[v3,07/17] scs: add support for stack usage debugging
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Message ID 20191031164637.48901-8-samitolvanen@google.com
State New
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Series
  • [v3,01/17] arm64: mm: avoid x18 in idmap_kpti_install_ng_mappings
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Commit Message

Sami Tolvanen Oct. 31, 2019, 4:46 p.m. UTC
Implements CONFIG_DEBUG_STACK_USAGE for shadow stacks.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
---
 kernel/scs.c | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

Comments

Kees Cook Nov. 1, 2019, 3:55 a.m. UTC | #1
On Thu, Oct 31, 2019 at 09:46:27AM -0700, samitolvanen@google.com wrote:
> Implements CONFIG_DEBUG_STACK_USAGE for shadow stacks.

Did I miss it, or is there no Kconfig section for this? I just realized
I can't find it. I was going to say "this commit log should explain
why/when this option is used", but then figured it might be explained in
the Kconfig ... but I couldn't find it. ;)

-Kees

> 
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> ---
>  kernel/scs.c | 39 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 39 insertions(+)
> 
> diff --git a/kernel/scs.c b/kernel/scs.c
> index 7780fc4e29ac..67c43af627d1 100644
> --- a/kernel/scs.c
> +++ b/kernel/scs.c
> @@ -167,6 +167,44 @@ int scs_prepare(struct task_struct *tsk, int node)
>  	return 0;
>  }
>  
> +#ifdef CONFIG_DEBUG_STACK_USAGE
> +static inline unsigned long scs_used(struct task_struct *tsk)
> +{
> +	unsigned long *p = __scs_base(tsk);
> +	unsigned long *end = scs_magic(tsk);
> +	uintptr_t s = (uintptr_t)p;
> +
> +	while (p < end && *p)
> +		p++;
> +
> +	return (uintptr_t)p - s;
> +}
> +
> +static void scs_check_usage(struct task_struct *tsk)
> +{
> +	static DEFINE_SPINLOCK(lock);
> +	static unsigned long highest;
> +	unsigned long used = scs_used(tsk);
> +
> +	if (used <= highest)
> +		return;
> +
> +	spin_lock(&lock);
> +
> +	if (used > highest) {
> +		pr_info("%s: highest shadow stack usage %lu bytes\n",
> +			__func__, used);
> +		highest = used;
> +	}
> +
> +	spin_unlock(&lock);
> +}
> +#else
> +static inline void scs_check_usage(struct task_struct *tsk)
> +{
> +}
> +#endif
> +
>  bool scs_corrupted(struct task_struct *tsk)
>  {
>  	return *scs_magic(tsk) != SCS_END_MAGIC;
> @@ -181,6 +219,7 @@ void scs_release(struct task_struct *tsk)
>  		return;
>  
>  	WARN_ON(scs_corrupted(tsk));
> +	scs_check_usage(tsk);
>  
>  	scs_account(tsk, -1);
>  	task_set_scs(tsk, NULL);
> -- 
> 2.24.0.rc0.303.g954a862665-goog
>
Sami Tolvanen Nov. 1, 2019, 4:32 p.m. UTC | #2
On Thu, Oct 31, 2019 at 8:55 PM Kees Cook <keescook@chromium.org> wrote:
>
> On Thu, Oct 31, 2019 at 09:46:27AM -0700, samitolvanen@google.com wrote:
> > Implements CONFIG_DEBUG_STACK_USAGE for shadow stacks.
>
> Did I miss it, or is there no Kconfig section for this? I just realized
> I can't find it. I was going to say "this commit log should explain
> why/when this option is used", but then figured it might be explained in
> the Kconfig ... but I couldn't find it. ;)

It's in lib/Kconfig.debug. But yes, I will add a commit message in v4.

Sami
Kees Cook Nov. 1, 2019, 7:02 p.m. UTC | #3
On Fri, Nov 01, 2019 at 09:32:54AM -0700, Sami Tolvanen wrote:
> On Thu, Oct 31, 2019 at 8:55 PM Kees Cook <keescook@chromium.org> wrote:
> >
> > On Thu, Oct 31, 2019 at 09:46:27AM -0700, samitolvanen@google.com wrote:
> > > Implements CONFIG_DEBUG_STACK_USAGE for shadow stacks.
> >
> > Did I miss it, or is there no Kconfig section for this? I just realized
> > I can't find it. I was going to say "this commit log should explain
> > why/when this option is used", but then figured it might be explained in
> > the Kconfig ... but I couldn't find it. ;)
> 
> It's in lib/Kconfig.debug. But yes, I will add a commit message in v4.

Oh duh -- it's an existing option. Cool; I'm all good. :)

Patch
diff mbox series

diff --git a/kernel/scs.c b/kernel/scs.c
index 7780fc4e29ac..67c43af627d1 100644
--- a/kernel/scs.c
+++ b/kernel/scs.c
@@ -167,6 +167,44 @@  int scs_prepare(struct task_struct *tsk, int node)
 	return 0;
 }
 
+#ifdef CONFIG_DEBUG_STACK_USAGE
+static inline unsigned long scs_used(struct task_struct *tsk)
+{
+	unsigned long *p = __scs_base(tsk);
+	unsigned long *end = scs_magic(tsk);
+	uintptr_t s = (uintptr_t)p;
+
+	while (p < end && *p)
+		p++;
+
+	return (uintptr_t)p - s;
+}
+
+static void scs_check_usage(struct task_struct *tsk)
+{
+	static DEFINE_SPINLOCK(lock);
+	static unsigned long highest;
+	unsigned long used = scs_used(tsk);
+
+	if (used <= highest)
+		return;
+
+	spin_lock(&lock);
+
+	if (used > highest) {
+		pr_info("%s: highest shadow stack usage %lu bytes\n",
+			__func__, used);
+		highest = used;
+	}
+
+	spin_unlock(&lock);
+}
+#else
+static inline void scs_check_usage(struct task_struct *tsk)
+{
+}
+#endif
+
 bool scs_corrupted(struct task_struct *tsk)
 {
 	return *scs_magic(tsk) != SCS_END_MAGIC;
@@ -181,6 +219,7 @@  void scs_release(struct task_struct *tsk)
 		return;
 
 	WARN_ON(scs_corrupted(tsk));
+	scs_check_usage(tsk);
 
 	scs_account(tsk, -1);
 	task_set_scs(tsk, NULL);