND_bytes(seed, sizeof(seed));

	/* Don't care if the seed doesn't exist */
	prng_check_seedfile(filename);
	
	if ((fd = open(filename, O_WRONLY|O_TRUNC|O_CREAT, 0600)) == -1) {
		debug("WARNING: couldn't access PRNG seedfile %.100s (%.100s)", 
		   filename, strerror(errno));
	} else {	
		if (atomicio(write, fd, &seed, sizeof(seed)) != sizeof(seed))
			fatal("problem writing PRNG seedfile %.100s (%.100s)", filename, 
				 strerror(errno));

		close(fd);
	}
}


Can anyone explain firstly why it does this, and secondly how I
can stop it?

Thanks,

Andrew Stribblehill
Systems programmer, IT Service, University of Durham, England

On Tue, 16 Jan 2001, Andrew Stribblehill wrote:
> I'm using OpenSSH 2.3.0p1. When my users use ssh for the first
> time, using rhosts authentication, entropy.c drops the privs in
> prng_write_seedfile() at the setuid(original_uid) line (line 550,
> approx):
 
> Can anyone explain firstly why it does this, and secondly how I
> can stop it?
 
Try the below patch, which causes seeds to be only written upon exit.

Index: entropy.c
==================================================================RCS file:
/var/cvs/openssh/entropy.c,v
retrieving revision 1.22
diff -u -r1.22 entropy.c
--- entropy.c	2000/11/24 23:09:32	1.22
+++ entropy.c	2001/01/16 22:29:37
@@ -601,12 +601,7 @@
 	debug("loading PRNG seed from file %.100s", filename);
 
 	if (!prng_check_seedfile(filename)) {
-		verbose("Random seed file not found, creating new");
-		prng_write_seedfile();
-		
-		/* Reseed immediatly */
-		(void)stir_from_system();
-		(void)stir_from_programs();
+		verbose("Random seed file not found or not valid, ignoring.");
 		return;
 	}
 


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