RDMA/umem: Avoid undefined behavior of rounddown_pow_of_two()

rounddown_pow_of_two() is undefined when the input is 0. Therefore we need
to avoid it in ib_umem_find_best_pgsz and return 0.  Otherwise, it could
result in not rejecting an invalid page size which eventually causes a
kernel oops due to the logical inconsistency.

Fixes: 3361c29e ("RDMA/umem: Use simpler logic for ib_umem_find_best_pgsz()")
Link: https://lore.kernel.org/r/20210113121703.559778-2-leon@kernel.orgSigned-off-by: Aharon Landau <aharonl@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/core/umem.c

@@ -135,7 +135,7 @@ unsigned long ib_umem_find_best_pgsz(struct ib_umem *umem,
 	 */
 	if (mask)
 		pgsz_bitmap &= GENMASK(count_trailing_zeros(mask), 0);
-	return rounddown_pow_of_two(pgsz_bitmap);
+	return pgsz_bitmap ? rounddown_pow_of_two(pgsz_bitmap) : 0;
 }
 EXPORT_SYMBOL(ib_umem_find_best_pgsz);