Re: [PATCH v10 09/12] arch/x86: enable task isolation functionality

[Andy Lutomirski]

On Tue, Mar 8, 2016 at 12:40 PM, Chris Metcalf <cmetcalf@xxxxxxxxxxxx> wrote:
> On 03/07/2016 03:55 PM, Andy Lutomirski wrote:
>>>>
>>>> Let task isolation users who want to detect when they screw up and do
>>>> >>a syscall do it with seccomp.
>>>
>>>
>>> >Can you give me more details on what you're imagining here?  Remember
>>> >that a key use case is that these applications can remove the syscall
>>> >prohibition voluntarily; it's only there to prevent unintended uses
>>> >(by third party libraries or just straight-up programming bugs).
>>> >As far as I can tell, seccomp does not allow you to go from "less
>>> >permissive" to "more permissive" settings at all, which means that as
>>> >it exists, it's not a good solution for this use case.
>>> >
>>> >Or were you thinking about a new seccomp API that allows this?
>>
>> I was.  This is at least the second time I've wanted a way to ask
>> seccomp to allow a layer to be removed.
>
>
> Andy,
>
> Please take a look at this draft patch that intends to enable seccomp
> as something that task isolation can use.

Kees, this sounds like it may solve your self-instrumentation problem.
Want to take a look?

--Andy

>
> The basic idea is to add a notion of "removable" seccomp filters.
> You can tag a filter that way when installing it (using the new
> SECCOMP_FILTER_FLAG_REMOVABLE flag bit for SECCOMP_SET_MODE_FILTER),
> and if the most recently-added filter is marked as removable, you can
> remove it with the new SECCOMP_POP_FILTER operation.  It is currently
> implemented to be incompatible with SECCOMP_FILTER_FLAG_TSYNC, which
> is plausible since the obvious use is for thread-local push and pop,
> but the API allows for future implementation by including a flag word
> with the pop_filter operation (now always zero).
>
> I did not make this supported via the prctl() since the "removable"
> flag requires seccomp(), so making pop work with prctl() seemed silly.
>
> One interesting result of this is that now it is no longer true
> that once current->seccomp.mode becomes non-zero, it may not be
> changed, since it can now be changed back to DISABLED when you push a
> removable filter and later pop it.
>
> My preference would be not to have to require all task-isolation users
> to also figure out all the complexities of creating BPF programs, so
> my intention is to have task isolation automatically generate a BPF
> program (just allowing prctl/exit/exit_group and failing everything
> else with SIGSYS).  To support having it work this way, I open up
> the seccomp stuff a little so that kernel clients can effectively
> push/pop a BPF program into seccomp:

That sounds like a great use case for the new libtaskisolation that
someone is surely writing :)

>
> long seccomp_push_filter(unsigned int flags, struct bpf_prog *fp)
> long seccomp_pop_filter(unsigned int flags);
>
> We mark filters from this API with a new "extern_prog" boolean in the
> seccomp_filter struct so the BPF program isn't freed when the
> seccomp_filter itself is freed.  Note that doing it this way avoids
> having to go through the substantial overhead of creating a brand-new
> BPF filter every time we enter task isolation mode.
>
> Not shown here is the additional code needed in task isolation to
> create a suitable BPF program and then push and pop it as we go in and
> out of task isolation mode.
>
> For what it's worth, I'm a little dubious about the tradeoff of adding
> a substantial chunk of code to seccomp to handle what the v10 task
> isolation code did with a single extra TIF flag test and a dozen lines
> of code that got called.  But given that you said there were other
> potential users for the "filter pop" idea, it may indeed make sense.
>
> This is still all untested, but I wanted to get your sense of whether
> this was even going in the right direction before spending more time
> on it.
>
> Thanks!
>
> diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
> index 2296e6b2f690..feeba7a23d20 100644
> --- a/include/linux/seccomp.h
> +++ b/include/linux/seccomp.h
> @@ -3,13 +3,15 @@
>   #include <uapi/linux/seccomp.h>
>  -#define SECCOMP_FILTER_FLAG_MASK      (SECCOMP_FILTER_FLAG_TSYNC)
> +#define SECCOMP_FILTER_FLAG_MASK       \
> +       (SECCOMP_FILTER_FLAG_TSYNC | SECCOMP_FILTER_FLAG_REMOVABLE)
>   #ifdef CONFIG_SECCOMP
>   #include <linux/thread_info.h>
>  #include <asm/seccomp.h>
>  +struct bpf_prog;
>  struct seccomp_filter;
>  /**
>   * struct seccomp - the state of a seccomp'ed process
> @@ -41,6 +43,8 @@ static inline int secure_computing(void)
>   extern u32 seccomp_phase1(struct seccomp_data *sd);
>  int seccomp_phase2(u32 phase1_result);
> +long seccomp_push_filter(unsigned int flags, struct bpf_prog *fp);
> +long seccomp_pop_filter(unsigned int flags);
>  #else
>  extern void secure_computing_strict(int this_syscall);
>  #endif
> diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
> index 0f238a43ff1e..6e65ac2a7262 100644
> --- a/include/uapi/linux/seccomp.h
> +++ b/include/uapi/linux/seccomp.h
> @@ -13,9 +13,11 @@
>  /* Valid operations for seccomp syscall. */
>  #define SECCOMP_SET_MODE_STRICT        0
>  #define SECCOMP_SET_MODE_FILTER        1
> +#define SECCOMP_POP_FILTER     2
>   /* Valid flags for SECCOMP_SET_MODE_FILTER */
>  #define SECCOMP_FILTER_FLAG_TSYNC      1
> +#define SECCOMP_FILTER_FLAG_REMOVABLE  2
>   /*
>   * All BPF programs must return a 32-bit value.
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 15a1795bbba1..c22eb3a56556 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -41,8 +41,9 @@
>   *         outside of a lifetime-guarded section.  In general, this
>   *         is only needed for handling filters shared across tasks.
>   * @prev: points to a previously installed, or inherited, filter
> - * @len: the number of instructions in the program
> - * @insnsi: the BPF program instructions to evaluate
> + * @prog: the BPF program to evaluate
> + * @removable: if this filter is removable with seccomp_pop_filter()
> + * @extern_prog: if @prog should not be freed in seccomp_free_filter()
>   *
>   * seccomp_filter objects are organized in a tree linked via the @prev
>   * pointer.  For any task, it appears to be a singly-linked list starting
> @@ -58,6 +59,8 @@ struct seccomp_filter {
>         atomic_t usage;
>         struct seccomp_filter *prev;
>         struct bpf_prog *prog;
> +       bool removable;
> +       bool extern_prog;
>  };
>   /* Limit any path through the tree to 256KB worth of instructions. */
> @@ -470,7 +473,8 @@ void get_seccomp_filter(struct task_struct *tsk)
>  static inline void seccomp_filter_free(struct seccomp_filter *filter)
>  {
>         if (filter) {
> -               bpf_prog_destroy(filter->prog);
> +               if (!filter->extern_prog)
> +                       bpf_prog_destroy(filter->prog);
>                 kfree(filter);
>         }
>  }
> @@ -722,6 +726,7 @@ long prctl_get_seccomp(void)
>   * seccomp_set_mode_strict: internal function for setting strict seccomp
>   *
>   * Once current->seccomp.mode is non-zero, it may not be changed.
> + * (other than to reset to DISABLED after removing the last removable
> filter).
>   *
>   * Returns 0 on success or -EINVAL on failure.
>   */
> @@ -749,33 +754,34 @@ out:
>   #ifdef CONFIG_SECCOMP_FILTER
>  /**
> - * seccomp_set_mode_filter: internal function for setting seccomp filter
> + * do_push_filter: internal function for setting seccomp filter
>   * @flags:  flags to change filter behavior
> - * @filter: struct sock_fprog containing filter
> + * @prepared: struct seccomp_filter to install
>   *
>   * This function may be called repeatedly to install additional filters.
>   * Every filter successfully installed will be evaluated (in reverse order)
>   * for each system call the task makes.
>   *
> - * Once current->seccomp.mode is non-zero, it may not be changed.
> + * Once current->seccomp.mode is non-zero, it may not be changed
> + * (other than to reset to DISABLED after removing the last removable
> filter).
>   *
>   * Returns 0 on success or -EINVAL on failure.
>   */
> -static long seccomp_set_mode_filter(unsigned int flags,
> -                                   const char __user *filter)
> +long do_push_filter(unsigned int flags, struct seccomp_filter *prepared)
>  {
>         const unsigned long seccomp_mode = SECCOMP_MODE_FILTER;
> -       struct seccomp_filter *prepared = NULL;
>         long ret = -EINVAL;
>         /* Validate flags. */
>         if (flags & ~SECCOMP_FILTER_FLAG_MASK)
>                 return -EINVAL;
>  -      /* Prepare the new filter before holding any locks. */
> -       prepared = seccomp_prepare_user_filter(filter);
> -       if (IS_ERR(prepared))
> -               return PTR_ERR(prepared);
> +       if (flags & SECCOMP_FILTER_FLAG_REMOVABLE) {
> +               /* The intended use case is for thread-local push/pop. */
> +               if (flags & SECCOMP_FILTER_FLAG_TSYNC)
> +                       goto out_free;
> +               prepared->removable = true;
> +       }
>         /*
>          * Make sure we cannot change seccomp or nnp state via TSYNC
> @@ -805,12 +811,87 @@ out_free:
>         seccomp_filter_free(prepared);
>         return ret;
>  }
> +
> +static long seccomp_set_mode_filter(unsigned int flags,
> +                                   const char __user *filter)
> +{
> +       struct seccomp_filter *prepared;
> +
> +       /* Prepare the new filter before holding any locks. */
> +       prepared = seccomp_prepare_user_filter(filter);
> +       if (IS_ERR(prepared))
> +               return PTR_ERR(prepared);
> +       return seccomp_push_filter(flags, prepared);
> +}
> +
> +long seccomp_push_filter(unsigned int flags, struct bpf_prog *fp)
> +{
> +       struct seccomp_filter *sfilter;
> +
> +       sfilter = kzalloc(sizeof(*sfilter), GFP_KERNEL);
> +       if (!sfilter)
> +               return ERR_PTR(-ENOMEM);
> +
> +       sfilter->prog = fp;
> +       sfilter->extern_prog = true;
> +       atomic_set(&sfilter->usage, 1);
> +
> +       return do_push_filter(flags, sfilter);
> +}
> +
> +/**
> + * seccomp_pop_filter: internal function for removing filter
> + * @flags:  flags to change pop behavior
> + *
> + * This function removes the most recently installed filter, if it was
> + * installed with the SECCOMP_FILTER_FLAG_REMOVABLE flag.  Any previously
> + * installed filters are left intact.
> + *
> + * If the last filter is removed, the seccomp state reverts to DISABLED.
> + *
> + * Returns 0 on success or -EINVAL on failure.
> + */
> +long seccomp_pop_filter(unsigned int flags)
> +{
> +       struct seccomp_filter *filter;
> +
> +       /* The intended use case is for temporary thread-local push/pop. */
> +       if (flags & SECCOMP_FILTER_FLAG_TSYNC)
> +               return -EINVAL;
> +
> +       spin_lock_irq(&current->sighand->siglock);
> +
> +       if (current->seccomp.mode != SECCOMP_MODE_FILTER)
> +               goto out;
> +
> +       filter = current->seccomp.filter;
> +       if (unlikely(WARN_ON(filter == NULL)) || !filter->removable)
> +               goto out;
> +
> +       if (filter->prev == NULL) {
> +               clear_tsk_thread_flag(current, TIF_SECCOMP);
> +               current->seccomp.mode = SECCOMP_MODE_DISABLED;
> +       }
> +
> +       current->seccomp.filter = filter->prev;
> +
> +       spin_unlock_irq(&current->sighand->siglock);
> +       seccomp_filter_free(filter);
> +       return 0;
> +out:
> +       spin_unlock_irq(&current->sighand->siglock);
> +       return -EINVAL;
> +}
>  #else
>  static inline long seccomp_set_mode_filter(unsigned int flags,
>                                            const char __user *filter)
>  {
>         return -EINVAL;
>  }
> +static inline long seccomp_pop_filter(unsigned int flags)
> +{
> +       return -EINVAL;
> +}
>  #endif
>   /* Common entry point for both prctl and syscall. */
> @@ -824,6 +905,8 @@ static long do_seccomp(unsigned int op, unsigned int
> flags,
>                 return seccomp_set_mode_strict();
>         case SECCOMP_SET_MODE_FILTER:
>                 return seccomp_set_mode_filter(flags, uargs);
> +       case SECCOMP_POP_FILTER:
> +               return seccomp_pop_filter(flags);
>         default:
>                 return -EINVAL;
>
>         }
>
> --
> Chris Metcalf, Mellanox Technologies
> http://www.mellanox.com
>



-- 
Andy Lutomirski
AMA Capital Management, LLC