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. Also, is there a reason you couldn't use text_poke() to modify the trampoline with a single flush?
