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SubjectRe: [PATCH 2/4] seccomp: Add SECCOMP_FILTER_FLAG_KILL_PROCESS
On Mon, Aug 7, 2017 at 6:23 PM, Tyler Hicks <tyhicks@canonical.com> wrote:
> On 08/02/2017 10:19 PM, Kees Cook wrote:
>> Right now, SECCOMP_RET_KILL kills the current thread. There have been
>> a few requests for RET_KILL to kill the entire process (the thread
>> group), but since seccomp's u32 return values are ABI, and ordered by
>> lowest value, with RET_KILL as 0, there isn't a trivial way to provide
>> an even smaller value that would mean the more restrictive action of
>> killing the thread group.
>>
>> Instead, create a filter flag that indicates that a RET_KILL from this
>> filter must kill the process rather than the thread. This can be set
>> (and not cleared) via the new SECCOMP_FILTER_FLAG_KILL_PROCESS flag.
>>
>> Pros:
>> - the logic for the filter action is contained in the filter.
>> - userspace can detect support for the feature since earlier kernels
>> will reject the new flag.
>> Cons:
>> - depends on adding an assignment to the seccomp_run_filters() loop
>> (previous patch).
>>
>> Alternatives to this approach with pros/cons:
>>
>> - Use a new test during seccomp_run_filters() that treats the RET_DATA
>> mask of a RET_KILL action as special. If a new bit is set in the data,
>> then treat the return value as -1 (lower than 0).
>> Pros:
>> - the logic for the filter action is contained in the filter.
>> Cons:
>> - added complexity to time-sensitive seccomp_run_filters() loop.
>> - there isn't a trivial way for userspace to detect if the kernel
>> supports the feature (earlier kernels will silently ignore the
>> RET_DATA and only kill the thread).
>
> I prefer using a filter flag over a special RET_DATA mask but, for
> completeness, I wanted to mention that SECCOMP_GET_ACTION_AVAIL
> operation could be extended to validate special RET_DATA masks. However,
> I don't think that is a clean design.
>
>> - Have SECCOMP_FILTER_FLAG_KILL_PROCESS attach to the seccomp struct
>> rather than the filter.
>> Pros:
>> - no change needed to seccomp_run_filters() loop.
>> Cons:
>> - the change in behavior technically originates external to the
>> filter, which allows for later filters to "enhance" a previously
>> applied filter's RET_KILL to kill the entire process, which may
>> be unexpected.
>>
>> Signed-off-by: Kees Cook <keescook@chromium.org>
>> ---
>> include/linux/seccomp.h | 3 ++-
>> include/uapi/linux/seccomp.h | 3 ++-
>> kernel/seccomp.c | 12 +++++++++++-
>> 3 files changed, 15 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
>> index ecc296c137cd..59d001ba655c 100644
>> --- a/include/linux/seccomp.h
>> +++ b/include/linux/seccomp.h
>> @@ -3,7 +3,8 @@
>>
>> #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_KILL_PROCESS)
>>
>> #ifdef CONFIG_SECCOMP
>>
>> diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
>> index 0f238a43ff1e..4b75d8c297b6 100644
>> --- a/include/uapi/linux/seccomp.h
>> +++ b/include/uapi/linux/seccomp.h
>> @@ -15,7 +15,8 @@
>> #define SECCOMP_SET_MODE_FILTER 1
>>
>> /* Valid flags for SECCOMP_SET_MODE_FILTER */
>> -#define SECCOMP_FILTER_FLAG_TSYNC 1
>> +#define SECCOMP_FILTER_FLAG_TSYNC 1
>> +#define SECCOMP_FILTER_FLAG_KILL_PROCESS 2
>>
>> /*
>> * All BPF programs must return a 32-bit value.
>> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
>> index 8bdcf01379e4..931eb9cbd093 100644
>> --- a/kernel/seccomp.c
>> +++ b/kernel/seccomp.c
>> @@ -44,6 +44,7 @@
>> * is only needed for handling filters shared across tasks.
>> * @prev: points to a previously installed, or inherited, filter
>> * @prog: the BPF program to evaluate
>> + * @kill_process: if true, RET_KILL will kill process rather than thread.
>> *
>> * 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
>> @@ -59,6 +60,7 @@ struct seccomp_filter {
>> refcount_t usage;
>> struct seccomp_filter *prev;
>> struct bpf_prog *prog;
>> + bool kill_process;
>
> Just a reminder to move bool up to be the 2nd member of the struct for
> improved struct packing. (You already said you were going to this while
> you were reviewing my logging patches)

Thanks! Yeah, done now.

>> };
>>
>> /* Limit any path through the tree to 256KB worth of instructions. */
>> @@ -448,6 +450,10 @@ static long seccomp_attach_filter(unsigned int flags,
>> return ret;
>> }
>>
>> + /* Set process-killing flag, if present. */
>> + if (flags & SECCOMP_FILTER_FLAG_KILL_PROCESS)
>> + filter->kill_process = true;
>> +
>> /*
>> * If there is an existing filter, make it the prev and don't drop its
>> * task reference.
>> @@ -658,7 +664,11 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
>> seccomp_init_siginfo(&info, this_syscall, data);
>> do_coredump(&info);
>> }
>> - do_exit(SIGSYS);
>> + /* Kill entire thread group if requested (or went haywire). */
>> + if (!match || match->kill_process)
>
> I found this conditional to be a little surprising. I would have expected:
>
> if (match && match->kill_process)
> do_group_exit(SIGSYS);
> else
> do_exit(SIGSYS);
>
> A resulting NULL match pointer is unexpected but callers of
> seccomp_run_filters() are expected to handle such a situation so it is
> possible. Are you preferring to fail closed as much as possible
> (considering that killing the process is more restrictive than killing
> the thread)?

Yeah, that was my thinking. The only way for this to happen is if we
have a totally impossible state: NULL filter in struct seccomp but
seccomp mode is non-zero. I'll add a comment above this.

> I don't know the specific situations that would result in the match
> pointer not being set in seccomp_run_filters() but I'm curious if
> existing, old userspace code that only expected the thread to be killed
> could potentially tickle such a situation and result in the entire
> thread group being unexpectedly killed.

There should be no way for userspace to reach this condition. It
shouldn't ever be possible to set the seccomp mode without a filter
attached.

> FWIW, I like the way the conditional is written and am only thinking out
> loud about the possible side effects.

Yup, very appreciated!

> Side note: I initially expected to see
> Documentation/userspace-api/seccomp_filter.rst being updated in this
> patch and then remembered that seccomp(2) isn't documented in that file
> and, therefore, it isn't trivial to add blurbs about filter flags in
> there. We should fix that after the dust settles on these two patch sets.

Hm, excellent point. Agreed, let's fix that as a separate step.

-Kees

--
Kees Cook
Pixel Security

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