On 11/30/19 1:52 AM, Daniel Borkmann wrote: > On 11/30/19 2:37 AM, Eric Dumazet wrote: >> On 11/29/19 2:29 PM, Daniel Borkmann wrote: >>> For the case where the interpreter is compiled out or when the prog is jited >>> it is completely unnecessary to set the BPF insn pages as read-only. In fact, >>> on frequent churn of BPF programs, it could lead to performance degradation of >>> the system over time since it would break the direct map down to 4k pages when >>> calling set_memory_ro() for the insn buffer on x86-64 / arm64 and there is no >>> reverse operation. Thus, avoid breaking up large pages for data maps, and only >>> limit this to the module range used by the JIT where it is necessary to set >>> the image read-only and executable. >> >> Interesting... But why the non JIT case would need RO protection ? > > It was done for interpreter around 5 years ago mainly due to concerns from security > folks that the BPF insn image could get corrupted (through some other bug in the > kernel) in post-verifier stage by an attacker and then there's nothing really that > would provide any sort of protection guarantees; pretty much the same reasons why > e.g. modules are set to read-only in the kernel. > >> Do you have any performance measures to share ? > > No numbers, and I'm also not aware of any reports from users, but it was recently > brought to our attention from mm folks during discussion of a different set: > > https://lore.kernel.org/lkml/1572171452-7958-2-git-send-email-rppt@xxxxxxxxxx/T/ > Thanks for the link ! Having RO protection as a debug feature would be useful. I believe we have CONFIG_STRICT_MODULE_RWX (and CONFIG_STRICT_KERNEL_RWX) for that already. Or are we saying we also want to get rid of them ?
