afs: Fix accessing YFS xattrs on a non-YFS server

If someone attempts to access YFS-related xattrs (e.g. afs.yfs.acl) on a
file on a non-YFS AFS server (such as OpenAFS), then the kernel will jump
to a NULL function pointer because the afs_fetch_acl_operation descriptor
doesn't point to a function for issuing an operation on a non-YFS
server[1].

Fix this by making afs_wait_for_operation() check that the issue_afs_rpc
method is set before jumping to it and setting -ENOTSUPP if not. This fix
also covers other potential operations that also only exist on YFS servers.

afs_xattr_get/set_yfs() then need to translate -ENOTSUPP to -ENODATA as the
former error is internal to the kernel.

The bug shows up as an oops like the following:

BUG: kernel NULL pointer dereference, address: 0000000000000000
[...]
Code: Unable to access opcode bytes at RIP 0xffffffffffffffd6.
[...]
Call Trace:
afs_wait_for_operation+0x83/0x1b0 [kafs]
afs_xattr_get_yfs+0xe6/0x270 [kafs]
__vfs_getxattr+0x59/0x80
vfs_getxattr+0x11c/0x140
getxattr+0x181/0x250
? __check_object_size+0x13f/0x150
? __fput+0x16d/0x250
__x64_sys_fgetxattr+0x64/0xb0
do_syscall_64+0x49/0xc0
entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x7fb120a9defe

This was triggered with "cp -a" which attempts to copy xattrs, including
afs ones, but is easier to reproduce with getfattr, e.g.:

getfattr -d -m ".*" /afs/openafs.org/

Fixes: e49c7b2f6de7 ("afs: Build an abstraction around an "operation" concept")
Reported-by: Gaja Sophie Peters <[e-mail]>
Signed-off-by: David Howells <[e-mail]>
Tested-by: Gaja Sophie Peters <[e-mail]>
Reviewed-by: Marc Dionne <[e-mail]>
Reviewed-by: Jeffrey Altman <[e-mail]>
cc: [e-mail]
Link: http://lists.infradead.org/pipermail/linux-afs/2021-March/003498.html [1]
Link: http://lists.infradead.org/pipermail/linux-afs/2021-March/003566.html # v1
Link: http://lists.infradead.org/pipermail/linux-afs/2021-March/003572.html # v2

fs/afs/fs_operation.c

···
 		if (test_bit(AFS_SERVER_FL_IS_YFS, &op->server->flags) &&
 		    op->ops->issue_yfs_rpc)
 			op->ops->issue_yfs_rpc(op);
-		else
+		else if (op->ops->issue_afs_rpc)
 			op->ops->issue_afs_rpc(op);
+		else
+			op->ac.error = -ENOTSUPP;
 
-		op->error = afs_wait_for_call_to_complete(op->call, &op->ac);
+		if (op->call)
+			op->error = afs_wait_for_call_to_complete(op->call, &op->ac);
 	}
 
 	switch (op->error) {

fs/afs/xattr.c

···
 			else
 				ret = -ERANGE;
 		}
+	} else if (ret == -ENOTSUPP) {
+		ret = -ENODATA;
 	}
 
 error_yacl:
···
 {
 	struct afs_operation *op;
 	struct afs_vnode *vnode = AFS_FS_I(inode);
+	int ret;
 
 	if (flags == XATTR_CREATE ||
 	    strcmp(name, "acl") != 0)
···
 		return afs_put_operation(op);
 
 	op->ops = &yfs_store_opaque_acl2_operation;
-	return afs_do_sync_operation(op);
+	ret = afs_do_sync_operation(op);
+	if (ret == -ENOTSUPP)
+		ret = -ENODATA;
+	return ret;
 }
 
 static const struct xattr_handler afs_xattr_yfs_handler = {