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From:   Kaiwan N Billimoria <kaiwan.billimoria@gmail.com>
Date:   Thu, 9 Nov 2017 10:13:26 +0530
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Subject: Re: [kernel-hardening] [PATCH v4] scripts: add leaking_addresses.pl
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> But I don't know if there is anything else than the profiling code
> that _really_ wants access to /proc/kallsyms in user space as a
> regular user.

Am unsure about this, but kprobes? (/jprobes/kretprobes), and by
extension, wrappers over this infra (like SystemTap)?
I (hazily) recollect a script I once wrote (years back though) that
collects kernel virtual addresses off of kallsyms for the purpose of
passing them to a 'helper' kernel module that uses kprobes. I realize
that 'modern' kprobes exposes APIs that just require the symbolic name
& that they're anyway at kernel privilege... but the point is, a
usermode script was picking up and passing the kernel addresses.

Also, what about kernel addresses exposed via System.map?
Oh, just checked, it's root rw only.. pl ignore.

> That said, that patch also fixes the /proc/kallsyms root check, in
> that now you can do:
>
>     sudo head < /proc/kallsyms
>
> and it still shows all zeroes - because the file was *opened* as a
> normal user. That's how UNIX file access security works, and how it is
> fundamentally supposed to work (ie passing a file descriptor to a sui
> program doesn't magically make it gain privileges).

Indeed.

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