Techwizz/kernel-fxtec-pro1x
1
0
Fork 0
Code Issues Pull requests Projects Releases Packages Wiki Activity

FROMLIST: kmod: make request_module() return an error when autoloading is disabled

Browse source 
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>
Reviewed-by: Jessica Yu <jeyu@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.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: NeilBrown <neilb@suse.com>
Link: https://lore.kernel.org/r/20200318230515.171692-2-ebiggers@kernel.org
Bug: 151589316
Change-Id: I5e04f85e12a4f85da23e53bc11da1ade565abcd6
Signed-off-by: Eric Biggers <ebiggers@google.com>
This commit is contained in:
Eric Biggers 2020-03-18 16:05:11 -07:00
parent 77333590dd
commit a898a4764c
1 changed files with 2 additions and 2 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

4
kernel/kmod.c
View file

@ -120,7 +120,7 @@ static int call_modprobe(char *module_name, int wait)
* invoke it.
*
* If module auto-loading support is disabled then this function
* becomes a no-operation.
* simply returns -ENOENT.
*/
int __request_module(bool wait, const char *fmt, ...)
{
@ -137,7 +137,7 @@ int __request_module(bool wait, const char *fmt, ...)
WARN_ON_ONCE(wait && current_is_async());
if (!modprobe_path[0])
return 0;
return -ENOENT;
va_start(args, fmt);
ret = vsnprintf(module_name, MODULE_NAME_LEN, fmt, args);