jffs2: validate symlink size in jffs2_do_read_inode_internal()

`csize' is read from disk and thus needs validation.  Otherwise a bogus
value 0xffffffff would turn the subsequent kmalloc(csize + 1, ...) into
kmalloc(0, ...), leading to out-of-bounds write.

This patch limits `csize' to JFFS2_MAX_NAME_LEN, which is also used
in jffs2_symlink().

Artem: we actually validate csize by checking CRC, so this 0xFFs cannot
come from empty flash region. But I guess an attacker could feed JFFS2
an image with random csize value, including 0xFFs.
Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

fs/jffs2/readinode.c

@@ -1266,6 +1266,12 @@ static int jffs2_do_read_inode_internal(struct jffs2_sb_info *c,
 			/* Symlink's inode data is the target path. Read it and
 			 * keep in RAM to facilitate quick follow symlink
 			 * operation. */
+			uint32_t csize = je32_to_cpu(latest_node->csize);
+			if (csize > JFFS2_MAX_NAME_LEN) {
+				mutex_unlock(&f->sem);
+				jffs2_do_clear_inode(c, f);
+				return -ENAMETOOLONG;
+			}
 			f->target = kmalloc(je32_to_cpu(latest_node->csize) + 1, GFP_KERNEL);
 			if (!f->target) {
 				JFFS2_ERROR("can't allocate %d bytes of memory for the symlink target path cache\n", je32_to_cpu(latest_node->csize));