CVE-2024-35871

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.