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riscv: process: Fix kernel gp leakage
stable inclusion from stable-v5.10.216 commit 9abc3e6f1116adb7a2d4fbb8ce20c37916976bf5 category: bugfix bugzilla: https://gitee.com/src-openeuler/kernel/issues/I9QG6A CVE: CVE-2024-35871 Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9abc3e6f1116adb7a2d4fbb8ce20c37916976bf5 -------------------------------- [ Upstream commit d14fa1fcf69db9d070e75f1c4425211fa619dfc8 ] 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. Fixes: 7db91e57 ("RISC-V: Task implementation") Cc: stable@vger.kernel.org Signed-off-by:Stefan O'Rear <sorear@fastmail.com> Reviewed-by: