On 03/09/2018 12:53 AM, Andy Lutomirski wrote: > On Thu, Mar 8, 2018 at 11:51 PM, Mickaël Salaün <mic@xxxxxxxxxxx> wrote: >> >> On 07/03/2018 02:21, Andy Lutomirski wrote: >>> On Tue, Mar 6, 2018 at 11:06 PM, Mickaël Salaün <mic@xxxxxxxxxxx> wrote: >>>> >>>> On 06/03/2018 23:46, Tycho Andersen wrote: >>>>> On Tue, Mar 06, 2018 at 10:33:17PM +0000, Andy Lutomirski wrote: >>>>>>>> Suppose I'm writing a container manager. I want to run "mount" in the >>>>>>>> container, but I don't want to allow moun() in general and I want to >>>>>>>> emulate certain mount() actions. I can write a filter that catches >>>>>>>> mount using seccomp and calls out to the container manager for help. >>>>>>>> This isn't theoretical -- Tycho wants *exactly* this use case to be >>>>>>>> supported. >>>>>>> >>>>>>> Well, I think this use case should be handled with something like >>>>>>> LD_PRELOAD and a helper library. FYI, I did something like this: >>>>>>> https://github.com/stemjail/stemshim >>>>>> >>>>>> I doubt that will work for containers. Containers that use user >>>>>> namespaces and, for example, setuid programs aren't going to honor >>>>>> LD_PRELOAD. >>>>> >>>>> Or anything that calls syscalls directly, like go programs. >>>> >>>> That's why the vDSO-like approach. Enforcing an access control is not >>>> the issue here, patching a buggy userland (without patching its code) is >>>> the issue isn't it? >>>> >>>> As far as I remember, the main problem is to handle file descriptors >>>> while "emulating" the kernel behavior. This can be done with a "shim" >>>> code mapped in every processes. Chrome used something like this (in a >>>> previous sandbox mechanism) as a kind of emulation (with the current >>>> seccomp-bpf ). I think it should be doable to replace the (userland) >>>> emulation code with an IPC wrapper receiving file descriptors through >>>> UNIX socket. >>>> >>> >>> Can you explain exactly what you mean by "vDSO-like"? >>> >>> When a 64-bit program does a syscall, it just executes the SYSCALL >>> instruction. The vDSO isn't involved at all. 32-bit programs usually >>> go through the vDSO, but not always. >>> >>> It could be possible to force-load a DSO into an entire container and >>> rig up seccomp to intercept all SYSCALLs not originating from the DSO >>> such that they merely redirect control to the DSO, but that seems >>> quite messy. >> >> vDSO is a code mapped for all processes. As you said, these processes >> may use it or not. What I was thinking about is to use the same concept, >> i.e. map a "shim" code into each processes pertaining to a particular >> hierarchy (the same way seccomp filters are inherited across processes). >> With a seccomp filter matching some syscall (e.g. mount, open), it is >> possible to jump back to the shim code thanks to SECCOMP_RET_TRAP. This >> shim code should then be able to emulate/patch what is needed, even >> faking a file opening by receiving a file descriptor through a UNIX >> socket. As did the Chrome sandbox, the seccomp filter may look at the >> calling address to allow the shim code to call syscalls without being >> catched, if needed. However, relying on SIGSYS may not fit with >> arbitrary code. Using a new SECCOMP_RET_EMULATE (?) may be used to jump >> to a specific process address, to emulate the syscall in an easier way >> than only relying on a {c,e}BPF program. >> > > This could indeed be done, but I think that Tycho's approach is much > cleaner and probably faster. > I like it too but how does this handle file descriptors?
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