Re: [PATCH v5 5/6] kprobes: Use text_alloc() and text_free()

[Masami Hiramatsu <mhiramat@kernel.org>]

On Tue, 28 Jul 2020 20:51:08 +0300
Ard Biesheuvel <ardb@kernel.org> wrote:

> On Tue, 28 Jul 2020 at 16:35, Masami Hiramatsu <mhiramat@kernel.org> wrote:
> >
> > On Tue, 28 Jul 2020 13:56:43 +0300
> > Ard Biesheuvel <ardb@kernel.org> wrote:
> >
> > > On Tue, 28 Jul 2020 at 11:17, Masami Hiramatsu <mhiramat@kernel.org> wrote:
> > > > > Masami or Peter should correct me if I am wrong, but it seems to me
> > > > > that the way kprobes uses these pages does not require them to be in
> > > > > relative branching range of the core kernel on any architecture, given
> > > > > that they are populated with individual instruction opcodes that are
> > > > > executed in single step mode, and relative branches are emulated (when
> > > > > needed)
> > > >
> > > > Actually, x86 and arm has the "relative branching range" requirements
> > > > for the jump optimized kprobes. For the other architectures, I think
> > > > we don't need it. Only executable text buffer is needed.
> > > >
> > >
> > > Thanks for the explanation. Today, arm64 uses the definition below.
> > >
> > > void *alloc_insn_page(void)
> > > {
> > >   return __vmalloc_node_range(PAGE_SIZE, 1, VMALLOC_START, VMALLOC_END,
> > >     GFP_KERNEL, PAGE_KERNEL_ROX, VM_FLUSH_RESET_PERMS,
> > >     NUMA_NO_NODE, __builtin_return_address(0));
> > > }
> > >
> > > Do you think we could use that as the generic implementation if we use
> > > MODULES_START/_END as the allocation window?
> >
> > Yes, but for the generic implementation, we don't need to consider the
> > relative branching range since we can override it for x86 and arm.
> > (and that will be almost same as module_alloc() default code)
> 
> Indeed. So having kprobes specific macros that default to
> VMALLOC_START/END but can be overridden would be sufficient.
> 
> > BTW, is PAGE_KERNEL_ROX flag available generically?
> >
> 
> Turns out that it is not :-(

Hmm, in that case, we need to use PAGE_KERNEL_EXEC.

In the result, may it be similar to this? :)

void * __weak module_alloc(unsigned long size)
{
        return __vmalloc_node_range(size, 1, VMALLOC_START, VMALLOC_END,
                        GFP_KERNEL, PAGE_KERNEL_EXEC, VM_FLUSH_RESET_PERMS,
                        NUMA_NO_NODE, __builtin_return_address(0));
}

The major difference between module_alloc() and kprobe's alloc_page_insn()
is the alloc_page_insn() makes the page ROX after allocating the pages *ONLY*
on x86 and arm64.

$ git grep -w alloc_insn_page -- arch
arch/arm64/kernel/probes/kprobes.c:void *alloc_insn_page(void)
arch/x86/kernel/kprobes/core.c:void *alloc_insn_page(void)

However since the module_alloc() owns its arch-dependent implementations
most of major architectures, if we implement independent text_alloc_kprobe(),
we need to make deadcopies of module_alloc() for each architecture.

$ git grep 'module_alloc(unsigned' arch/
arch/arm/kernel/module.c:void *module_alloc(unsigned long size)
arch/arm64/kernel/module.c:void *module_alloc(unsigned long size)
arch/mips/kernel/module.c:void *module_alloc(unsigned long size)
arch/nds32/kernel/module.c:void *module_alloc(unsigned long size)
arch/nios2/kernel/module.c:void *module_alloc(unsigned long size)
arch/parisc/kernel/module.c:void *module_alloc(unsigned long size)
arch/riscv/kernel/module.c:void *module_alloc(unsigned long size)
arch/s390/kernel/module.c:void *module_alloc(unsigned long size)
arch/sparc/kernel/module.c:void *module_alloc(unsigned long size)
arch/unicore32/kernel/module.c:void *module_alloc(unsigned long size)
arch/x86/kernel/module.c:void *module_alloc(unsigned long size)

It seems that some constrains for module_alloc() exists for above
architectures.

Anyway, for kprobe's text_alloc() requirements are
- It must be executable for the arch which uses a single-step out-of-line.
  (and need to be registered to KASAN?)
- It must be ROX if implemented (currently only for x86 and arm64)
- It must be in the range of relative branching only for x86 and arm.

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>