Subject: CVE-2024-35871: riscv: process: Fix kernel gp leakage

From: Greg Kroah-Hartman <[email-masked]>

Description
===========

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

riscv: process: Fix kernel gp leakage

childregs represents the registers which are active for the new thread
in user context. For a kernel thread, childregs->gp is never used since
the kernel gp is not touched by switch_to. For a user mode helper, the
gp value can be observed in user space after execve or possibly by other
means.

[From the email thread]

The /* Kernel thread */ comment is somewhat inaccurate in that it is also used
for user_mode_helper threads, which exec a user process, e.g. /sbin/init or
when /proc/sys/kernel/core_pattern is a pipe. Such threads do not have
PF_KTHREAD set and are valid targets for ptrace etc. even before they exec.

childregs is the *user* context during syscall execution and it is observable
from userspace in at least five ways:

1. kernel_execve does not currently clear integer registers, so the starting
   register state for PID 1 and other user processes started by the kernel has
   sp = user stack, gp = kernel __global_pointer$, all other integer registers
   zeroed by the memset in the patch comment.

   This is a bug in its own right, but I'm unwilling to bet that it is the only
   way to exploit the issue addressed by this patch.

2. ptrace(PTRACE_GETREGSET): you can PTRACE_ATTACH to a user_mode_helper thread
   before it execs, but ptrace requires SIGSTOP to be delivered which can only
   happen at user/kernel boundaries.

3. /proc/*/task/*/syscall: this is perfectly happy to read pt_regs for
   user_mode_helpers before the exec completes, but gp is not one of the
   registers it returns.

4. PERF_SAMPLE_REGS_USER: LOCKDOWN_PERF normally prevents access to kernel
   addresses via PERF_SAMPLE_REGS_INTR, but due to this bug kernel addresses
   are also exposed via PERF_SAMPLE_REGS_USER which is permitted under
   LOCKDOWN_PERF. I have not attempted to write exploit code.

5. Much of the tracing infrastructure allows access to user registers. I have
   not attempted to determine which forms of tracing allow access to user
   registers without already allowing access to kernel registers.

The Linux kernel CVE team has assigned CVE-2024-35871 to this issue.

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

	Issue introduced in 4.15 with commit 7db91e57a0ac and fixed in 5.10.216 with commit 9abc3e6f1116
	Issue introduced in 4.15 with commit 7db91e57a0ac and fixed in 5.15.154 with commit dff6072124f6
	Issue introduced in 4.15 with commit 7db91e57a0ac and fixed in 6.1.85 with commit f6583444d7e7
	Issue introduced in 4.15 with commit 7db91e57a0ac and fixed in 6.6.26 with commit 00effef72c98
	Issue introduced in 4.15 with commit 7db91e57a0ac and fixed in 6.8.5 with commit d8dcba0691b8
	Issue introduced in 4.15 with commit 7db91e57a0ac and fixed in 6.9 with commit d14fa1fcf69d

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-2024-35871
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/kernel/process.c

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/9abc3e6f1116adb7a2d4fbb8ce20c37916976bf5
	https://git.kernel.org/stable/c/dff6072124f6df77bfd36951fbd88565746980ef
	https://git.kernel.org/stable/c/f6583444d7e78dae750798552b65a2519ff3ca84
	https://git.kernel.org/stable/c/00effef72c98294edb1efa87ffa0f6cfb61b36a4
	https://git.kernel.org/stable/c/d8dcba0691b8e42bddb61aab201e4d918a08e5d9
	https://git.kernel.org/stable/c/d14fa1fcf69db9d070e75f1c4425211fa619dfc8