futex: Fix UaF between futex_key_to_node_opt() and vma_replace_policy()

futex: Fix UaF between futex_key_to_node_opt() and vma_replace_policy()

During futex_key_to_node_opt() execution, vma->vm_policy is read under
speculative mmap lock and RCU. Concurrently, mbind() may call
vma_replace_policy() which frees the old mempolicy immediately via
kmem_cache_free().

This creates a race where __futex_key_to_node() dereferences a freed
mempolicy pointer, causing a use-after-free read of mpol->mode.

[  151.412631] BUG: KASAN: slab-use-after-free in __futex_key_to_node (kernel/futex/core.c:349)
[  151.414046] Read of size 2 at addr ffff888001c49634 by task e/87

[  151.415969] Call Trace:

[  151.416732]  __asan_load2 (mm/kasan/generic.c:271)
[  151.416777]  __futex_key_to_node (kernel/futex/core.c:349)
[  151.416822]  get_futex_key (kernel/futex/core.c:374 kernel/futex/core.c:386 kernel/futex/core.c:593)

Fix by adding rcu to __mpol_put().

Fixes: c042c505210d ("futex: Implement FUTEX2_MPOL")
Reported-by: Hao-Yu Yang <naup96721@gmail.com>
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Hao-Yu Yang <naup96721@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Link: https://patch.msgid.link/20260324174418.GB1850007@noisy.programming.kicks-ass.net