From: Steven Price <firstname.lastname@example.org> To: Amanieu d'Antras <email@example.com> Cc: Arnd Bergmann <firstname.lastname@example.org>, Ryan Houdek <Houdek.Ryan@fex-emu.org>, Catalin Marinas <email@example.com>, Will Deacon <firstname.lastname@example.org>, Mark Rutland <email@example.com>, David Laight <David.Laight@aculab.com>, Mark Brown <firstname.lastname@example.org>, Linux ARM <email@example.com>, Linux Kernel Mailing List <firstname.lastname@example.org> Subject: Re: [RESEND PATCH v4 8/8] arm64: Allow 64-bit tasks to invoke compat syscalls Date: Fri, 21 May 2021 09:51:01 +0100 [thread overview] Message-ID: <email@example.com> (raw) In-Reply-To: <CA+y5pbRwgpctUOBzzscT9XMN9LM2qraPNg6K6onFcpQaaFDYkQ@mail.gmail.com> On 19/05/2021 17:14, Amanieu d'Antras wrote: > On Wed, May 19, 2021 at 4:30 PM Steven Price <firstname.lastname@example.org> wrote: >> Perhaps I'm missing something, but surely some syscalls that would be >> native on 32 bit will have to be translated by Tango to 64 bit syscalls >> to do the right thing? E.g. from the previous patch compat sigreturn >> isn't available. > > That's correct. > > Tango handles syscalls in 3 different ways: > - ~20 syscalls are completely emulated in userspace or through 64-bit > syscalls. E.g. sigaction, sigreturn, clone, exit. > - Another ~50 syscalls have various forms of pre/post-processing, but > are otherwise passed on to the kernel compat syscall handler. E.g. > open, mmap, ptrace. > - The remaining syscalls are passed on to the kernel compat syscall > handler directly. > > The first group of ~20 syscalls will effectively bypass the > user-specified seccomp filter: any 64-bit syscalls used to emulate > them will be whitelisted. I consider this an acceptable limitation to > Tango's seccomp support since I see no viable way of supporting > seccomp filtering for these syscalls. I agree it's difficult - the only 'solution' I can see is like I said to emulate the BPF code in user space. >> In those cases to correctly emulate seccomp, isn't Tango is going to >> have to implement the seccomp filter in user space? > > I have not implemented user-mode seccomp emulation because it can > trivially be bypassed by spawning a 64-bit child process which runs > outside Tango. Even when spawning another translated process, the > user-mode filter will not be preserved across an execve. Clearly if you have user-mode seccomp emulation then you'd hook execve and either install the real BPF filter (if spawning a 64 bit child outside Tango) or ensure that the user-mode emulation is passed on to the child (if running within Tango). You already have a potential 'issue' here of a 64 bit process setting up a seccomp filter and then execve()ing a 32 bit (Tango'd) process. The set of syscalls needed for the system which supports AArch32 natively is going to be different from the syscalls needed for Tango. (Fundamentally this is a major limitation with the whole seccomp syscall filtering approach). >> I guess the question comes down to how big a hole is >> syscall_in_tango_whitelist() - if Tango only requires a small set of >> syscalls then there is still some security benefit, but otherwise this >> doesn't seem like a particularly big benefit considering you're already >> going to need the BPF infrastructure in user space. > > Currently Tango only whitelists ~50 syscalls, which is small enough to > provide security benefits and definitely better than not supporting > seccomp at all. Agreed, and I don't want to imply that this approach is necessarily wrong. But given that the approach of getting the kernel to do the compat syscall filtering is not perfect, I'm not sure in itself it's a great justification for needing the kernel to support all the compat syscalls. One other thought: I suspect in practise there aren't actually many variations in the BPF programs used with seccomp. It may well be quite possible to convert the 32-bit syscall filtering programs to filter the equivalent 64-bit syscalls that Tango would use. Sadly this would be fragile if a program used a BPF program which didn't follow the "normal" pattern. Steve _______________________________________________ linux-arm-kernel mailing list email@example.com http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next prev parent reply other threads:[~2021-05-21 8:53 UTC|newest] Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-05-18 9:06 [RESEND PATCH v4 0/8] " Amanieu d'Antras 2021-05-18 9:06 ` [RESEND PATCH v4 1/8] mm: Add arch_get_mmap_base_topdown macro Amanieu d'Antras 2021-05-18 9:06 ` [RESEND PATCH v4 2/8] hugetlbfs: Use arch_get_mmap_* macros Amanieu d'Antras 2021-05-18 9:06 ` [RESEND PATCH v4 3/8] mm: Support mmap_compat_base with the generic layout Amanieu d'Antras 2021-05-18 9:06 ` [RESEND PATCH v4 4/8] arm64: Separate in_compat_syscall from is_compat_task Amanieu d'Antras 2021-05-18 9:06 ` [RESEND PATCH v4 5/8] arm64: mm: Use HAVE_ARCH_COMPAT_MMAP_BASES Amanieu d'Antras 2021-05-18 9:06 ` [RESEND PATCH v4 6/8] arm64: Add a compat syscall flag to thread_info Amanieu d'Antras 2021-05-18 9:06 ` [RESEND PATCH v4 7/8] arm64: Forbid calling compat sigreturn from 64-bit tasks Amanieu d'Antras 2021-05-18 9:06 ` [RESEND PATCH v4 8/8] arm64: Allow 64-bit tasks to invoke compat syscalls Amanieu d'Antras 2021-05-18 13:02 ` Arnd Bergmann 2021-05-18 20:26 ` David Laight 2021-05-18 22:41 ` Ryan Houdek 2021-05-18 23:51 ` Amanieu d'Antras 2021-05-19 15:30 ` Steven Price 2021-05-19 16:14 ` Amanieu d'Antras 2021-05-21 8:51 ` Steven Price [this message] 2021-05-21 19:18 ` Amanieu d'Antras 2021-05-24 11:20 ` Steven Price 2021-05-24 12:38 ` David Laight 2021-05-18 23:52 ` Ryan Houdek
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