On Mar 8, 2018, at 7:06 PM, Linus Torvalds [off-list ref] wrote:
Honestly, that "read twice" thing may be what scuttles this.
Initially, I thought it was a non-issue, because anybody who controls
the module subdirectory enough to rewrite files would be in a position
to just execute the file itself directly instead.
On further consideration, I think there’s another showstopper. This patch is a potentially severe ABI break. Right now, loading a module *copies* it into memory and does not hold a reference to the underlying fs. With the patch applied, all kinds of use cases can break in gnarly ways. Initramfs is maybe okay, but initrd may be screwed. If you load an ET_EXEC module from initrd, then umount it, then clear the ramdisk, something will go horribly wrong. Exactly what goes wrong depends on whether userspace notices that umount() failed. Similarly, if you load one of these modules over a network and then lose your connection, you have a problem.
The “read twice” thing is also bad for another reason: containers. Suppose I have a setup where a container can load a signed module blob. With the read twice code, the container can race and run an entirely different blob outside the container.