Re: [PATCH 1/2] ARM: entry-common: fix forgotten set of thread_info->syscall

[Kees Cook]

On Wed, Jan 21, 2015 at 5:24 PM, Roman Peniaev <r.peniaev@xxxxxxxxx> wrote:
> On Thu, Jan 22, 2015 at 8:32 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>> On Tue, Jan 20, 2015 at 3:04 PM, Russell King - ARM Linux
>> <linux@xxxxxxxxxxxxxxxx> wrote:
>>> On Tue, Jan 20, 2015 at 10:45:19PM +0000, Russell King - ARM Linux wrote:
>>>> Well, the whole question is this: is restarting a system call like
>>>> usleep() really a separate system call, or is it a kernel implementation
>>>> detail?
>>>>
>>>> If you wanted seccomp to see this, what would be the use case?  Why
>>>> would seccomp want to block this syscall?  Does it make sense for
>>>> seccomp to block this syscall when it doesn't block something like
>>>> usleep() and then have usleep() fail just because the thread received
>>>> a signal?
>>>>
>>>> I personally regard the whole restart system call thing as a purely
>>>> kernel internal thing which should not be exposed to userland.  If
>>>> we decide that it should be exposed to userland, then it becomes part
>>>> of the user ABI, and it /could/ become difficult if we needed to
>>>> change it in future - and I'd rather not get into the "oh shit, we
>>>> can't change it because that would break app X" crap.
>>>
>>> Here's a scenario where it could become a problem:
>>>
>>> Let's say that we want to use seccomp to secure some code which issues
>>> system calls.  We determine that the app uses system calls which don't
>>> result in the restart system call being issued, so we decide to ask
>>> seccomp to block the restart system call.  Some of these system calls
>>> that the app was using are restartable system calls.
>>>
>>> When these system calls are restarted, what we see via ptrace etc is
>>> that the system call simply gets re-issued as its own system call.
>>>
>>> In a future kernel version, we decide that we could really do with one
>>> of those system calls using the restart block feature, so we arrange
>>> for it to set up the restart block, and return -ERESTART_BLOCK.  That's
>>> fine for most applications, but this app now breaks.
>>>
>>> The side effect of that breakage is that we have to revert that kernel
>>> change - because we've broken userland, and that's simply not allowed.
>>>
>>> Now look at the alternative: we don't make the restart syscall visible.
>>> This means that we hide that detail, and we actually reflect the
>>> behaviour that we've had for the other system call restart mechanisms,
>>> and we don't have to fear userspace breakage as a result of switching
>>> from one restart mechanism to another.
>>>
>>> I am very much of the opinion that we should be trying to limit the
>>> exposure of inappropriate kernel internal details to userland, because
>>> userland has a habbit of becoming reliant on them, and when it does,
>>> it makes kernel maintanence unnecessarily harder.
>>
>> I totally agree with you. :) My question here is more about what we
>> should do with what we currently have since we have some unexpected
>> combinations.
>>
>> There is already an __NR_restart_syscall syscall and it seems like
>> it's already part of the userspace ABI:
>>  - it is possible to call it from userspace directly
>>  - seccomp "sees" it
>>  - ptrace doesn't see it
>>
>> Native ARM64 hides the restart from both seccomp and ptrace, and this
>> seems like the right idea, except that restart_syscall is still
>> callable from userspace. I don't think there's a way to make that
>> vanish, which means we'll always have an exposed syscall. If anything
>> goes wrong with it (which we've been quite close to recently[1]),
>> there would be no way to have seccomp filter it.
>>
>> So, at the least, I'd like arm64 to NOT hide restart_syscall from
>> seccomp, and at best I'd like both arm and arm64 to (somehow) entirely
>> remove restart_syscall from the userspace ABI so it wouldn't need to
>> be filtered, and wouldn't become a weird ABI hiccup as you've
>> described.
>>
>> I fail to imagine a way to remove restart_syscall from userspace, so
>> I'm left with wanting parity of behavior between ARM and ARM64 (and
>> x86). What's the right way forward?
>
> My sufferings are an opposite of what seccompt expects: currently I do
> not see any possible way (from userspace) to get syscall number which was
> restarted, because at some given time userspace checks the procfs
> syscall file and sees NR_restart there, without any chance to understand
> what exactly was restarted (I am talking about some kind of debugging
> tool which does dead-lock analysis of stuck tasks).
>
> I totally agree with Russell not to provide kernel guts to userspace,
> but it is already done.  Too late.
>
> So probably there is a need to split syscall on two numbers:
> real and effective.  Real number we have right now on x86.
>
> And this should be done for both ptrace and procfs syscall file.
> (am I right that only for ARM we already have PTRACE_SET_SYSCALL?
>  seems we can add also real/effective getter)

ARM's syscall "get" is via PTRACE_GETREGSET with NT_PRSTATUS, reading ARM_r7:

int syscall_get(pid_t tracee) {
        struct iovec iov;
        struct pt_regs;

        iov.iov_base = &regs;
        iov.iov_len = sizeof(regs);
        if (ptrace(PTRACE_GETREGSET, tracee, NT_PRSTATUS, &iov) < 0) {
               perror("PTRACE_GETREGSET, NT_PRSTATUS");
               return -1;
        }
        return regs.ARM_r7;
}

ARM's syscall "set" is via PTRACE_SET_SYSCALL:

int syscall_set(int syscall, pid_t tracee) {
        return ptrace(PTRACE_SET_SYSCALL, tracee, NULL, syscall);
}

Landing in 3.19, ARM64 has get/set via PTRACE_[GS]ETREGSET with
NT_ARM_SYSTEM_CALL:

int syscall_get(pid_t tracee) {
        struct iovec iov;
        int syscall;

        iov.iov_base = &syscall;
        iov.iov_len = sizeof(syscall);
        if (ptrace(PTRACE_GETREGSET, tracee,
                  NT_ARM_SYSTEM_CALL, &iov) < 0) {
                perror("PTRACE_GETREGSET, NT_ARM_SYSTEM_CALL");
                return -1;
        }
        return syscall;
}

int syscall_set(int syscall, pid_t tracee) {
        iov.iov_base = &syscall;
        iov.iov_len = sizeof(syscall);
        return ptrace(PTRACE_SETREGSET, tracee,
                  NT_ARM_SYSTEM_CALL, &iov);
}

Prior to 3.19, ARM64 could use PTRACE_[GS]ETREGSET, NT_STATUS on
struct user_pt_regs and regs[8].

-Kees

-- 
Kees Cook
Chrome OS Security
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