riscv: process: Fix kernel gp leakage

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.