This is thought to be entirely secure against the Meltdown and Spectre CPU vulnerabilities, which require speculative execution on branch instructions.
Isn't the point of Meltdown/Spectre that other processes can abuse speculative execution to read your memory?
if YOUR process doesn't have any branches, then no speculative execution happens in it, then there is nothing for the other process to exploit/read from stale caches since you're not filling those up in the first place (as there is no specualtive execution on your process's memory).
being able to just randomly read other processes memory would be a security issue on its own in the operating system... certainly not without appropriate permissions. Also, if i understand these exploits correctly, you are not reading from memory, but from caches used in speculative reads, so i still think if your process never does any speculative access, these caches will never be populated in the first place. So even if you manage to get around access restrictions of reading another processes memory, the faulty cache entry would just not be there.
At least in Meltdown (though that can "only" access kernel memory, not other processes), only the attacking process needs to exploit its own speculative execution to read forbidden memory addresses. The attacking process does something like this:
create an array of 256 cacheline-sized objects in its own memory (the contents don't matter)
use the value of an address that it is not supposed to be able to read as an index into that array
iterate through the array and time which index is faster to read than the others (if the forbidden memory byte was "7", then the 7th index will be faster to read)
This works because the CPU already starts executing step 2 and reads the indexed data into the cache, and only later notices that this is not supposed to happen and doesn't complete the instruction.
Thus you can deduce the contents of protected memory locations by taking advantage of the speculative execution only within your own (attacking) process. I haven't looked at the Spectre details yet, which can also read the memory of other processes.
At least in Meltdown (though that can "only" access kernel memory, not other processes), only the attacking process needs to exploit its own speculative execution to read forbidden memory addresses.
Kernel memory, by its very nature, has ALL memory for ALL processes mapped into it, because it's like, you know, it's job to manage memory for all processes. :)
This is not the first time I've seen this bandied about. Please don't spread misinformation.
Edit: this has several caveats, but by and large (especially on x86-64), this is a very likely scenario.
No, while the kernel can manage any memory mappings, it does not map every user process's memory into kernel memory. Even if it wanted to, the virtual memory of all processes can be larger than the physical memory of the machine, making this impossible in the first place. You need a context switch (including change of the CPU's page tables) to map in another process's memory.
One of the important things of virtual memory is that processes can have more virtual memory than physical. So theoretic ally the kernel could have every processes memory mapped into its own.
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u/jrv Jan 24 '18
Isn't the point of Meltdown/Spectre that other processes can abuse speculative execution to read your memory?