> 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.
