CVE-2025-38434

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

Revert "riscv: Define TASK_SIZE_MAX for __access_ok()"

This reverts commit ad5643cf2f69 ("riscv: Define TASK_SIZE_MAX for __access_ok()").

This commit changes TASK_SIZE_MAX to be LONG_MAX to optimize access_ok(), because the previous TASK_SIZE_MAX (default to TASK_SIZE) requires some computation.

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

Addresses between TASK_SIZE and LONG_MAX 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: get_user_pages_fast() happily accepts addresses between TASK_SIZE and LONG_MAX. futex(), for instance, uses get_user_pages_fast(). This causes the problem reported by Robert [1].

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

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

This vulnerability does not affect Red Hat Enterprise Linux as the affected RISC-V architecture code is not present in supported Red Hat kernel packages. The flaw stems from improper user-space memory boundary validation within upstream RISC-V implementations, which could permit local processes to supply out-of-range user memory addresses to kernel memory handling routines. Because Red Hat products do not include this architecture-specific change, there is no operational or security risk to Red Hat deployments.