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The main idea behind randomising the EL2 VA is that we usually have a few spare bits between the most significant bit of the VA mask and the most significant bit of the linear mapping. Those bits could be a bunch of zeroes, and could be useful to move things around a bit. Of course, the more memory you have, the less randomisation you get... Alternatively, these bits could be the result of KASLR, in which case they are already random. But it would be nice to have a *different* randomization, just to make the job of a potential attacker a bit more difficult. Inserting these random bits is a bit involved. We don't have a spare register (short of rewriting all the kern_hyp_va call sites), and the immediate we want to insert is too random to be used with the ORR instruction. The best option I could come up with is the following sequence: and x0, x0, #va_mask ror x0, x0, #first_random_bit add x0, x0, #(random & 0xfff) add x0, x0, #(random >> 12), lsl #12 ror x0, x0, #(63 - first_random_bit) making it a fairly long sequence, but one that a decent CPU should be able to execute without breaking a sweat. It is of course NOPed out on VHE. The last 4 instructions can also be turned into NOPs if it appears that there is no free bits to use. Acked-by: Catalin Marinas <catalin.marinas@arm.com> Reviewed-by: James Morse <james.morse@arm.com> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com> |
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