Re: [External] [PATCH] riscv: process: Fix kernel gp leakage

[yunhui cui]

Hi Stefan,

On Thu, Mar 28, 2024 at 12:54 AM Stefan O'Rear <sorear@xxxxxxxxxxxx> wrote:
>
> On Wed, Mar 27, 2024, at 4:43 AM, yunhui cui wrote:
> > Hi Stefan,
> >
> > On Wed, Mar 27, 2024 at 2:14 PM Stefan O'Rear <sorear@xxxxxxxxxxxx> wrote:
> >>
> >> 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.
> >>
> >> Fixes: 7db91e57a0ac ("RISC-V: Task implementation")
> >> Signed-off-by: Stefan O'Rear <sorear@xxxxxxxxxxxx>
> >> ---
> >>  arch/riscv/kernel/process.c | 3 ---
> >>  1 file changed, 3 deletions(-)
> >>
> >> diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
> >> index 92922dbd5b5c..51042f48da17 100644
> >> --- a/arch/riscv/kernel/process.c
> >> +++ b/arch/riscv/kernel/process.c
> >> @@ -27,8 +27,6 @@
> >>  #include <asm/vector.h>
> >>  #include <asm/cpufeature.h>
> >>
> >> -register unsigned long gp_in_global __asm__("gp");
> >> -
> >>  #if defined(CONFIG_STACKPROTECTOR) && !defined(CONFIG_STACKPROTECTOR_PER_TASK)
> >>  #include <linux/stackprotector.h>
> >>  unsigned long __stack_chk_guard __read_mostly;
> >> @@ -207,7 +205,6 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
> >>         if (unlikely(args->fn)) {
> >>                 /* Kernel thread */
> >>                 memset(childregs, 0, sizeof(struct pt_regs));
> >> -               childregs->gp = gp_in_global;
> >>                 /* Supervisor/Machine, irqs on: */
> >>                 childregs->status = SR_PP | SR_PIE;
> >>
> >> --
> >> 2.40.1
> >>
> >>
> > Can you help express in more detail what the problem was before fixing it?
>
> It's a KASLR bypass, since gp_in_global is the address of the kernel symbol
> __global_pointer$.
>
> The /* Kernel thread */ comment is somewhat inaccurate in that it is also used
> for user_mode_helper threads, which exec a user process, e.g. /sbin/init or
> when /proc/sys/kernel/core_pattern is a pipe. Such threads do not have
> PF_KTHREAD set and are valid targets for ptrace etc. even before they exec.
>
> childregs is the *user* context during syscall execution and it is observable
> from userspace in at least five ways:
>
> 1. kernel_execve does not currently clear integer registers, so the starting
>    register state for PID 1 and other user processes started by the kernel has
>    sp = user stack, gp = kernel __global_pointer$, 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.
>
> Does this help? How much of this should be in the commit message?

Fine enough, Thanks.

Thanks,
Yunhui