CVE-2026-99016: riscv: futex: untag the user pointer before the atomic access

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

riscv: futex: untag the user pointer before the atomic access

accessok() checks untaggedaddr(uaddr), but archfutexatomicopinuser() and futexatomiccmpxchginatomic() hand the raw uaddr to the inline asm amoswap/amoadd/.../lr.w+sc.w through the "+m" (uaddr) operand. When the tagged address ABI is enabled (CONFIGRISCVISASUPM, prctl PRSETTAGGEDADDRCTRL with PMLEN != 0), those two addresses differ: a user pointer whose top PMLEN bits hold a tag passes accessok() after untagging, but the atomic is then performed on the still-tagged raw address.

Supervisor-mode data accesses are not subject to the U-mode pointer masking (that is governed by menvcfg.PMM, which the kernel does not set), so hardware does not strip the tag for the kernel's own access. With Sv57 and PMLEN=16 the tag bits overlap the canonical-address bits, so a tagged pointer can name a canonical kernel virtual address (e.g. in the linear map) whose untagged form is a valid user address. An unprivileged process can thus make FUTEXWAKEOP perform an atomic read-modify-write on an arbitrary kernel address, with the matching FUTEXOPCMP result serving as a read oracle.

getuser()/putuser()/rawcopy{to,from}user() already untag the pointer after the accessok() check; do the same in the futex helpers so the atomic operates on the address that was actually validated.