BPF_STX Kernel Write Exploit

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bpf has a little virtual machine that executes packet filters. The machine includes a "scratch area" which is stored as an array on the stack. There are two instructions that write to that array:

       case BPF_ST:
           mem[pc->k] = A;                                                
           continue;                                                      
       
       case BPF_STX:
           mem[pc->k] = X;
           continue;          

bpf_validate runs first to check the program, and handles BPF_ST correctly, but forgets to handle BPF_STX:

       /*
        * Check that memory operations use valid addresses.
        */
       if ((BPF_CLASS(p->code) == BPF_ST ||
            (BPF_CLASS(p->code) == BPF_LD &&
             (p->code & 0xe0) == BPF_MEM)) &&
           p->k >= BPF_MEMWORDS)
           return 0;

This allows arbitrary locations on the stack to be modified.

This bug was actually fixed in FreeBSD. [1]

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