selinux: KASAN: slab-out-of-bounds in xattr_getsecurity

Call trace:
 [<ffffff9203a8d7a8>] dump_backtrace+0x0/0x428
 [<ffffff9203a8dbf8>] show_stack+0x28/0x38
 [<ffffff920409bfb8>] dump_stack+0xd4/0x124
 [<ffffff9203d187e8>] print_address_description+0x68/0x258
 [<ffffff9203d18c00>] kasan_report.part.2+0x228/0x2f0
 [<ffffff9203d1927c>] kasan_report+0x5c/0x70
 [<ffffff9203d1776c>] check_memory_region+0x12c/0x1c0
 [<ffffff9203d17cdc>] memcpy+0x34/0x68
 [<ffffff9203d75348>] xattr_getsecurity+0xe0/0x160
 [<ffffff9203d75490>] vfs_getxattr+0xc8/0x120
 [<ffffff9203d75d68>] getxattr+0x100/0x2c8
 [<ffffff9203d76fb4>] SyS_fgetxattr+0x64/0xa0
 [<ffffff9203a83f70>] el0_svc_naked+0x24/0x28

If user get root access and calls security.selinux setxattr() with an
embedded NUL on a file and then if some process performs a getxattr()
on that file with a length greater than the actual length of the string,
it would result in a panic.

To fix this, add the actual length of the string to the security context
instead of the length passed by the userspace process.

Signed-off-by: Sachin Grover <sgrover@codeaurora.org>
Cc: stable@vger.kernel.org
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/selinux/ss/services.c

@@ -1494,7 +1494,7 @@ static int security_context_to_sid_core(struct selinux_state *state,
 				      scontext_len, &context, def_sid);
 	if (rc == -EINVAL && force) {
 		context.str = str;
-		context.len = scontext_len;
+		context.len = strlen(str) + 1;
 		str = NULL;
 	} else if (rc)
 		goto out_unlock;