Andrew Cooper <andrew.coop...@citrix.com> wrote: > On 16/02/2018 00:25, Nadav Amit wrote: >> Dave Hansen <dave.han...@linux.intel.com> wrote: >> >>> On 02/15/2018 08:35 AM, Nadav Amit wrote: >>>> I removed the PTI disabling while SMEP is unsupported, although I >>>> must admit I did not fully understand why it is required. >>> Do you mean you don't fully understand how PTI gives SMEP-like behavior >>> on non-SMEP hardware? >> No. I understand how it provide SMEP-like behavior, and I understand the >> value >> of SMEP by itself. >> >> However, I do not understand why SMEP-like protection is required to protect >> processes that run in compatibility-mode from Meltdown/Spectre attacks. As >> far as I understand, the process should not be able to manipulate the kernel >> to execute code in the low 4GB. > > Being 32bit is itself sufficient protection against Meltdown (as long as > there nothing interesting of the kernels mapped below the 4G boundary). > > However, a 32bit compatibility process try to attack with Spectre/SP2 to > redirect speculation back into userspace, at which point (if successful) > the pipeline will be speculating in 64bit mode, and Meltdown is back on > the table. SMEP will block this attack vector, irrespective of other > SP2 defences the kernel may employ, but a fully SP2-defended kernel > doesn't require SMEP to be safe in this case.
Based on Jann Horn’s description of the branch predictor, it basically only holds the lowest 31-bits of the target address. There might be a subtle problem if the prediction wrapsaround, but excluding this case, I do not see how Spectre v2 can be used to jump into running user code.
