Re: [PATCH v2 09/20] x86/kprobes: instruction pages initialization enhancements

Borislav Petkov
2019-02-11T18:22:30Z

Only nitpicks:

> Subject: Re: [PATCH v2 09/20] x86/kprobes: instruction pages initialization enhancements

Subject needs a verb.

On Mon, Jan 28, 2019 at 04:34:11PM -0800, Rick Edgecombe wrote:
> From: Nadav Amit <namit@vmware.com>
>
> Make kprobes instruction pages read-only (and executable) after they are
> set to prevent them from mistaken or malicious modifications.
>
> This is a preparatory patch for a following patch that makes module
> allocated pages non-executable and sets the page as executable after
> allocation.
>
> While at it, do some small cleanup of what appears to be unnecessary
> masking.
>
> Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
> Signed-off-by: Nadav Amit <namit@vmware.com>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> ---
> arch/x86/kernel/kprobes/core.c | 24 ++++++++++++++++++++----
> 1 file changed, 20 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> index 4ba75afba527..fac692e36833 100644
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -431,8 +431,20 @@ void *alloc_insn_page(void)
> void *page;
>
> page = module_alloc(PAGE_SIZE);
> - if (page)
> - set_memory_ro((unsigned long)page & PAGE_MASK, 1);
> + if (page == NULL)
> + return NULL;

Null tests we generally do like this:

if (! ...

like in the rest of this file.

> +
> + /*
> + * First make the page read-only, and then only then make it executable

s/then only then/only then/

ditto below.

> + * to prevent it from being W+X in between.
> + */
> + set_memory_ro((unsigned long)page, 1);
> +
> + /*
> + * TODO: Once additional kernel code protection mechanisms are set, ensure
> + * that the page was not maliciously altered and it is still zeroed.
> + */
> + set_memory_x((unsigned long)page, 1);
>
> return page;
> }
> @@ -440,8 +452,12 @@ void *alloc_insn_page(void)
> /* Recover page to RW mode before releasing it */
> void free_insn_page(void *page)
> {
> - set_memory_nx((unsigned long)page & PAGE_MASK, 1);
> - set_memory_rw((unsigned long)page & PAGE_MASK, 1);
> + /*
> + * First make the page non-executable, and then only then make it
> + * writable to prevent it from being W+X in between.
> + */
> + set_memory_nx((unsigned long)page, 1);
> + set_memory_rw((unsigned long)page, 1);
> module_memfree(page);
> }
>
> --

--
Regards/Gruss,
Boris.

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