[bug#43371] doc: prevent host/container nscd mismatch
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  • [bug#43371] doc: prevent host/container nscd mismatch
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Commit Message

Edouard Klein Sept. 13, 2020, 10:30 a.m. UTC
doc/guix.texi: (Name Service Switch) add a workaround for bug #41575
---
 doc/guix.texi | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

Comments

Ludovic Courtès Sept. 13, 2020, 9:05 p.m. UTC | #1
Hi,

edk@beaver-labs.com skribis:

> doc/guix.texi: (Name Service Switch) add a workaround for bug #41575
> ---
>  doc/guix.texi | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/doc/guix.texi b/doc/guix.texi
> index a6e14ea177..a9472e680e 100644
> --- a/doc/guix.texi
> +++ b/doc/guix.texi
> @@ -1706,6 +1706,20 @@ this binary incompatibility problem because those @code{libnss_*.so}
>  files are loaded in the @command{nscd} process, not in applications
>  themselves.
>  
> +For applications running in containers (@pxref{Invokin guix container}),
> +however, @code{nscd} may leak information from the host to the container.
> +If there is a configuration mismatch between the two ---e.g., the host
> +has no @code{sshd} user while the container needs one--- then it may be

I find the example is hard to understand.  How about: “applications in
the container could end up looking users in the host”?

> +worthwhile to limit which kind of information the host's @code{nscd}
> +daemon may give to the container by adding the following to
> +@code{/etc/nscd.conf}.
> +
> +@example
> +        enable-cache            passwd          no
> +        enable-cache            group           no
> +        enable-cache            netgroup        no
> +@end example

Actually, perhaps the better fix is to never use the host’s nscd?  We
could change ‘containerized-operating-system’ accordingly.

That would allow guest OSes to work correctly regardless of the host’s
nscd config, which seems like an improvement.

Thoughts?

Ludo’.
conjaroy Sept. 14, 2020, 1:05 a.m. UTC | #2
Hello Ludo',

A separate nscd per container also seems like a reasonable option. However,
for the sake of machines hosting many long-lived containers, perhaps we
should consider reducing the cache size: currently it's 32MB for each name
service type, with an expiration of 12-24 hours:

https://git.savannah.gnu.org/cgit/guix.git/tree/gnu/services/base.scm?id=1042d269a723360a02b19a2baafef1e24a3bfc73#n1115

Cheers,

Jason

On Sun, Sep 13, 2020 at 5:05 PM Ludovic Courtès <ludo@gnu.org> wrote:

> Hi,
>
> edk@beaver-labs.com skribis:
>
> > doc/guix.texi: (Name Service Switch) add a workaround for bug #41575
> > ---
> >  doc/guix.texi | 16 +++++++++++++++-
> >  1 file changed, 15 insertions(+), 1 deletion(-)
> >
> > diff --git a/doc/guix.texi b/doc/guix.texi
> > index a6e14ea177..a9472e680e 100644
> > --- a/doc/guix.texi
> > +++ b/doc/guix.texi
> > @@ -1706,6 +1706,20 @@ this binary incompatibility problem because those
> @code{libnss_*.so}
> >  files are loaded in the @command{nscd} process, not in applications
> >  themselves.
> >
> > +For applications running in containers (@pxref{Invokin guix container}),
> > +however, @code{nscd} may leak information from the host to the
> container.
> > +If there is a configuration mismatch between the two ---e.g., the host
> > +has no @code{sshd} user while the container needs one--- then it may be
>
> I find the example is hard to understand.  How about: “applications in
> the container could end up looking users in the host”?
>
> > +worthwhile to limit which kind of information the host's @code{nscd}
> > +daemon may give to the container by adding the following to
> > +@code{/etc/nscd.conf}.
> > +
> > +@example
> > +        enable-cache            passwd          no
> > +        enable-cache            group           no
> > +        enable-cache            netgroup        no
> > +@end example
>
> Actually, perhaps the better fix is to never use the host’s nscd?  We
> could change ‘containerized-operating-system’ accordingly.
>
> That would allow guest OSes to work correctly regardless of the host’s
> nscd config, which seems like an improvement.
>
> Thoughts?
>
> Ludo’.
>
Edouard Klein Sept. 14, 2020, 7:24 a.m. UTC | #3
Hi !

Ludovic Courtès writes:

> Hi,
>
> edk@beaver-labs.com skribis:
>
>> doc/guix.texi: (Name Service Switch) add a workaround for bug #41575
>> ---
>>  doc/guix.texi | 16 +++++++++++++++-
>>  1 file changed, 15 insertions(+), 1 deletion(-)
>>
>> diff --git a/doc/guix.texi b/doc/guix.texi
>> index a6e14ea177..a9472e680e 100644
>> --- a/doc/guix.texi
>> +++ b/doc/guix.texi
>> @@ -1706,6 +1706,20 @@ this binary incompatibility problem because those @code{libnss_*.so}
>>  files are loaded in the @command{nscd} process, not in applications
>>  themselves.
>>  
>> +For applications running in containers (@pxref{Invokin guix container}),
>> +however, @code{nscd} may leak information from the host to the container.
>> +If there is a configuration mismatch between the two ---e.g., the host
>> +has no @code{sshd} user while the container needs one--- then it may be
>
> I find the example is hard to understand.  How about: “applications in
> the container could end up looking users in the host”?
>
>> +worthwhile to limit which kind of information the host's @code{nscd}
>> +daemon may give to the container by adding the following to
>> +@code{/etc/nscd.conf}.
>> +
>> +@example
>> +        enable-cache            passwd          no
>> +        enable-cache            group           no
>> +        enable-cache            netgroup        no
>> +@end example
>
> Actually, perhaps the better fix is to never use the host’s nscd?  We
> could change ‘containerized-operating-system’ accordingly.
>

I think this would be best, but I did not know where to make this
change, so I just edited the doc instead. I don't know if containers
need the host's nscd to avoid the libc issues mentionned in the doc, but
if they dont, then prevening them from accessing the host's nscd seems
logical and would solve the problem. And we wouldn't need to amend the
doc at all.

> That would allow guest OSes to work correctly regardless of the host’s
> nscd config, which seems like an improvement.
>
> Thoughts?
>
> Ludo’.
Ludovic Courtès Sept. 14, 2020, 7:26 a.m. UTC | #4
Hi,

conjaroy <conjaroy@gmail.com> skribis:

> A separate nscd per container also seems like a reasonable option. However,
> for the sake of machines hosting many long-lived containers, perhaps we
> should consider reducing the cache size: currently it's 32MB for each name
> service type, with an expiration of 12-24 hours:
>
> https://git.savannah.gnu.org/cgit/guix.git/tree/gnu/services/base.scm?id=1042d269a723360a02b19a2baafef1e24a3bfc73#n1115

Good point.

In that case, we can have ‘containerized-operating-system’ provide its
own NSS configuration with a reduced cache size (or without cache since
there’s caching happening on the host for host name lookups, for
instance).

WDYT?  Would you like to give it a try?

Thanks,
Ludo’.
conjaroy Sept. 14, 2020, 10:53 p.m. UTC | #5
Sure, I'm happy to take a stab at this.

Jason

On Mon, Sep 14, 2020 at 3:28 AM Ludovic Courtès <ludo@gnu.org> wrote:

> In that case, we can have ‘containerized-operating-system’ provide its
> own NSS configuration with a reduced cache size (or without cache since
> there’s caching happening on the host for host name lookups, for
> instance).
>
> WDYT?  Would you like to give it a try?
>
> Thanks,
> Ludo’.
>
>
>
>

Patch
diff mbox series

diff --git a/doc/guix.texi b/doc/guix.texi
index a6e14ea177..a9472e680e 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -1706,6 +1706,20 @@  this binary incompatibility problem because those @code{libnss_*.so}
 files are loaded in the @command{nscd} process, not in applications
 themselves.
 
+For applications running in containers (@pxref{Invokin guix container}),
+however, @code{nscd} may leak information from the host to the container.
+If there is a configuration mismatch between the two ---e.g., the host
+has no @code{sshd} user while the container needs one--- then it may be
+worthwhile to limit which kind of information the host's @code{nscd}
+daemon may give to the container by adding the following to
+@code{/etc/nscd.conf}.
+
+@example
+        enable-cache            passwd          no
+        enable-cache            group           no
+        enable-cache            netgroup        no
+@end example
+
 @subsection X11 Fonts
 
 @cindex fonts
@@ -27582,7 +27596,7 @@  that should be preferably killed.
 
 @item @code{avoid-regexp} (default: @code{#f})
 A regular expression (as a string) to match the names of the processes
-that should @emph{not} be killed.
+that should @emph{not} be kcoilled.
 
 @item @code{memory-report-interval} (default: @code{0})
 The interval in seconds at which a memory report is printed.  It is