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Comment by j_coder | Hacker News Reader

Isn't possible for the kernel to patch all clflush instructions when the software is loaded to keep a circular list of all evicted addresses that would be evicted again on the interrupt that happens when the protected address is read? This way the the timing attack would not be possible.

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self modifying code (which exists) would take a massive performance hit. any time a page is marked +X, the kernel would have to mark it -W, and then on page fault the kernel would have to check if userspace was changing something to a clflush instruction.

oh, and x86 has variable length instructions - the same byte stream can decode as different instructions depending on where you start - so i doubt it's possible at all on x86 without a massive performance hit (you'd have to keep track of every jump instruction in the entire address space...)

You are right.

The best approach is to evict all user space pages from cache when an invalid page access happens if the page fault was caused by the software trying to read/write kernel space pages.

Massive performance hit but only to misbehaved software. Normal software will not have the performance hit of the current solution.

Kernel could even switch to unmapped kernel pages solution if too many read/write attempts.

clflush only makes the attack easier. There are other ways to flush the cache. Besides: code is mutable. You can just make a clflush instruction out of thin air without the loader's involvement.

For software that requires self-modifying code to run the existing Linux kernel patch would apply (performance penalty). If there is other ways to flush the cache it is necessary to evict the entire software memory on the interrupt.

So in all cases just evict the entire process memory from the cache when the interrupt is raised when reading from a protected memory. The performance penalty would apply only to misbehaved code.

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