In the Linux kernel, the following vulnerability has been resolved:

RDMA/rxe: validate access flags before swapping the MR's PD

rxe_rereg_user_mr() reassigns mr->ibmr.pd first and only then
validates the IB_MR_REREG_ACCESS argument:

	if (flags & IB_MR_REREG_PD) {
		rxe_put(old_pd);
		rxe_get(pd);
		mr->ibmr.pd = ibpd;
	}

	if (flags & IB_MR_REREG_ACCESS) {
		if (access & ~RXE_ACCESS_SUPPORTED_MR)
			return ERR_PTR(-EOPNOTSUPP);
		mr->access = access;
	}

Both flags pass the entry check because RXE_MR_REREG_SUPPORTED is
IB_MR_REREG_PD | IB_MR_REREG_ACCESS, so a caller can reach the access
check with mr->ibmr.pd already reassigned.

mr->ibmr.pd is owned by the core, which adjusts pd->usecnt only on the
success path: ib_uverbs_rereg_mr() jumps to put_new_uobj on a driver error
without undoing the reassignment, so mr->pd == new_pd while the usecnts
still charge the MR to orig_pd. ib_dereg_mr_user() then decrements
new_pd, whose count can reach zero while a memory window still references
it; uverbs_free_pd() frees the PD on that count alone and rxe_mw_cleanup()
writes to freed memory:

  BUG: KASAN: slab-use-after-free in __rxe_put+0x31/0xa0
  Write of size 4 at addr ffff8881301dd690 by task rxe_poc/591
   __rxe_put+0x31/0xa0
   rxe_mw_cleanup+0x42/0x200
   __rxe_cleanup+0x115/0x370
   rxe_dealloc_mw+0x4c/0x80
  Allocated by task 591:
   ib_uverbs_alloc_pd+0x258/0x540
  Freed by task 591:
   ib_dealloc_pd_user+0x174/0x210
   uverbs_free_pd+0x8d/0xc0
   ib_uverbs_dealloc_pd+0x18e/0x1d0

Validate the access flags before mutating any state so the callback either
applies every requested change or none.