Re: [PATCH v3 1/5] kmod: make request_module() return an error when autoloading is disabled

[Jessica Yu <jeyu@kernel.org>]

+++ Eric Biggers [14/03/20 14:34 -0700]:
>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.
>
>Acked-by: Luis Chamberlain <mcgrof@kernel.org>
>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: NeilBrown <neilb@suse.com>
>Signed-off-by: Eric Biggers <ebiggers@google.com>

Reviewed-by: Jessica Yu <jeyu@kernel.org>