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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Greg Kroah-Hartman <gregkh@kernel.org>

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.

The Linux kernel CVE team has assigned CVE-2026-99016 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.13 with commit 2e1743085887ba3f553c2bb472a75a3ff744b242 and fixed in 6.18.57 with commit 2e5c80bbc062a8dda33e687ebf7be8f074ee4f7d
	Issue introduced in 6.13 with commit 2e1743085887ba3f553c2bb472a75a3ff744b242 and fixed in 7.2.11 with commit 8e48d8d51ab37b7decfbff3e2844e027a52ad6ea
	Issue introduced in 6.13 with commit 2e1743085887ba3f553c2bb472a75a3ff744b242 and fixed in 7.3 with commit 405290be962b32eec842776fbe3bdf81f15b1676

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-99016
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	arch/riscv/include/asm/futex.h


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/2e5c80bbc062a8dda33e687ebf7be8f074ee4f7d
	https://git.kernel.org/stable/c/8e48d8d51ab37b7decfbff3e2844e027a52ad6ea
	https://git.kernel.org/stable/c/405290be962b32eec842776fbe3bdf81f15b1676