sort(1): Simplify and bound random seeding

Bound input file processing length to avoid the issue reported in [1].  For
simplicity, only allow regular file and character device inputs.  For
character devices, only allow /dev/random (and /dev/urandom symblink).

32 bytes of random is perfectly sufficient to seed MD5; we don't need any
more.  Users that want to use large files as seeds are encouraged to truncate
those files down to an appropriate input file via tools like sha256(1).

(This does not change the sort algorithm of sort -R.)

[1]: https://lists.freebsd.org/pipermail/freebsd-hackers/2018-August/053152.html

PR:		230792
Reported by:	Ali Abdallah <aliovx AT gmail.com>
Relnotes:	yes

usr.bin/sort/sort.c

@@ -36,6 +36,7 @@ __FBSDID("$FreeBSD$");
 
 #include <err.h>
 #include <errno.h>
+#include <fcntl.h>
 #include <getopt.h>
 #include <limits.h>
 #include <locale.h>
@@ -61,13 +62,7 @@ nl_catd catalog;
 
 #define	OPTIONS	"bcCdfghik:Mmno:RrsS:t:T:uVz"
 
-#define DEFAULT_RANDOM_SORT_SEED_FILE ("/dev/random")
-#define MAX_DEFAULT_RANDOM_SEED_DATA_SIZE (1024)
-
 static bool need_random;
-static const char *random_source = DEFAULT_RANDOM_SORT_SEED_FILE;
-static const void *random_seed;
-static size_t random_seed_size;
 
 MD5_CTX md5_ctx;
 
@@ -907,55 +902,76 @@ fix_obsolete_keys(int *argc, char **argv)
 }
 
 /*
- * Set random seed
+ * Seed random sort
  */
 static void
-set_random_seed(void)
+get_random_seed(const char *random_source)
 {
-	if (strcmp(random_source, DEFAULT_RANDOM_SORT_SEED_FILE) == 0) {
-		FILE* fseed;
-		MD5_CTX ctx;
-		char rsd[MAX_DEFAULT_RANDOM_SEED_DATA_SIZE];
-		size_t sz = 0;
-
-		fseed = openfile(random_source, "r");
-		while (!feof(fseed)) {
-			int cr;
-
-			cr = fgetc(fseed);
-			if (cr == EOF)
-				break;
-
-			rsd[sz++] = (char) cr;
-
-			if (sz >= MAX_DEFAULT_RANDOM_SEED_DATA_SIZE)
-				break;
-		}
-
-		closefile(fseed, random_source);
-
-		MD5Init(&ctx);
-		MD5Update(&ctx, rsd, sz);
+	char randseed[32];
+	struct stat fsb, rsb;
+	ssize_t rd;
+	int rsfd;
+
+	rsfd = -1;
+	rd = sizeof(randseed);
+
+	if (random_source == NULL) {
+		if (getentropy(randseed, sizeof(randseed)) < 0)
+			err(EX_SOFTWARE, "getentropy");
+		goto out;
+	}
 
-		random_seed = MD5End(&ctx, NULL);
-		random_seed_size = strlen(random_seed);
+	rsfd = open(random_source, O_RDONLY | O_CLOEXEC);
+	if (rsfd < 0)
+		err(EX_NOINPUT, "open: %s", random_source);
+
+	if (fstat(rsfd, &fsb) != 0)
+		err(EX_SOFTWARE, "fstat");
+
+	if (!S_ISREG(fsb.st_mode) && !S_ISCHR(fsb.st_mode))
+		err(EX_USAGE,
+		    "random seed isn't a regular file or /dev/random");
+
+	/*
+	 * Regular files: read up to maximum seed size and explicitly
+	 * reject longer files.
+	 */
+	if (S_ISREG(fsb.st_mode)) {
+		if (fsb.st_size > (off_t)sizeof(randseed))
+			errx(EX_USAGE, "random seed is too large (%jd >"
+			    " %zu)!", (intmax_t)fsb.st_size,
+			    sizeof(randseed));
+		else if (fsb.st_size < 1)
+			errx(EX_USAGE, "random seed is too small ("
+			    "0 bytes)");
+
+		memset(randseed, 0, sizeof(randseed));
+
+		rd = read(rsfd, randseed, fsb.st_size);
+		if (rd < 0)
+			err(EX_SOFTWARE, "reading random seed file %s",
+			    random_source);
+		if (rd < (ssize_t)fsb.st_size)
+			errx(EX_SOFTWARE, "short read from %s", random_source);
+	} else if (S_ISCHR(fsb.st_mode)) {
+		if (stat("/dev/random", &rsb) < 0)
+			err(EX_SOFTWARE, "stat");
+
+		if (fsb.st_dev != rsb.st_dev ||
+		    fsb.st_ino != rsb.st_ino)
+			errx(EX_USAGE, "random seed is a character "
+			    "device other than /dev/random");
+
+		if (getentropy(randseed, sizeof(randseed)) < 0)
+			err(EX_SOFTWARE, "getentropy");
+	}
 
-	} else {
-		MD5_CTX ctx;
-		char *b;
+out:
+	if (rsfd >= 0)
+		close(rsfd);
 
-		MD5Init(&ctx);
-		b = MD5File(random_source, NULL);
-		if (b == NULL)
-			err(2, NULL);
-
-		random_seed = b;
-		random_seed_size = strlen(b);
-	}
 	MD5Init(&md5_ctx);
-	if(random_seed_size>0) {
-		MD5Update(&md5_ctx, random_seed, random_seed_size);
-	}
+	MD5Update(&md5_ctx, randseed, rd);
 }
 
 /*
@@ -965,6 +981,7 @@ int
 main(int argc, char **argv)
 {
 	char *outfile, *real_outfile;
+	char *random_source = NULL;
 	int c, result;
 	bool mef_flags[NUMBER_OF_MUTUALLY_EXCLUSIVE_FLAGS] =
 	    { false, false, false, false, false, false };
@@ -1225,7 +1242,7 @@ main(int argc, char **argv)
 	}
 
 	if (need_random)
-		set_random_seed();
+		get_random_seed(random_source);
 
 	/* Case when the outfile equals one of the input files: */
 	if (strcmp(outfile, "-")) {