[v7,09/14] x86/kprobes: Instruction pages initialization enhancements
diff mbox series

Message ID 20181205013408.47725-10-namit@vmware.com
State New
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Series
  • x86/alternative: text_poke() enhancements
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Commit Message

Nadav Amit Dec. 5, 2018, 1:34 a.m. UTC
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.

While at it, do some small cleanup of what appears to be unnecessary
masking.

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Nadav Amit <namit@vmware.com>
---
 arch/x86/kernel/kprobes/core.c | 24 ++++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

Comments

Masami Hiramatsu Dec. 6, 2018, 1:09 p.m. UTC | #1
On Tue, 4 Dec 2018 17:34:03 -0800
Nadav Amit <namit@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@kernel.org>


Thanks!

> 
> Cc: Masami Hiramatsu <mhiramat@kernel.org>
> Signed-off-by: Nadav Amit <namit@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
>

Patch
diff mbox series

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);
 }