linux-kernel - Re: [PATCH 3/8] seccomp: Introduce SECCOMP_PIN

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Message-ID: <CAG48ez0-jSSaw85=ku35UM3vMe98Vz97B68LsUoNd8ftwpunkQ@mail.gmail.com>
Date:   Wed, 17 Jun 2020 17:25:07 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     kernel list <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>,
        Christian Brauner <christian@...uner.io>,
        Sargun Dhillon <sargun@...gun.me>,
        Tycho Andersen <tycho@...ho.ws>,
        "zhujianwei (C)" <zhujianwei7@...wei.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Shuah Khan <shuah@...nel.org>,
        Matt Denton <mpdenton@...gle.com>,
        Chris Palmer <palmer@...gle.com>,
        Jeffrey Vander Stoep <jeffv@...gle.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Hehuazhen <hehuazhen@...wei.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH 3/8] seccomp: Introduce SECCOMP_PIN_ARCHITECTURE

On Tue, Jun 16, 2020 at 9:49 AM Kees Cook <keescook@...omium.org> wrote:
> For systems that provide multiple syscall maps based on architectures
> (e.g. AUDIT_ARCH_X86_64 and AUDIT_ARCH_I386 via CONFIG_COMPAT), allow
> a fast way to pin the process to a specific syscall mapping, instead of
> needing to generate all filters with an architecture check as the first
> filter action.

This seems reasonable; but can we maybe also add X86-specific handling
for that X32 mess? AFAIK there are four ways to do syscalls with
AUDIT_ARCH_X86_64:

1. normal x86-64 syscall, X32 bit unset (native case)
2. normal x86-64 syscall, X32 bit set (for X32 code calling syscalls
with no special X32 version)
3. x32-specific syscall, X32 bit unset (never happens legitimately)
4. x32-specific syscall, X32 bit set (for X32 code calling syscalls
with special X32 version)

(I got this wrong when I wrote the notes on x32 in the seccomp manpage...)

Can we add a flag for AUDIT_ARCH_X86_64 that says either "I want
native x64-64" (enforcing case 1) or "I want X32" (enforcing case 2 or
4, and in case 2 checking that the syscall has no X32 equivalent)? (Of
course, if the kernel is built without X32 support, we can leave out
these extra checks.)

> +static long seccomp_pin_architecture(void)
> +{
> +#ifdef CONFIG_COMPAT
> +       u32 arch = syscall_get_arch(current);
> +
> +       /* How did you even get here? */
> +       if (current->seccomp.arch && current->seccomp.arch != arch)
> +               return -EBUSY;
> +
> +       current->seccomp.arch = arch;
> +#endif
> +       return 0;
> +}

Are you intentionally writing this such that SECCOMP_PIN_ARCHITECTURE
only has an effect once you've installed a filter, and propagation to
other threads happens when a filter is installed with TSYNC? I guess
that is a possible way to design the API, but it seems like something
that should at least be pointed out explicitly.