Re: [PATCH v2 1/4] kmod: make request_module() return an error when autoloading is disabled

[Luis Chamberlain <mcgrof@kernel.org>]

On Thu, Mar 12, 2020 at 01:25:49PM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> It's long been possible to disable kernel module autoloading completely
> (while still allowing manual module insertion) by setting
> /proc/sys/kernel/modprobe to the empty string.  This can be preferable
> to setting it to a nonexistent file since it avoids the overhead of an
> attempted execve(), avoids potential deadlocks, and avoids the call to
> security_kernel_module_request() and thus on SELinux-based systems
> eliminates the need to write SELinux rules to dontaudit module_request.
> 
> However, when module autoloading is disabled in this way,
> request_module() returns 0.  This is broken because callers expect 0 to
> mean that the module was successfully loaded.
> 
> Apparently this was never noticed because this method of disabling
> module autoloading isn't used much, and also most callers don't use the
> return value of request_module() since it's always necessary to check
> whether the module registered its functionality or not anyway.  But
> improperly returning 0 can indeed confuse a few callers, for example
> get_fs_type() in fs/filesystems.c where it causes a WARNING to be hit:
> 
> 	if (!fs && (request_module("fs-%.*s", len, name) == 0)) {
> 		fs = __get_fs_type(name, len);
> 		WARN_ONCE(!fs, "request_module fs-%.*s succeeded, but still no fs?\n", len, name);
> 	}
> 
> This is easily reproduced with:
> 
> 	echo > /proc/sys/kernel/modprobe
> 	mount -t NONEXISTENT none /
> 
> It causes:
> 
> 	request_module fs-NONEXISTENT succeeded, but still no fs?
> 	WARNING: CPU: 1 PID: 1106 at fs/filesystems.c:275 get_fs_type+0xd6/0xf0
> 	[...]
> 
> This should actually use pr_warn_once() rather than WARN_ONCE(), since
> it's also user-reachable if userspace immediately unloads the module.
> Regardless, request_module() should correctly return an error when it
> fails.  So let's make it return -ENOENT, which matches the error when
> the modprobe binary doesn't exist.
> 
> I've also sent patches to document and test this case.
> 
> Cc: stable@vger.kernel.org
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Jeff Vander Stoep <jeffv@google.com>
> Cc: Jessica Yu <jeyu@kernel.org>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Luis Chamberlain <mcgrof@kernel.org>
> Cc: NeilBrown <neilb@suse.com>
> Signed-off-by: Eric Biggers <ebiggers@google.com>

Acked-by: Luis Chamberlain <mcgrof@kernel.org>

  Luis