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    SubjectRe: [PATCH v5 2/3] seccomp_filters: system call filtering using BPF
    On Tue, Jan 31, 2012 at 4:04 AM, Will Drewry <> wrote:
    > On Mon, Jan 30, 2012 at 7:42 PM, Indan Zupancic <> wrote:
    >> On Mon, January 30, 2012 23:26, Will Drewry wrote:
    >>> Do you think something along the lines of 2 kB is sane for a config-less change?
    >> Yes, especially if there is some way to get rid of it anyway,
    >> like disabling SECCOMP or some option under CONFIG_EMBEDDED.
    >> But it seems you need at least a hidden config option which
    >> depends on the stuff you need.
    > Disabling SECCOMP would definitely do it.
    >>> Doing exactly that.  I've been tinkering with the best way to minimize
    >>> the impact to the existing BPF evaluator.  Right now, I'm adding a
    >>> very small number of new instructions to the sk_buff specific code
    >>> path, but I haven't yet benchmarked - just disasssembled.
    >> I would do all the checking in sk_chk_filter(), so you know that when
    >> you do run the filter, you can't hit the sk_buff paths. This doesn't
    >> cause any slow down for the networking path.
    > Ah sorry - I was referring to the intrusion of a load_pointer function
    > pointer.  I want to leave the current networking path as untouched as
    > possible.  For checking, I agree -- a quick change to sk_chk_filter or
    > even just a small helper function that scans for any codes in the
    > ancillary range will do the trick.

    I see now. I can do all the fixup in sk_chk_filter. Clever! Sorry for
    not catching on faster :)

    >>> I agree. I will post the next series with a proposed integration. If
    >>> there is a lot of resistance, then the difference will be going from a
    >>> 2kB changes to a 3kB change.
    >> Let's see how it goes.
    >>>> I think you should go on a quest to make sure (almost) all archs have that file,
    >>>> before this patch can be merged. At least the archs that have ptrace support.
    >>> I'm an idiot.  CONFIG_HAVE_ARCH_TRACEHOOK covers asm/syscall.h
    >>> So I have two choices:
    >>> 1. allow seccomp with filtering on these platforms by fail if an
    >>> argument is accessed
    >>> 2. return ENOSYS when a filter is attempted to be installed on
    >>> platforms with no tracehook support.
    >> I vote for:
    >> 3. Add tracehook support to all archs.
    > I don't see these #3 as mutually exclusive :)  tracehook requires:
    > - task_pt_regs()          in asm/processor.h or asm/ptrace.h
    > - arch_has_single_step()  if there is hardware single-step support
    > - arch_has_block_step()   if there is hardware block-step support
    > - asm/syscall.h           supplying asm-generic/syscall.h interface
    > - linux/regset.h          user_regset interfaces
    > - CORE_DUMP_USE_REGSET    #define'd in linux/elf.h
    > -TIF_SYSCALL_TRACE       calls tracehook_report_syscall_{entry,exit}
    > - TIF_NOTIFY_RESUME       calls tracehook_notify_resume()
    > - signal delivery         calls tracehook_signal_handler()
    >> Maybe not all archs, but at least some more. That way, every time someone
    >> adds something tracehook specific, more archs support it.
    > Well the other arch I want this on specifically for my purposes is
    > arm, but someone recently posted a partial asm/syscall.h for arm, but
    > I didn't see that one go anywhere just yet.  (I know syscall_get_nr
    > can be tricky on arm because it doesn't (didn't) have a way of
    > tracking in-syscall state.)
    > ref:
    >> syscall.h has no TRACEHOOK defines or anything though.
    > Nope - it is just part of what is expected.
    >> Only syscall_rollback() looks tricky. I have no clue what the difference
    >> between syscall_get_error() and syscall_get_return_value() is. But you
    >> only need to add syscall_get_nr() and syscall_[gs]et_arguments(), which
    >> should be possible for all archs.
    > It seems even syscall_get_nr can have some wrinkles :)
    > ref:
    >> How many archs don't support tracehook?
    > 14 out of 26.  However, 5 of those still have asm/syscall.h
    >>> I think #2 is the nicest user contract, but #1 allows working filters
    >>> even on less hospitable arches even if they can't user arguments
    >>> (yet).  I'm coding it up as #2, but it is easy to switch to #1.
    >> If you don't support the arch, don't compile any code at all, and
    >> let prctl(2) return EINVAL. You don't want to return ENOSYS.
    > I was thinking of the inline prctl handler, but EINVAL makes sense.
    >>>> Yeah, I figured that out later on. It's quite nifty, but I find the recursion
    >>>> within kref_put() slightly worrisome. Perhaps the code would be cleaner if this
    >>>> was avoided and done differently, but I can't think of a good alternative. I'll
    >>>> wait for the new version to see if I can find a way.
    >>> Thanks - sure.  Since kref_put is just an atomic_dec_and_test followed
    >>> by a call to the function pointer, I wasn't too concerned.  Without
    >>> changing how the relationships are handled, I'm not sure how to
    >>> approach it differently (and still avoid races). IIRC, this isn't much
    >>> different to how namespaces work, they just use their own atomic
    >>> counters.
    >> Well, the thing is, this recursion is controlled by user space depending
    >> on how many filters they have installed. What is preventing them to force
    >> you out of stack?
    > Hrm true.  The easiest option is to just convert it to iterative by
    > not using kref_t, but I'll look more closely.
    >> So perhaps add at least some arbitrary filter limit to avoid this?
    > Definitely possible -- probably as a sysctl.  I'm not quite sure what
    > number makes sense yet, but I'll look at breaking the recursion first.
    >  Thanks!
    >>> I'll clarify a bit.  My original ptrace integration worked such that a
    >>> tracer may only intercept syscall failures if it attached prior to the
    >>> failing filter being installed.  I did it this way to avoid using
    >>> ptrace to escape system call filtering.  However, since I don't have
    >>> that as part of the patch series, it doesn't make sense to keep it. (I
    >>> tracked a tracer pid_struct in the filters.)  If it needs to come back
    >>> with later patchsets, then it can be tackled then!
    >> The problem of that is that filters can be shared between processes with
    >> different ptracers. And you have all the hassle of keeping it up to date.
    >> I think seccomp should always come first and just trust ptrace. This
    >> because it can deny all ptrace() calls for filtered tasks, so the only
    >> untrusted tasks doing ptrace() are outside of seccomp's filtering control.
    >> And those could do the same system calls themselves.
    >> The case where there is one task being filtered and allowed to do ptrace,
    >> but not some other syscall, ptracing another filtered task which isn't
    >> allowed to do ptrace, but allowed to do that other syscall, is quite far
    >> fetched I think. If you really want to handle this, then you could run
    >> the ptracer's filters against the tracee's post-ptrace syscall state.
    >> This is best done in the ptracer's context, just before continuing the
    >> system call. (You really want Oleg's SIKILL immediate patch then.)
    >> What about:
    >> 1) Seccomp filters can deny a syscall by killing the task.
    >> 2) Seccomp can deny a syscall and let it return an error value.
    >>   I know you're not fond of this one. It's just a performance
    >>   optimisation as sometimes a lot of denied but harmless syscalls
    >>   are called by glibc all the time, like getxattr(). Hardcoding
    >>   the kill policy seems wrong when it can be avoided. If this is
    >>   too hard then skip it, but if it's easy to add then please do.
    >>   I'll take a look at this with your next patch version.
    > It's easy on x86 harder on other arches.  I would suggest saving
    > changing the __secure_computing signature until after the core
    > functionality lands, but that's just me.
    >> 3) Seccomp can allow a syscall to proceed normally.
    >> 4) Seccomp can set a hint to skip ptrace syscall events for this syscall.
    >>   A filter sets this by returning a specific value.
    >> 5) Ptrace always gets a syscall event when it asked for it.
    >> 6) Ptrace can set an option to honour seccomp's hint and to not get all
    >>   syscall events.
    >> This way all seccomp needs to do is to set some flags which ptrace can check.
    > I like the use of flags/options to trigger ptrace handling.  If I were
    > to stack rank these for pursuit after the core functionality lands,
    > it'd be to add #6 (and its deps) then #2.  With #6, #2 can be
    > simulated (by having a supervisor that changes the syscall number to
    > -1), but that is much less ideal than just returning SECCOMP_ERROR
    > instead of SECCOMP_ALLOW/DENY and letting an error code get bubbled
    > up.
    > thanks!
    > will
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