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

Overview

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.

Affected

• Linux >= 2e1743085887ba3f553c2bb472a75a3ff744b242 < 2e5c80bbc062a8dda33e687ebf7be8f074ee4f7d
• Linux >= 2e1743085887ba3f553c2bb472a75a3ff744b242 < 8e48d8d51ab37b7decfbff3e2844e027a52ad6ea
• Linux >= 2e1743085887ba3f553c2bb472a75a3ff744b242 < 405290be962b32eec842776fbe3bdf81f15b1676
• Linux 6.13

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.