Re: [PATCH v2] keys: Protect the type name while describing a key

[Jarkko Sakkinen <jarkko@kernel.org>]

On Fri, Oct 09, 2026 at 01:11:03PM +0800, Cen Zhang wrote:
> 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
> 
> Fixes: aa9d4437893f ("KEYS: Fix the size of the key description passed to/from userspace")
> Assisted-by: LLM
> Signed-off-by: Cen Zhang <zzzccc427@gmail.com>
> ---
> Changes in v2:
> - Trim the Oops report to the fault location and vsnprintf frame.
> 
> Link to v1: https://lore.kernel.org/r/pm-key-management-objects-candidate-0009-v3-43bbf21d1c452f23d829@gmail.com
> 
> diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
> index d14ace88e529cb77de5386f1f3a99905d4b7688c..5a5efa26ed0ece3a5ab22e37ad7bda7a5a744b83 100644
> --- a/security/keys/keyctl.c
> +++ b/security/keys/keyctl.c
> @@ -677,12 +677,14 @@ long keyctl_describe_key(key_serial_t keyid,
>  
>  	/* calculate how much information we're going to return */
>  	ret = -ENOMEM;
> +	down_read(&key->sem);
>  	infobuf = kasprintf(GFP_KERNEL,
>  			    "%s;%d;%d;%08x;",
>  			    key->type->name,
>  			    from_kuid_munged(current_user_ns(), key->uid),
>  			    from_kgid_munged(current_user_ns(), key->gid),
>  			    key->perm);
> +	up_read(&key->sem);
>  	if (!infobuf)
>  		goto error2;
>  	infolen = strlen(infobuf);

This would be even clear without scenario as it is quite obvious that
key->type can swap but I do appreciate having one of the scenarios
documented. Just saying that this is much better commit that would
have gone through :-)

Great work, thank you.

Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>

Br, Jarkko