[PATCH v9 3/8] powerpc/kprobes: Mark newly allocated probes as RO

[Jordan Niethe]

On Tue, Mar 16, 2021 at 5:44 PM Christophe Leroy
<christophe.leroy at csgroup.eu> wrote:
>
>
>
> Le 16/03/2021 à 04:17, Jordan Niethe a écrit :
> > From: Russell Currey <ruscur at russell.cc>
> >
> > With CONFIG_STRICT_KERNEL_RWX=y and CONFIG_KPROBES=y, there will be one
> > W+X page at boot by default.  This can be tested with
> > CONFIG_PPC_PTDUMP=y and CONFIG_PPC_DEBUG_WX=y set, and checking the
> > kernel log during boot.
> >
>
> This text is confusing. I don't understand what is the status before the patch, and what is the
> status after.
Before the patch kprobes is allocating W+X pages. This can be seen in
the kernel log with those debug options on.
After the patch kprobes  no longer allocate W+X pages.
I will reword it to more clear.
>
> "there will be one ...", does it mean after the patch ?
No, before, after there will be none.
>
> > Add an arch specific insn page allocator which returns RO pages if
> > STRICT_KERNEL_RWX is enabled. This page is only written to with
> > patch_instruction() which is able to write RO pages.
>
> "an" or "the" arch specific insn page allocator ?
Hmm, will go with "the arch specific insn page allocator for powerpc".
>
> >
> > Reviewed-by: Daniel Axtens <dja at axtens.net>
> > Signed-off-by: Russell Currey <ruscur at russell.cc>
> > Signed-off-by: Christophe Leroy <christophe.leroy at c-s.fr>
> > [jpn: Reword commit message, switch from vmalloc_exec(), add
> >        free_insn_page()]
> > Signed-off-by: Jordan Niethe <jniethe5 at gmail.com>
> > ---
> > v9: - vmalloc_exec() no longer exists
> >      - Set the page to RW before freeing it
> > ---
> >   arch/powerpc/kernel/kprobes.c | 22 ++++++++++++++++++++++
> >   1 file changed, 22 insertions(+)
> >
> > diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
> > index 01ab2163659e..bb7e4d321988 100644
> > --- a/arch/powerpc/kernel/kprobes.c
> > +++ b/arch/powerpc/kernel/kprobes.c
> > @@ -25,6 +25,8 @@
> >   #include <asm/sections.h>
> >   #include <asm/inst.h>
> >   #include <linux/uaccess.h>
> > +#include <linux/set_memory.h>
> > +#include <linux/vmalloc.h>
> >
> >   DEFINE_PER_CPU(struct kprobe *, current_kprobe) = NULL;
> >   DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);
> > @@ -103,6 +105,26 @@ kprobe_opcode_t *kprobe_lookup_name(const char *name, unsigned int offset)
> >       return addr;
> >   }
> >
> > +void *alloc_insn_page(void)
> > +{
> > +     void *page = vmalloc(PAGE_SIZE);
>
> Can't do that on book3s/32, see https://github.com/linuxppc/linux/commit/6ca05532 and
> https://github.com/linuxppc/linux/commit/7fbc22ce
>
> Should do:
>         return __vmalloc_node_range(size, 1, MODULES_VADDR, MODULES_END, GFP_KERNEL,
>                                     PAGE_KERNEL_ROX, VM_FLUSH_RESET_PERMS, NUMA_NO_NODE,
>                                     __builtin_return_address(0));
>
>
> To keep it simple, you'll probably need to define MODULES_VADDR and MODULES_END as resp
> VMALLOC_START and VMALLOC_END when they are not defined, maybe in asm/pgtable.h
>
> > +
> > +     if (!page)
> > +             return NULL;
> > +
> > +     set_memory_ro((unsigned long)page, 1);
> > +     set_memory_x((unsigned long)page, 1);
> > +
> > +     return page;
> > +}
> > +
> > +void free_insn_page(void *page)
> > +{
> > +     set_memory_nx((unsigned long)page, 1);
> > +     set_memory_rw((unsigned long)page, 1);
> > +     vfree(page);
> > +}
> > +
> >   int arch_prepare_kprobe(struct kprobe *p)
> >   {
> >       int ret = 0;
> >