write: doesn't check for tty group

write(1) selects a wrong tty, because there is not a proper
check of tty group ownership:

	$ write kzak
	write: kzak is logged in more than once; writing to tty7
	write: /dev/tty7: Permission denied

	$ ls -la /dev/tty7
	crw--w---- 1 root root 4, 7 2008-07-04 00:32 /dev/tty7
	                  ^^^^

	$ ls -la /usr/bin/write
	-rwxr-sr-x 1 root tty 11864 2008-04-02 16:24 /usr/bin/write
	      ^           ^^^

We have to check for tty group owner, because we don't have
permissions to write to arbitrary tty.

Fixed version:

	$ write kzak
	write: kzak is logged in more than once; writing to pts/6
	                                                    ^^^^
	Message from test@nb on pts/7 at 15:22 ...

	^C

	$ ls -la /dev/pts/6
	crw--w---- 1 kzak tty 136, 6 2008-07-07 15:35 /dev/pts/6
	                  ^^^

Addresses-Red-Hat-Bugzilla: #454252
Signed-off-by: Karel Zak <kzak@redhat.com>

1 files changed, 7 insertions, 1 deletions

diff --git a/misc-utils/write.c b/misc-utils/write.c
index 46b8f0acf..276fe96d4 100644
--- a/misc-utils/write.c
+++ b/misc-utils/write.c
@@ -72,6 +72,8 @@ static void done(int);
 int term_chk(char *, int *, time_t *, int);
 int utmp_chk(char *, char *);
 
+static gid_t myegid;
+
 int
 main(int argc, char **argv) {
 	time_t atime;
@@ -83,6 +85,8 @@ main(int argc, char **argv) {
 	bindtextdomain(PACKAGE, LOCALEDIR);
 	textdomain(PACKAGE);
 
+	myegid = getegid();
+
 	/* check that sender has write enabled */
 	if (isatty(fileno(stdin)))
 		myttyfd = fileno(stdin);
@@ -267,7 +271,9 @@ int term_chk(char *tty, int *msgsokP, time_t *atimeP, int showerror)
 			    "write: %s: %s\n", path, strerror(errno));
 		return(1);
 	}
-	*msgsokP = (s.st_mode & (S_IWRITE >> 3)) != 0;	/* group write bit */
+
+	/* group write bit and group ownership */
+	*msgsokP = (s.st_mode & (S_IWRITE >> 3)) && myegid == s.st_gid;
 	*atimeP = s.st_atime;
 	return(0);
 }