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

[Palmer Dabbelt]

On Tue, 02 Apr 2024 02:21:15 PDT (-0700), alex at ghiti.fr wrote:
> 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.

I just pasted it all in the commit, it seems generally useful for people 
running into the commit.  With the Link tags maybe it's less important 
these days, but I always just err on the side of putting more stuff in 
the commit messages.

> You can add:
>
> Reviewed-by: Alexandre Ghiti <alexghiti at rivosinc.com>
>
> And this should go to -fixes.

It's queued up for the tester.

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