Re: [GIT PULL] sigqueue cache fix

[Linus Torvalds]

On Sun, Jun 27, 2021 at 10:14 PM Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> The most fundamental race we can have is this:

No. It's this (all on the same CPU):

   sigqueue_cache_or_free():

       if (!READ_ONCE(current->sigqueue_cache))
                     <-- Interrupt happens here
               WRITE_ONCE(current->sigqueue_cache, q);

and then the interrupt sends a SIGCONT, which ends up flushing
previous process control signals, which ends up freeing them, which
ends up in sigqueue_cache_or_free() again, at which point you have

       if (!READ_ONCE(current->sigqueue_cache))
               WRITE_ONCE(current->sigqueue_cache, q);

again.

And both the original and the interrupting one sees a NULL
current->sigqueue_cache, so both of them will do that WRITE_ONCE(),
and when the interrupt returns, the original case will overwrite the
value that the interrupt free'd.

Boom - memory leak.

It does seem to be very small race window, and it's "only" a memory
leak, but it's a very simple example of how this cache was broken even
on UP.

              Linus