Re: [GIT PULL] sigqueue cache fix

[Ingo Molnar]

* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> 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);

Indeed - I was under the impression that this cannot happen, because 
interrupts are disabled - but I was wrong:

__sigqueue_free() is the only user of sigqueue_cache_or_free().

Callers of __sigqueue_free():

 - flush_sigqueue():
    # flush_signals() is holding the siglock & disables IRQs
    # __exit_signal() isn't holding the siglock but has IRQs disabled
    # selinux_bprm_committed_creds() is holding the siglock & disables IRQs

 - __flush_itimer_signals()
    # Its single caller is holding the siglock & disables IRQs

 - collect_signal()
    # Its single caller is holding the siglock & disables IRQs

 - dequeue_synchronous_signal()
    # Its single caller is holding the siglock & disables IRQs

 - flush_sigqueue_mask():
    # All callers are holding the siglock & disable IRQs

 - sigqueue_free()
    ...    

Boom, the last one on the list, sigqueue_free(), does __sigqueue_free() 
while not holding the siglock and not disabling interrupts. :-/

It does it in various syscall paths in the POSIX timers code through 
release_posix_timer(), with interrupts clearly enabled.

> 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.

Yeah - a clear Producer <-> Producer IRQ preemptability race that can leak 
freed sigqueue structures.

Thanks for catching this ...

But even if release_posix_timer() is changed to call sigqueue_free() with 
IRQs disabled, or sigqueue_free() disables interrupts itself, I think we 
need to be mindful of the Consumer <-> Producer SMP races, which only 
appear to be safe due to an accidental barrier by free_uid().

Plus a lockdep_assert_irqs_disabled() would have helped a lot in catching 
this sooner.

Thanks,

	Ingo