> On Jan 6, 2020, at 12:25 PM, Edgecombe, Rick P <rick.p.edgecombe@xxxxxxxxx> wrote: > > On Sat, 2020-01-04 at 09:49 +0900, Andy Lutomirski wrote: >>>> On Jan 4, 2020, at 8:47 AM, KP Singh <kpsingh@xxxxxxxxxxxx> wrote: >>> >>> From: KP Singh <kpsingh@xxxxxxxxxx> >>> >>> The image for the BPF trampolines is allocated with >>> bpf_jit_alloc_exe_page which marks this allocated page executable. This >>> means that the allocated memory is W and X at the same time making it >>> susceptible to WX based attacks. >>> >>> Since the allocated memory is shared between two trampolines (the >>> current and the next), 2 pages must be allocated to adhere to W^X and >>> the following sequence is obeyed where trampolines are modified: >> >> Can we please do better rather than piling garbage on top of garbage? >> >>> >>> - Mark memory as non executable (set_memory_nx). While module_alloc for >>> x86 allocates the memory as PAGE_KERNEL and not PAGE_KERNEL_EXEC, not >>> all implementations of module_alloc do so >> >> How about fixing this instead? >> >>> - Mark the memory as read/write (set_memory_rw) >> >> Probably harmless, but see above about fixing it. >> >>> - Modify the trampoline >> >> Seems reasonable. It’s worth noting that this whole approach is suboptimal: >> the “module” allocator should really be returning a list of pages to be >> written (not at the final address!) with the actual executable mapping to be >> materialized later, but that’s a bigger project that you’re welcome to ignore >> for now. (Concretely, it should produce a vmap address with backing pages but >> with the vmap alias either entirely unmapped or read-only. A subsequent healer >> would, all at once, make the direct map pages RO or not-present and make the >> vmap alias RX.) >>> - Mark the memory as read-only (set_memory_ro) >>> - Mark the memory as executable (set_memory_x) >> >> No, thanks. There’s very little excuse for doing two IPI flushes when one >> would suffice. >> >> As far as I know, all architectures can do this with a single flush without >> races x86 certainly can. The module freeing code gets this sequence right. >> Please reuse its mechanism or, if needed, export the relevant interfaces. > > So if I understand this right, some trampolines have been added that are > currently set as RWX at modification time AND left that way during runtime? The > discussion on the order of set_memory_() calls in the commit message made me > think that this was just a modification time thing at first. I’m not sure what the status quo is. We really ought to have a genuinely good API for allocation and initialization of text. We can do so much better than set_memory_blahblah. FWIW, I have some ideas about making kernel flushes cheaper. It’s currently blocked on finding some time and on tglx’s irqtrace work. > > Also, is there a reason you couldn't use text_poke() to modify the trampoline > with a single flush? > Does text_poke to an IPI these days?
