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SubjectRe: For review: seccomp_user_notif(2) manual page
On Wed, Sep 30, 2020 at 01:07:38PM +0200, Michael Kerrisk (man-pages) wrote:
> [...] I did :-)

Yay! Thank you!

> [...]
> Overview
> In conventional usage of a seccomp filter, the decision about how
> to treat a particular system call is made by the filter itself.
> The user-space notification mechanism allows the handling of the
> system call to instead be handed off to a user-space process.
> The advantages of doing this are that, by contrast with the sec‐
> comp filter, which is running on a virtual machine inside the
> kernel, the user-space process has access to information that is
> unavailable to the seccomp filter and it can perform actions that
> can't be performed from the seccomp filter.

I might clarify a bit with something like (though maybe the
target/supervisor paragraph needs to be moved to the start):

This is used for performing syscalls on behalf of the target,
rather than having the supervisor make security policy decisions
about the syscall, which would be inherently race-prone. The
target's syscall should either be handled by the supervisor or
allowed to continue normally in the kernel (where standard security
policies will be applied).

I'll comment more later, but I've run out of time today and I didn't see
anyone mention this detail yet in the existing threads... :)

--
Kees Cook

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