New transient execution attack on Boom.

[JaewonHur]

Type of issue: bug report

Impact: rtl refactoring

Development Phase: proposal

Hi,
I found a new transient execution attack on risc-v boom.
The attack relies on the bug #558, which is a performance bug originally.
But the same bug can also be used to transiently poison the BIM table using a transiently accessed secret.

The attached PoC attack is a Meltdown type of attack where a supervisor-mode software transiently leaks
a secret from the machine-mode software (i.e., either a firmware or an enclave).
The attack is based on two vulnerabilities: 1) boom transiently executes load instruction before checking
PMP violation, and 2) BIM table can be transiently updated using the accessed value.
The attack is quite slow than using D-cache as a side channel, but it still works and almost correctly retrieves
the secret value (i.e., 0xdeadbeef).

• Used boom commit: d77c2c3
• How to reproduce the attack:

       /* in the given directory */
       make clean; make
       <path to simulator-chipyard-SmallBoomConfig> ./exploit.riscv

This can be mitigated by fixing either one of two bugs above.

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