On Mon, Oct 29, 2018 at 01:11:24PM +0100, Arnd Bergmann wrote: > On 10/29/18, Will Deacon <will.dea...@arm.com> wrote: > > On Mon, Oct 15, 2018 at 01:16:00PM +0200, Anders Roxell wrote: > > >> -static int __kprobes patch_text(kprobe_opcode_t *addr, u32 opcode) > >> +void *alloc_insn_page(void) > >> { > >> - void *addrs[1]; > >> - u32 insns[1]; > >> + void *page; > >> > >> - addrs[0] = (void *)addr; > >> - insns[0] = (u32)opcode; > >> + page = vmalloc_exec(PAGE_SIZE); > >> + if (page) > >> + set_memory_ro((unsigned long)page & PAGE_MASK, 1); > > > > This looks a bit strange to me -- you're allocating PAGE_SIZE bytes so > > that we can adjust the permissions, yet we can't guarantee that page is > > actually page-aligned and therefore end up explicitly masking down. > > > > In which case allocating an entire page isn't actually helping us, and > > we could end up racing with somebody else changing permission on the > > same page afaict. > > > > I think we need to ensure we really have an entire page, perhaps using > > vmap() instead? Or have I missed some subtle detail here? > > I'm fairly sure that vmalloc() and vmalloc_exec() is guaranteed to be page > aligned everywhere. The documentation is a bit vague here, but I'm > still confident enough that we can make that assumption based on > > /** > * vmalloc_exec - allocate virtually contiguous, executable memory > * @size: allocation size > * > * Kernel-internal function to allocate enough pages to cover @size > * the page level allocator and map them into contiguous and > * executable kernel virtual space. > * > * For tight control over page level allocator and protection flags > * use __vmalloc() instead. > */ > void *vmalloc_exec(unsigned long size)
FWIW, I did a bit of digging and I agree with your conclusion. vmalloc() allocations end up getting installed in map_vm_area() via __vmalloc_area_node(), which allocates things a page at a time. So we can simplify this patch to drop the masking when calling set_memory_ro(). Will