CVE-2026-99016 riscv: futex: untag the user pointer before the atomic access
- Exploited in the wild
- Not known
- Fix
- Available
What is CVE-2026-99016?
CVE-2026-99016 is a high-severity vulnerability in Linux, published on 23 Oct 2026.
Official description
In the Linux kernel, the following vulnerability has been resolved:
riscv: futex: untag the user pointer before the atomic access
access_ok() checks untagged_addr(uaddr), but arch_futex_atomic_op_inuser() and futex_atomic_cmpxchg_inatomic() 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 (CONFIG_RISCV_ISA_SUPM, prctl PR_SET_TAGGED_ADDR_CTRL with PMLEN != 0), those two addresses differ: a user pointer whose top PMLEN bits hold a tag passes access_ok() 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 FUTEX_WAKE_OP perform an atomic read-modify-write on an arbitrary kernel address, with the matching FUTEX_OP_CMP_* result serving as a read oracle.
get_user()/put_user()/raw_copy_{to,from}_user() already untag the pointer after the access_ok() check; do the same in the futex helpers so the atomic operates on the address that was actually validated.
Which versions are affected by CVE-2026-99016?
How do I fix CVE-2026-99016?
Upgrade to a fixed version.
Is CVE-2026-99016 being exploited?
There is no known exploitation of CVE-2026-99016 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2026-99016, DEBIAN-CVE-2026-99016, GHSA-5mv4-wx47-m6wh, UBUNTU-CVE-2026-99016.