CVE-2026-53145: drm/gem: Try to fix change_handle ioctl, attempt 4

In the Linux kernel, the following vulnerability has been resolved:

drm/gem: Try to fix changehandle ioctl, attempt 4

[airlied: just added some comments on how to reenable] On-list because the cat is out of the bag and we're clearly not good enough to figure this out in private. The story thus far:

5e28b7b94408 ("drm: Set old handle to NULL before prime swap in changehandle") tried to fix a race condition between the gemclose and gemchangehandle ioctls, but got a few things wrong:

- There's a confusion with the local variable handle, which is actually the new handle, and so the two-stage trick was actually applied to the wrong idr slot. 7164d78559b0 ("drm/gem: fix race between changehandle and handledelete") tried to fix that by adding yet another code block, but forgot to add the error handling. Which meant we now have two paths, both kinda wrong.

- dc366607c41c ("drm: Replace old pointer to new idr") tried to apply another fix, but inconsistently, again because of the handle confusion - this would be the right fix (kinda, somewhat, it's a mess) if we'd do the two-stage approach for the new handle. Except that wasn't the intent of the original fix.

We also didn't have an igt merged for the original ioctl, which is a big no-go. This was attempted to address off-list in the original bugfix, and amd QA people claimed the bug was fixed now. Very clearly that's not the case. Here's my attempt to sort this out:

- Rename the local variable to newhandle, the old aliasing with args->handle is just too dangerously confusing.

- Merge the gem obj lookup with the two-stage idrreplace so that we avoid getting ourselves confused there.

- This means we don't have a surplus temporary reference anymore, only an inherited from the idr. A concurrent gemclose on the newhandle could steal that. Fix that with the same two-stage approach createtail uses. This is a bit overkill as documented in the comment, but I also don't trust my ability to understand this all correctly, so go with the established pattern we have from other ioctls instead for maximum paranoia.

- Adjust error paths. I've tried to make the error and success paths common, because they are identical except for which handle is removed and on which we call idrreplace to (re)install the object again. But that made things messier to read, so I've left it at the more verbose version, which unfortunately hides the symmetry in the entire code flow a bit.

- While at it, also replace the 7 space indent with 1 tab.

And finally, because I flat out don't trust my abilities here at all anymore:

- Disable the ioctl until we have the igt situation and everything else sorted out on-list and with full consensus.

v2:

Sashiko noticed that I didn't handle the error path for idrreplace correctly, it must be checked with ISERRORNULL like in gemhandledelete. So yeah, definitely should just the existing paths 1:1 because this is endless amounts of tricky.

Also add the Fixes: line for the original ioctl, I forgot that too.