killchain-compendium/Exploits/CPUs/Meltdown.md

Meltdown

• PoC from IAIK

• Each page table entries has the following flag values

  • SUP (user or kernel mode)
  • READ
  • WRITE

• Flags are checked by the MMU

• Pipelined steps of an instruction are the following

  • Fetch
  • Decode
  • Execute
  • Memory access
  • Register write back

• A pipeline can execute one of each steps concurrently every clock cycle.

• The MMU may jump between instruction queued in the pipeline to make use of time while the current instruction stalls.

• Speculative execution is the reordering of instructions to speed up execution inside the pipeline.

  • If a condition is executed speculatively, kernel memory may be executed. No error is thrown, because the is no assurance if the condition will actually be executed in the end.

• Kernel Page Table Isolation can be used to mitigate meltdown.