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

[Alexandre Ghiti]

Hi Stefan,

On 27/03/2024 17:53, Stefan O'Rear 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 at fastmail.com> 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 at fastmail.com>
>>> ---
>>>   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.


So as I  did not this know this path really well, I played a bit and I 
can confirm that usermode processes reach userspace with the gp = kernel:

Thread 1 hit Breakpoint 12, 0x00007fff82487fc4 in ?? ()
1: x/i $pc
=> 0x7fff82487fc4:    mv    a0,sp
3: /x $gp = 0xffffffff817fee50


>
>     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?


I'd put them all, but up to you, at least the first usecase that I was 
able to reproduce should be added to the commit log.

You can add:

Reviewed-by: Alexandre Ghiti <alexghiti at rivosinc.com>

And this should go to -fixes.

Thanks,

Alex


>
> -s
>
>> Thanks,
>> Yunhui
>>
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