linux-kernel - Re: [PATCH v6 1/9] kernel: Support TIF_SYSCALL

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Message-ID: <202009221243.6BC5635E@keescook>
Date:   Tue, 22 Sep 2020 12:44:21 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Gabriel Krisman Bertazi <krisman@...labora.com>
Cc:     luto@...nel.org, tglx@...utronix.de, x86@...nel.org,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        willy@...radead.org, linux-kselftest@...r.kernel.org,
        shuah@...nel.org, kernel@...labora.com
Subject: Re: [PATCH v6 1/9] kernel: Support TIF_SYSCALL_INTERCEPT flag

On Fri, Sep 04, 2020 at 04:31:39PM -0400, Gabriel Krisman Bertazi wrote:
> Convert TIF_SECCOMP into a generic TI flag for any syscall interception
> work being done by the kernel.  The actual type of work is exposed by a
> new flag field outside of thread_info.  This ensures that the
> syscall_intercept field is only accessed if struct seccomp has to be
> accessed already, such that it doesn't incur in a much higher cost to
> the seccomp path.
> 
> In order to avoid modifying every architecture at once, this patch has a
> transition mechanism, such that architectures that define TIF_SECCOMP
> continue to work by ignoring the syscall_intercept flag, as long as they
> don't support other syscall interception mechanisms like the future
> syscall user dispatch.  When migrating TIF_SECCOMP to
> TIF_SYSCALL_INTERCEPT, they should adopt the semantics of checking the
> syscall_intercept flag, like it is done in the common entry syscall
> code, or even better, migrate to the common syscall entry code.

Can we "eat" all the other flags like ptrace, audit, etc, too? Doing
this only for seccomp seems strange.

-- 
Kees Cook