On Wed, Mar 11, 2020 at 04:32:21AM +0000, Luis Chamberlain wrote:
On Tue, Mar 10, 2020 at 03:37:31PM -0700, Eric Biggers wrote:
From: Eric Biggers ebiggers@google.com
It's long been possible to disable kernel module autoloading completely by setting /proc/sys/kernel/modprobe to the empty string. This can be preferable
preferable but ... not documented. Or was this documented or recommended somewhere?
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.
Not that I know of, though I didn't look too hard. proc(5) mentions /proc/sys/kernel/modprobe but doesn't mention the empty string case.
In any case, it's been supported for a long time, and it's useful for the reasons I mentioned.
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.
However this is implicitly not true. For instance, as Neil recently chased down -- blacklisting a module today returns 0 as well, and so this corner case is implicitly set to return 0.
That sounds like another similar bug, but in the modprobe program instead of in the kernel. Do you have a link to the discussion about it?
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 [...]
Thanks for reporting this.
Arguably this warning is broken and should be removed, since the module could have been unloaded already.
No, the warning is present *because* debuggins issues for when the module which did not load is a rootfs is *really* hard to debug. Then, if the culprit of the issue is a userspace modprobe bug (it happens) this makes debugging *very* difficult as you won't know what failed at all, you just get a silent failed boot.
I meant that it's broken to use WARN_ON(), because it's a userspace triggerable condition. WARN_ON() is for kernel bugs only. Of course, if it's a useful warning, it can still be left in as pr_warn().
However, request_module() should also 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.
This is a user experience change though, and I wouldn't have on my radar who would use this, and expects the old behaviour. Josh, would you by chance?
I'd like this to be more an RFC first so we get vetted parties to review. I take it this and Neil's case are cases we should revisit now, properly document as we didn't before, ensure we don't break anything, and also extend the respective kmod selftests to ensure we don't break these corner cases in the future.
This patch only affects kernel internals, not the userspace API. So I don't see why it would be controversial? I already went through all callers of request_module() that check its return value, and they all appear to work better with -ENOENT, since they assume that 0 means the module was loaded.
Incorrectly returning 0 typically causes unnecessary work (checking again whether the module's functionality is available) or misleading log messages. In fact, I can't think of a situation where kernel code would *want* 0 returned in this case, as it's ambiguous with the module being successfully loaded.
Sure, I'll check whether it would be possible to add a test for this case in lib/test_kmod.c and tools/testing/selftests/kmod/.
- Eric