Messages in this thread | ![/](/images/icornerl.gif) | | From | "Reshetova, Elena" <> | Subject | RE: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall | Date | Thu, 2 May 2019 08:07:22 +0000 |
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> From: Reshetova, Elena > > Sent: 30 April 2019 18:51 > ... > > I guess this is true, so I did a quick implementation now to estimate the > > performance hits. > > Here are the preliminary numbers (proper ones will take a bit more time): > > > > base: Simple syscall: 0.1761 microseconds > > get_random_bytes (4096 bytes per-cpu buffer): 0.1793 microsecons > > get_random_bytes (64 bytes per-cpu buffer): 0.1866 microsecons > > > > It does not make sense to go less than 64 bytes since this seems to be > > Chacha20 block size, so if we go lower, we will trash useful bits. > > You can go even higher than 4096 bytes, but even this looks like > > okish performance to me. > > > > Below is a snip of what I quickly did (relevant parts) to get these numbers. > > I do initial population of per-cpu buffers in late_initcall, but > > practice shows that rng might not always be in good state by then. > > So, we might not have really good randomness then, but I am not sure > > if this is a practical problem since it only applies to system boot and by > > the time it booted, it already issued enough syscalls that buffer gets refilled > > with really good numbers. > > Alternatively we can also do it on the first syscall that each cpu gets, but I > > am not sure if that is always guaranteed to have a good randomness. > ... > > +unsigned char random_get_byte(void) > > +{ > > + struct rnd_buffer *buffer = &get_cpu_var(stack_rand_offset); > > + unsigned char res; > > + > > + if (buffer->byte_counter >= RANDOM_BUFFER_SIZE) { > > + get_random_bytes(&(buffer->buffer), sizeof(buffer->buffer)); > > + buffer->byte_counter = 0; > > + } > > + > > + res = buffer->buffer[buffer->byte_counter]; > > + buffer->buffer[buffer->byte_counter] = 0; > > + buffer->byte_counter ++; > > + put_cpu_var(stack_rand_offset); > > + return res; > > +} > > +EXPORT_SYMBOL(random_get_byte); > > You'll almost certainly get better code if you copy buffer->byte_counter > to a local 'unsigned long' variable. >
Noted, will fix, thank you for the suggestion!
I haven't yet worked on this code for a "proper version", so I think many things would need polishing for both speed and code size.
Best Regards, Elena.
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