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Date: Tue, 15 Mar 2011 18:36:05 -0600
From: RB <>
Subject: Re: tcpdump vagaries

On Tue, Mar 15, 2011 at 17:07, Solar Designer <> wrote:
> One of my concerns is that if the sniffer is to create additional files
> after dropping root, the directory holding those files will need to be
> writable by the non-root pseudo-user.

Whether or not it's writable, where tcpdump falls down in this case is
that it doesn't make that check early enough.  If the first file were
created after dropping privileges (as opposed to after), this wouldn't
be an issue, as the user immediately gets feedback that the file
cannot be created.  This encroaches on bug territory, and I'll have to
see what the progress is on accepting Dmitry's (and any similar)

> Yet those files would then be
> examined by root, which allows for certain attacks (via (sym)links to
> device files, etc.)

I'm not sure what you're suggesting here; beyond the privileges to
capture the packets, no elevated ones would (or should) be required to
analyze the resultant files, as long as the user has proper group

> I'm afraid there's no perfect solution to this,
> although we might try to do "something" - e.g., have the sniffer create
> the subdirectory prior to dropping root, set perms to 1730 with the
> pseudo-user's group, and our kernel is to be hardened again to distrust
> symlinks in +t directories like 2.4.x-ow kernels were.

Perhaps; I personally think digging too far into the potential failure
cases takes away from allowing the administrator to make precise
choices and tends to make software excessively complex.  As long as
privileges are dropped as early as possible, particularly prior to
file creation, it should be up to the user to solve any permissions
problems that arise.

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