Vulnerability CVE-2024-35871
Published: 2024-05-19

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.

 References:
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
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