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From: "Amanieu d'Antras" <amanieu@gmail.com>
To: Steven Price <steven.price@arm.com>
Cc: Arnd Bergmann <arnd@kernel.org>,
	Ryan Houdek <Houdek.Ryan@fex-emu.org>,
	 Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	 Mark Rutland <mark.rutland@arm.com>,
	David Laight <David.Laight@aculab.com>,
	 Mark Brown <broonie@kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	 Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [RESEND PATCH v4 8/8] arm64: Allow 64-bit tasks to invoke compat syscalls
Date: Fri, 21 May 2021 20:18:25 +0100	[thread overview]
Message-ID: <CA+y5pbSbky2kS+O5j9bn57nROdYaYeHcd2R-46X1cc388PKOvg@mail.gmail.com> (raw)
In-Reply-To: <1c2bd27a-1c96-f154-ed18-f33630b109b1@arm.com>

On Fri, May 21, 2021 at 9:51 AM Steven Price <steven.price@arm.com> wrote:
> >> 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).

Spawning another process is just an example. Fundamentally, Tango is
not intended or designed to be a sandbox around the 32-bit code. For
example, many of the newer ioctls use u64 instead of a pointer type to
avoid the need for a compat_ioctl handler. This means that such ioctls
could be abused to read/write any address in the process address
space, including the code that is performing the usermode seccomp
emulation.

> 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).

The specific example I had in mind here is Android which installs a
global seccomp filter on the zygote process from which app processes
are forked from. This filter is designed for mixed arm32/arm64 systems
and therefore has syscall whitelists for both AArch32 and AArch64.
This filter allows 32-bit processes to spawn 64-bit processes and
vice-versa: for example, many 32-bit apps will invoke another 32-bit
executable via system() which uses a 64-bit /system/bin/sh.

> >> 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.

I feel that exposing all compat syscalls to 64-bit processes is better
than the alternative of only exposing a subset of them. Of the top of
my head I can think of quite a few compat syscalls that cannot be
fully emulated in userspace and would need to be exposed in the
kernel:
- mmap/mremap/shmat/io_setup: anything that allocates VM space needs
to return a pointer in the low 4GB.
- ioctl: too many variants to reasonably maintain a separate compat
layer in userspace.
- getdents/lseek: ext4 uses 32-bit directory offsets for 32-bit processes.
- get_robust_list/set_robust_list: different in-memory ABI for
32/64-bit processes.
- open: don't force O_LARGEFILE for 32-bit processes.
- io_uring_create: different in-memory ABI for 32/64-bit processes.
- (and possibly many others)

Also consider the churn involved when adding a new syscall which
behaves differently in compat processes: rather than just using
in_compat_syscall() or wiring up a COMPAT_SYSCALL_DEFINE, a compat
variant of this syscall would also need to be added to the 64-bit
syscall table to support translation layers like Tango and FEX.

> 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.

This might work for simple filters that only look at the syscall
number, but becomes much harder when the filter also inspects the
syscall arguments.

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  reply	other threads:[~2021-05-21 19:21 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-18  9:06 [RESEND PATCH v4 0/8] arm64: Allow 64-bit tasks to invoke compat syscalls 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
2021-05-21 19:18             ` Amanieu d'Antras [this message]
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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