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Network Security Tools by Justin Clarke, Nitesh Dhanjani

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Forcing Access to sys_call_table

Because sys_call_table is no longer exported in the 2.6 kernels, we can access it only by brute force. LKMs have access to kernel memory, so it is possible to gain access to sys_call_table by comparing known locations with exported system calls. Although sys_call_table itself is not exported, a few system calls such as sys_read() and sys_write( ) are still exported and available to LKMs. To demonstrate how to get access to sys_call_table in the 2.6 kernels, we will write a simple LKM that intercepts sys_open( ) and prevents anyone from opening the /tmp/test file.

Tip

Although we intercept sys_open( ) in this section to prevent someone from opening a file, it is not completely foolproof. This is because the root user still has access to the raw disk device, which determined users can manipulate directly.

We’ll walk through the critical bits here, but you’ll find the full source code for intercept_open.c in the next section. Notice that the my_init( ) function is called during initialization. This function attempts to gain access to sys_call_table by starting at the address of system_utsname. The system_utsname structure contains a list of system information and is known to exist before the system call table. Therefore, the function starts at the location of system_utsname and iterates 1,024 (MAX_TRY) times. It advances a byte every time and compares the current location with that of sys_read(), whose address is assumed to be available to the LKM. ...

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