RDMA/siw: Fix a use after free in siw_alloc_mr

Our code analyzer reported a UAF.

In siw_alloc_mr(), it calls siw_mr_add_mem(mr,..). In the implementation of
siw_mr_add_mem(), mem is assigned to mr->mem and then mem is freed via
kfree(mem) if xa_alloc_cyclic() failed. Here, mr->mem still point to a
freed object. After, the execution continue up to the err_out branch of
siw_alloc_mr, and the freed mr->mem is used in siw_mr_drop_mem(mr).

My patch moves "mr->mem = mem" behind the if (xa_alloc_cyclic(..)<0) {}
section, to avoid the uaf.

Fixes: 2251334d ("rdma/siw: application buffer management")
Link: https://lore.kernel.org/r/20210426011647.3561-1-lyl2019@mail.ustc.edu.cnSigned-off-by: NLv Yunlong <lyl2019@mail.ustc.edu.cn>
Reviewed-by: NBernard Metzler <bmt@zurich.ihm.com>
Signed-off-by: NJason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/sw/siw/siw_mem.c

@@ -106,8 +106,6 @@ int siw_mr_add_mem(struct siw_mr *mr, struct ib_pd *pd, void *mem_obj,
 	mem->perms = rights & IWARP_ACCESS_MASK;
 	kref_init(&mem->ref);
 
-	mr->mem = mem;
-
 	get_random_bytes(&next, 4);
 	next &= 0x00ffffff;
 
@@ -116,6 +114,8 @@ int siw_mr_add_mem(struct siw_mr *mr, struct ib_pd *pd, void *mem_obj,
 		kfree(mem);
 		return -ENOMEM;
 	}
+
+	mr->mem = mem;
 	/* Set the STag index part */
 	mem->stag = id << 8;
 	mr->base_mr.lkey = mr->base_mr.rkey = mem->stag;