[PATCH v7 09/14] x86/kprobes: Instruction pages initialization enhancements
Masami Hiramatsu
mhiramat at kernel.org
Thu Dec 6 13:09:23 UTC 2018
On Tue, 4 Dec 2018 17:34:03 -0800
Nadav Amit <namit at vmware.com> wrote:
> This patch is a preparatory patch for a following patch that makes
> module allocated pages non-executable. The patch sets the page as
> executable after allocation.
>
> In the future, we may get better protection of executables. For example,
> by using hypercalls to request the hypervisor to protect VM executable
> pages from modifications using nested page-tables. This would allow
> us to ensure the executable has not changed between allocation and
> its write-protection.
Sounds interesting!
>
> While at it, do some small cleanup of what appears to be unnecessary
> masking.
Looks good to me.
Acked-by: Masami Hiramatsu <mhiramat at kernel.org>
Thanks!
>
> Cc: Masami Hiramatsu <mhiramat at kernel.org>
> Signed-off-by: Nadav Amit <namit at vmware.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 c33b06f5faa4..ca0118d3b3e8 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;
> +
> + /*
> + * First make the page read-only, and then only then make it executable
> + * 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);
> }
>
> --
> 2.17.1
>
--
Masami Hiramatsu <mhiramat at kernel.org>
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