Re: [PATCH 1/7] kthread: Don't allocate kthread_struct for init and umh

[Eric W. Biederman]

chill <maximkabox13@xxxxxxxxx> writes:

> this looks like a real uaf vulnerability and can be executed by the user

The potential to use memory after it has been freed appears completely
real.  As such it is a bug and it should definitely be fixed.  That is
as far as I can see.

What I don't see, and I am very bad at this so I could be missing
something, is what bad thing kthread_is_per_cpu could be tricked into
doing.

I see a window of a single instruction which reads a single bit
that normally will return false.  If that bit instead reads true
it looks like the scheduler will simply decide to not run the
process on another cpu.


So I will put this change in linux-next.  It will be tested and I will
send it to Linus when the merge window for v5.19 opens.  After Linus
merges this I expect after a week or so it will be backported to the
various stable kernels.  Not that it needs to go farther than about
v5.17 where I introduced the bug.

Eric