Commit message: get_rock_ridge_filename(): handle malformed NM entries

Payloads of NM entries are not supposed to contain NUL.  When we run
into such, only the part prior to the first NUL goes into the
concatenation (i.e. the directory entry name being encoded by a bunch
of NM entries).  We do stop when the amount collected so far + the
claimed amount in the current NM entry exceed 254.  So far, so good,
but what we return as the total length is the sum of *claimed*
sizes, not the actual amount collected.  And that can grow pretty
large - not unlimited, since you'd need to put CE entries in
between to be able to get more than the maximum that could be
contained in one isofs directory entry / continuation chunk and
we are stop once we'd encountered 32 CEs, but you can get about 8Kb
easily.  And that's what will be passed to readdir callback as the
name length.  8Kb __copy_to_user() from a buffer allocated by
__get_free_page()

Cc: stable@vger.kernel.org # 0.98pl6+ (yes, really)
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
CVE: CVE-2016-4913
CWE: CWE-200
Vulnerability description: The get_rock_ridge_filename function in fs/isofs/rock.c in the Linux kernel before 4.5.5 mishandles NM (aka alternate name) entries containing \0 characters, which allows local users to obtain sensitive information from kernel memory or possibly have unspecified other impact via a crafted isofs filesystem.

Project: linux
File: rock.c

int get_rock_ridge_filename(struct iso_directory_record *de,
			    char *retname, struct inode *inode)
{
	struct rock_state rs;
	struct rock_ridge *rr;
	int sig;
 	int retnamlen = 0;
 	int truncate = 0;
 	int ret = 0;
 
 	if (!ISOFS_SB(inode->i_sb)->s_rock)
 		return 0;
	*retname = 0;

	init_rock_state(&rs, inode);
	setup_rock_ridge(de, inode, &rs);
repeat:

	while (rs.len > 2) { /* There may be one byte for padding somewhere */
		rr = (struct rock_ridge *)rs.chr;
		/*
		 * Ignore rock ridge info if rr->len is out of range, but
		 * don't return -EIO because that would make the file
		 * invisible.
		 */
		if (rr->len < 3)
			goto out;	/* Something got screwed up here */
		sig = isonum_721(rs.chr);
		if (rock_check_overflow(&rs, sig))
			goto eio;
		rs.chr += rr->len;
		rs.len -= rr->len;
		/*
		 * As above, just ignore the rock ridge info if rr->len
		 * is bogus.
		 */
		if (rs.len < 0)
			goto out;	/* Something got screwed up here */

		switch (sig) {
		case SIG('R', 'R'):
			if ((rr->u.RR.flags[0] & RR_NM) == 0)
				goto out;
			break;
		case SIG('S', 'P'):
			if (check_sp(rr, inode))
				goto out;
			break;
		case SIG('C', 'E'):
			rs.cont_extent = isonum_733(rr->u.CE.extent);
			rs.cont_offset = isonum_733(rr->u.CE.offset);
			rs.cont_size = isonum_733(rr->u.CE.size);
			break;
		case SIG('N', 'M'):
			if (truncate)
				break;
			if (rr->len < 5)
				break;
			/*
			 * If the flags are 2 or 4, this indicates '.' or '..'.
			 * We don't want to do anything with this, because it
			 * screws up the code that calls us.  We don't really
			 * care anyways, since we can just use the non-RR
			 * name.
			 */
			if (rr->u.NM.flags & 6)
				break;

			if (rr->u.NM.flags & ~1) {
				printk("Unsupported NM flag settings (%d)\n",
 					rr->u.NM.flags);
 				break;
 			}
			if ((strlen(retname) + rr->len - 5) >= 254) {
 				truncate = 1;
 				break;
 			}
			strncat(retname, rr->u.NM.name, rr->len - 5);
			retnamlen += rr->len - 5;
 			break;
 		case SIG('R', 'E'):
 			kfree(rs.buffer);
			return -1;
		default:
			break;
		}
	}
	ret = rock_continue(&rs);
	if (ret == 0)
		goto repeat;
	if (ret == 1)
		return retnamlen; /* If 0, this file did not have a NM field */
out:
	kfree(rs.buffer);
	return ret;
eio:
	ret = -EIO;
	goto out;
}