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Adapted from a patch by Paul Holzinger: when pasta spawns a command,
operating without a pre-existing user and network namespace, it needs
to wait for the tap device to be configured and its handler ready,
before the command is actually executed.
Otherwise, something like:
pasta --config-net nslookup passt.top
usually fails as the nslookup command is issued before the network
interface is ready.
We can't adopt a simpler approach based on SIGSTOP and SIGCONT here:
the child runs in a separate PID namespace, so it can't send SIGSTOP
to itself as the kernel sees the child as init process and blocks
the delivery of the signal.
We could send SIGSTOP from the parent, but this wouldn't avoid the
possible condition where the child isn't ready to wait for it when
the parent sends it, also raised by Paul -- and SIGSTOP can't be
blocked, so it can never be pending.
Use SIGUSR1 instead: mask it before clone(), so that the child starts
with it blocked, and can safely wait for it. Once the parent is
ready, it sends SIGUSR1 to the child. If SIGUSR1 is sent before the
child is waiting for it, the kernel will queue it for us, because
it's blocked.
Reported-by: Paul Holzinger <pholzing@redhat.com>
Fixes: 1392bc5ca002 ("Allow pasta to take a command to execute")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
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When in passt mode, or pasta mode spawning a command, we create a userns
for ourselves. This is used both to isolate the pasta/passt process itself
and to run the spawned command, if any.
Since eed17a47 "Handle userns isolation and dropping root at the same time"
we've handled both cases the same, configuring the UID and GID mappings in
the new userns to map whichever UID we're running as to root within the
userns.
This mapping is desirable when spawning a shell or other command, so that
the user gets a root shell with reasonably clear abilities within the
userns and netns. It's not necessarily essential, though. When not
spawning a shell, it doesn't really have any purpose: passt itself doesn't
need to be root and can operate fine with an unmapped user (using some of
the capabilities we get when entering the userns instead).
Configuring the uid_map can cause problems if passt is running with any
capabilities in the initial namespace, such as CAP_NET_BIND_SERVICE to
allow it to forward low ports. In this case the kernel makes files in
/proc/pid owned by root rather than the starting user to prevent the user
from interfering with the operation of the capability-enhanced process.
This includes uid_map meaning we are not able to write to it.
Whether this behaviour is correct in the kernel is debatable, but in any
case we might as well avoid problems by only initializing the user mappings
when we really want them.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
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passt/pasta can interact with user namespaces in a number of ways:
1) With --netns-only we'll remain in our original user namespace
2) With --userns or a PID option to pasta we'll join either the given
user namespace or that of the PID
3) When pasta spawns a shell or command we'll start a new user namespace
for the command and then join it
4) With passt we'll create a new user namespace when we sandbox()
ourself
However (3) and (4) turn out to have essentially the same effect. In both
cases we create one new user namespace. The spawned command starts there,
and passt/pasta itself will live there from sandbox() onwards.
Because of this, we can simplify user namespace handling by moving the
userns handling earlier, to the same point we drop root in the original
namespace. Extend the drop_user() function to isolate_user() which does
both.
After switching UID and GID in the original userns, isolate_user() will
either join or create the userns we require. When we spawn a command with
pasta_start_ns()/pasta_setup_ns() we no longer need to create a userns,
because we're already made one. sandbox() likewise no longer needs to
create (or join) an userns because we're already in the one we need.
We no longer need c->pasta_userns_fd, since the fd is only used locally
in isolate_user(). Likewise we can replace c->netns_only with a local
in conf(), since it's not used outside there.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
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conf_ns_open() opens file descriptors for the namespaces pasta needs, but
it doesnt really have anything to do with configuration any more. For
better clarity, move it to pasta.c and rename it pasta_open_ns(). This
makes the symmetry between it and pasta_start_ns() more clear, since these
represent the two basic ways that pasta can operate, either attaching to
an existing namespace/process or spawning a new one.
Since its no longer validating options, the errors it could return
shouldn't cause a usage message. Just exit directly with an error instead.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
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When not given an existing PID or network namspace to attach to, pasta
spawns a shell. Most commands which can spawn a shell in an altered
environment can also run other commands in that same environment, which can
be useful in automation.
Allow pasta to do the same thing; it can be given an arbitrary command to
run in the network and user namespace which pasta creates. If neither a
command nor an existing PID or netns to attach to is given, continue to
spawn a default shell, as before.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
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...at the moment, just for consistency with packet.h, icmp.h,
tcp.h and udp.h.
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
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This should be convenient for users managing filesystem-bound network
namespaces: monitor the base directory of the namespace and exit if
the namespace given as PATH or NAME target is deleted. We can't add
an inotify watch directly on the namespace directory, that won't work
with nsfs.
Add an option to disable this behaviour, --no-netns-quit.
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
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SPDX tags don't replace license files. Some notices were missing and
some tags were not according to the SPDX specification, too.
Now reuse --lint from the REUSE tool (https://reuse.software/) passes.
Reported-by: Martin Hauke <mardnh@gmx.de>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
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...I forgot to add this earlier.
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
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