keys: Protect the type name while describing a key

In the Linux kernel, the following vulnerability has been resolved:

keys: Protect the type name while describing a key

keyctl_describe_key() must keep the key type's name alive until
kasprintf() has consumed it. The retained key reference keeps the key
and its description alive, but does not pin the module providing its
type. The name is passed to kasprintf() without holding key->sem.

kvasprintf() uses the saved name pointer in two formatting passes, with
a GFP_KERNEL allocation between them. With AF_RXRPC=m, a task that
passes View permission before type retirement can overlap a privileged
module unload in this order:

1. KEYCTL_DESCRIBE looks up the key, saves key->type->name in the
kasprintf() arguments and completes the first formatting pass.
2. While the formatter is delayed before its second pass,
af_rxrpc_exit() calls unregister_key_type(), which removes the type
from the registry and waits for key_gc_keytype().
3. The collector takes key->sem for writing, retypes the retained key
to key_type_dead and completes retirement. Unregistration and module
exit return, allowing free_module() to release the type and name.
4. The formatter resumes its second pass and reads the saved name from
released module storage, which can fault in string().

Hold key->sem for reading across kasprintf(). The collector then cannot
retype the key or complete retirement until both formatting passes have
finished. If retirement wins the lock first, formatting uses the core
key_type_dead name instead. Release the semaphore immediately after
kasprintf(), including on allocation failure, before copying to
userspace.

Oops report as below:

BUG: unable to handle page fault for address: fffffbfff8054794
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page
RIP: 0010:string+0x23f/0x470
Call Trace:
vsnprintf+0x330/0x1110