futex: Provide infrastructure to plug the non contended robust futex unlock race

When the FUTEX_ROBUST_UNLOCK mechanism is used for unlocking (PI-)futexes,
then the unlock sequence in user space looks like this:

  1)	robust_list_set_op_pending(mutex);
  2)	robust_list_remove(mutex);

  	lval = gettid();
  3)	if (atomic_try_cmpxchg(&mutex->lock, lval, 0))
  4)		robust_list_clear_op_pending();
  	else
  5)		sys_futex(OP | FUTEX_ROBUST_UNLOCK, ....);

That still leaves a minimal race window between #3 and #4 where the mutex
could be acquired by some other task, which observes that it is the last
user and:

  1) unmaps the mutex memory
  2) maps a different file, which ends up covering the same address

When then the original task exits before reaching #5 then the kernel robust
list handling observes the pending op entry and tries to fix up user space.

In case that the newly mapped data contains the TID of the exiting thread
at the address of the mutex/futex the kernel will set the owner died bit in
that memory and therefore corrupt unrelated data.

On X86 this boils down to this simplified assembly sequence:

		mov		%esi,%eax	// Load TID into EAX
        	xor		%ecx,%ecx	// Set ECX to 0
   #3		lock cmpxchg	%ecx,(%rdi)	// Try the TID -> 0 transition
	.Lstart:
		jnz    		.Lend
   #4 		movq		%rcx,(%rdx)	// Clear list_op_pending
	.Lend:

If the cmpxchg() succeeds and the task is interrupted before it can clear
list_op_pending in the robust list head (#4) and the task crashes in a
signal handler or gets killed then it ends up in do_exit() and subsequently
in the robust list handling, which then might run into the unmap/map issue
described above.

This is only relevant when user space was interrupted and a signal is
pending. The fix-up has to be done before signal delivery is attempted
because:

   1) The signal might be fatal so get_signal() ends up in do_exit()

   2) The signal handler might crash or the task is killed before returning
      from the handler. At that point the instruction pointer in pt_regs is
      not longer the instruction pointer of the initially interrupted unlock
      sequence.

The right place to handle this is in __exit_to_user_mode_loop() before
invoking arch_do_signal_or_restart() as this covers obviously both
scenarios.

As this is only relevant when the task was interrupted in user space, this
is tied to RSEQ and the generic entry code as RSEQ keeps track of user
space interrupts unconditionally even if the task does not have a RSEQ
region installed. That makes the decision very lightweight:

       if (current->rseq.user_irq && within(regs, csr->unlock_ip_range))
       		futex_fixup_robust_unlock(regs, csr);

futex_fixup_robust_unlock() then invokes a architecture specific function
to return the pending op pointer or NULL. The function evaluates the
register content to decide whether the pending ops pointer in the robust
list head needs to be cleared.

Assuming the above unlock sequence, then on x86 this decision is the
trivial evaluation of the zero flag:

	return regs->eflags & X86_EFLAGS_ZF ? regs->dx : NULL;

Other architectures might need to do more complex evaluations due to LLSC,
but the approach is valid in general. The size of the pointer is determined
from the matching range struct, which covers both 32-bit and 64-bit builds
including COMPAT.

The unlock sequence is going to be placed in the VDSO so that the kernel
can keep everything synchronized, especially the register usage. The
resulting code sequence for user space is:

   if (__vdso_futex_robust_list$SZ_try_unlock(lock, tid, &pending_op) != tid)
 	err = sys_futex($OP | FUTEX_ROBUST_UNLOCK,....);

Both the VDSO unlock and the kernel side unlock ensure that the pending_op
pointer is always cleared when the lock becomes unlocked.

Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: André Almeida <andrealmeid@igalia.com>
Link: https://patch.msgid.link/20260602090535.773669210@kernel.org

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