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

Summary

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.

Timeline

1. Oct 23, 2026, 12:20 PM UTCCVE published
2. Oct 23, 2026, 12:45 PM UTCFirst seen by cve.now
3. Oct 24, 2026, 2:10 PM UTCRecord last updated

Affected versions

• Linux Linux

  From 2e1743085887ba3f553c2bb472a75a3ff744b242 · fixed in 2e5c80bbc062a8dda33e687ebf7be8f074ee4f7d; From 2e1743085887ba3f553c2bb472a75a3ff744b242 · fixed in 8e48d8d51ab37b7decfbff3e2844e027a52ad6ea; From 2e1743085887ba3f553c2bb472a75a3ff744b242 · fixed in 405290be962b32eec842776fbe3bdf81f15b1676

• Linux Linux

  6.13

• KernelLinux

  From 6.13.0 · fixed in 6.18.57; From 6.19.0 · fixed in 7.2.11

References 3