CVE-2025-38434

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

Revert "riscv: Define TASKSIZEMAX for _accessok()"

This reverts commit ad5643cf2f69 ("riscv: Define TASKSIZEMAX for _accessok()").

This commit changes TASKSIZEMAX to be LONGMAX to optimize accessok(), because the previous TASKSIZEMAX (default to TASK_SIZE) requires some computation.

The reasoning was that all user addresses are less than LONGMAX, and all kernel addresses are greater than LONGMAX. Therefore access_ok() can filter kernel addresses.

Addresses between TASKSIZE and LONGMAX are not valid user addresses, but access_ok() let them pass. That was thought to be okay, because they are not valid addresses at hardware level.

Unfortunately, one case is missed: getuserpagesfast() happily accepts addresses between TASKSIZE and LONGMAX. futex(), for instance, uses getuserpagesfast(). This causes the problem reported by Robert [1].

Therefore, revert this commit. TASKSIZEMAX is changed to the default: TASK_SIZE.

This unfortunately reduces performance, because TASKSIZE is more expensive to compute compared to LONGMAX. But correctness first, we can think about optimization later, if required.