CVE-2024-35871: 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 switchto. 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 usermodehelper threads, which exec a user process, e.g. /sbin/init or when /proc/sys/kernel/corepattern is a pipe. Such threads do not have PFKTHREAD 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. kernelexecve 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 globalpointer$, 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(PTRACEGETREGSET): you can PTRACEATTACH to a usermodehelper 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 ptregs for usermodehelpers before the exec completes, but gp is not one of the registers it returns.

4. PERFSAMPLEREGSUSER: LOCKDOWNPERF normally prevents access to kernel addresses via PERFSAMPLEREGSINTR, but due to this bug kernel addresses are also exposed via PERFSAMPLEREGSUSER which is permitted under LOCKDOWNPERF. 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.