isofs: bound Rock Ridge symlink components to the SL record

get_symlink_chunk() and the SL handling in
parse_rock_ridge_inode_internal() walk the variable-length components of
a Rock Ridge "SL" (symbolic link) record.  Each component is a two-byte
header (flags, len) followed by len bytes of text, so it occupies
slp->len + 2 bytes.  Both loops read slp->len and advance to the next
component, and get_symlink_chunk() additionally does
memcpy(rpnt, slp->text, slp->len), but neither checks that the component
lies within the SL record before dereferencing it.

A crafted SL record whose component declares a len that runs past the
record (rr->len) therefore triggers an out-of-bounds read of up to 255
bytes.  When the record sits at the tail of its backing buffer - for
example a small kmalloc()ed continuation block reached through a CE
record - the read crosses the allocation; get_symlink_chunk() then
copies the out-of-bounds bytes into the symlink body returned to user
space by readlink(), disclosing adjacent kernel memory.

ISO 9660 images are routinely mounted from untrusted removable media -
desktop environments auto-mount them (e.g. via udisks2) without
CAP_SYS_ADMIN - so the record contents are attacker-controlled.

Reject any component that does not fit in the remaining record bytes
before using it.  In get_symlink_chunk() return NULL, like the existing
output-buffer (plimit) checks, so a malformed record makes readlink()
fail with -EIO rather than silently returning a truncated target; in
parse_rock_ridge_inode_internal() stop the inode-size walk.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Suggested-by: Michael Bommarito <michael.bommarito@gmail.com>
Signed-off-by: Bryam Vargas <hexlabsecurity@proton.me>
Link: https://patch.msgid.link/20260607011823.217748-1-hexlabsecurity@proton.me
Signed-off-by: Jan Kara <jack@suse.cz>

Author: bryamzxz

fs/isofs/rock.c

@@ -466,6 +466,9 @@ parse_rock_ridge_inode_internal(struct iso_directory_record *de,
 				inode->i_size = symlink_len;
 				while (slen > 1) {
 					rootflag = 0;
+					/* keep the component within the SL record */
+					if (slp->len + 2 > slen)
+						goto eio;
 					switch (slp->flags & ~1) {
 					case 0:
 						inode->i_size +=
@@ -621,6 +624,14 @@ static char *get_symlink_chunk(char *rpnt, struct rock_ridge *rr, char *plimit)
 	slp = &rr->u.SL.link;
 	while (slen > 1) {
 		rootflag = 0;
+		/*
+		 * A component is slp->len + 2 bytes (a two-byte header plus
+		 * len bytes of text).  If it does not fit in the bytes left in
+		 * the SL record the record is malformed: fail like the plimit
+		 * checks below so readlink() returns -EIO, not a truncated path.
+		 */
+		if (slp->len + 2 > slen)
+			return NULL;
 		switch (slp->flags & ~1) {
 		case 0:
 			if (slp->len > plimit - rpnt)