linux-kernel - System call interposition/unprotecting the table

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Date:	Mon, 13 Aug 2007 18:05:35 -0400
From:	hinoue@...l.carleton.ca
To:	linux-kernel@...r.kernel.org
Subject: System call interposition/unprotecting the table

I have a question about changing the page attributes of the
system call table.

I am writing a kernel module that does some system call interposition.
This works fine on my debian system, but apparently the stock Fedora
kernel (2.6.22) has the system call table write protected.  So I would like
the module to add write permissions to the system call table before
it modifies it.

This is the code in my init_module that is problematic:

// Storing the original call
orig_kill = sys_call_table[__NR_kill];

// Change to write
pg = virt_to_page(sys_call_table);
change_page_attr(pg, 1, PAGE_KERNEL);
global_flush_tlb();

// Test write, should change nothing, but oopses instead
sys_call_table[__NR_kill] = (void*)orig_kill;

I imagine that I'm doing something obviously wrong; I've only been looking
at kernel code for a couple weeks.  Can someone please explain what my
error is?

-Hajime Inoue
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