On Mon, May 09, 2022 at 04:51:35PM +0300, Alexander Popov wrote:

Hello Mark!

On 27.04.2022 20:31, Mark Rutland wrote:

quoted

Currently we over-estimate the region of stack which must be erased.

To determine the region to be erased, we scan downards for a contiguous
block of poison values (or the low bound of the stack). There are a few
minor problems with this today:

* When we find a block of poison values, we include this block within
   the region to erase.

   As this is included within the region to erase, this causes us to
   redundantly overwrite 'STACKLEAK_SEARCH_DEPTH' (128) bytes with
   poison.

Right, this can be improved.

quoted

* As the loop condition checks 'poison_count <= depth', it will run an
   additional iteration after finding the contiguous block of poison,
   decrementing 'erase_low' once more than necessary.

Actually, I think the current code is correct.

I'm intentionally searching one poison value more than
STACKLEAK_SEARCH_DEPTH to avoid the corner case. See the BUILD_BUG_ON
assertion in stackleak_track_stack() that describes it:

/*
 * Having CONFIG_STACKLEAK_TRACK_MIN_SIZE larger than
 * STACKLEAK_SEARCH_DEPTH makes the poison search in
 * stackleak_erase() unreliable. Let's prevent that.
 */
BUILD_BUG_ON(CONFIG_STACKLEAK_TRACK_MIN_SIZE > STACKLEAK_SEARCH_DEPTH);

I had read that, but as written that doesn't imply that it's necessary to scan
one more element than STACKLEAK_SEARCH_DEPTH, nor why. I'm more than happy to
change the logic, but I think we need a very clear explanation as to why we
need to scan the specific number of bytes we scan, and we should account for
that *within* STACKLEAK_SEARCH_DEPTH for clarity.

quoted

   As this is included within the region to erase, this causes us to
   redundantly overwrite an additional unsigned long with poison.

* As we always decrement 'erase_low' after checking an element on the
   stack, we always include the element below this within the region to
   erase.

   As this is included within the region to erase, this causes us to
   redundantly overwrite an additional unsigned long with poison.

   Note that this is not a functional problem. As the loop condition
   checks 'erase_low > task_stack_low', we'll never clobber the
   STACK_END_MAGIC. As we always decrement 'erase_low' after this, we'll
   never fail to erase the element immediately above the STACK_END_MAGIC.

Right, I don't see any bug in the current erasing code.

When I wrote the current code, I carefully checked all the corner cases. For
example, on the first stack erasing, the STACK_END_MAGIC was not
overwritten, but the memory next to it was erased. Same for the beginning of
the stack: I carefully checked that no unpoisoned bytes were left on the
thread stack.

quoted

In total, this can cause us to erase `128 + 2 * sizeof(unsigned long)`
bytes more than necessary, which is unfortunate.

This patch reworks the logic to find the address immediately above the
poisoned region, by finding the lowest non-poisoned address. This is
factored into a stackleak_find_top_of_poison() helper both for clarity
and so that this can be shared with the LKDTM test in subsequent
patches.

You know, I wrote stackleak_erase() in very plain C. I wanted a compiler to
generate assembly that is very close to the original asm version. I worried
that compilers might do weird stuff with the local variables and the stack
pointer.

So I checked stackleak for gcc versions 4.8, 5, 6, 7, 8, 9, and 10 on
x86_64, i386 and arm64. This is my project that helped with this work:
https://github.com/a13xp0p0v/kernel-build-containers

I've used the kernel.org cross toolchains, as published at:

  https://mirrors.edge.kernel.org/pub/tools/crosstool/

Mark, in this patch series you use many local variables and helper functions.
Honestly, this worries me. For example, compilers can (and usually do)
ignore the presence of the 'inline' specifier for the purpose of
optimization.

I've deliberately used `__always_inline` rather than regular `inline` to
prevent code being placed out-of-line. As mentioned in oether replies it has a
stronger semantic.

Thanks,
Mark.

Thanks!

quoted

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Alexander Popov <redacted>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Kees Cook <redacted>
---
  include/linux/stackleak.h | 26 ++++++++++++++++++++++++++
  kernel/stackleak.c        | 18 ++++--------------
  2 files changed, 30 insertions(+), 14 deletions(-)

diff --git a/include/linux/stackleak.h b/include/linux/stackleak.h
index 467661aeb4136..c36e7a3b45e7e 100644
--- a/include/linux/stackleak.h
+++ b/include/linux/stackleak.h

@@ -42,6 +42,32 @@ stackleak_task_high_bound(const struct task_struct *tsk)
  	return (unsigned long)task_pt_regs(tsk);
  }
+/*
+ * Find the address immediately above the poisoned region of the stack, where
+ * that region falls between 'low' (inclusive) and 'high' (exclusive).
+ */
+static __always_inline unsigned long
+stackleak_find_top_of_poison(const unsigned long low, const unsigned long high)
+{
+	const unsigned int depth = STACKLEAK_SEARCH_DEPTH / sizeof(unsigned long);
+	unsigned int poison_count = 0;
+	unsigned long poison_high = high;
+	unsigned long sp = high;
+
+	while (sp > low && poison_count < depth) {
+		sp -= sizeof(unsigned long);
+
+		if (*(unsigned long *)sp == STACKLEAK_POISON) {
+			poison_count++;
+		} else {
+			poison_count = 0;
+			poison_high = sp;
+		}
+	}
+
+	return poison_high;
+}
+
  static inline void stackleak_task_init(struct task_struct *t)
  {
  	t->lowest_stack = stackleak_task_low_bound(t);

diff --git a/kernel/stackleak.c b/kernel/stackleak.c
index ba346d46218f5..afd54b8e10b83 100644
--- a/kernel/stackleak.c
+++ b/kernel/stackleak.c

@@ -74,20 +74,10 @@ static __always_inline void __stackleak_erase(void)
  {
  	const unsigned long task_stack_low = stackleak_task_low_bound(current);
  	const unsigned long task_stack_high = stackleak_task_high_bound(current);
-	unsigned long erase_low = current->lowest_stack;
-	unsigned long erase_high;
-	unsigned int poison_count = 0;
-	const unsigned int depth = STACKLEAK_SEARCH_DEPTH / sizeof(unsigned long);
-
-	/* Search for the poison value in the kernel stack */
-	while (erase_low > task_stack_low && poison_count <= depth) {
-		if (*(unsigned long *)erase_low == STACKLEAK_POISON)
-			poison_count++;
-		else
-			poison_count = 0;
-
-		erase_low -= sizeof(unsigned long);
-	}
+	unsigned long erase_low, erase_high;
+
+	erase_low = stackleak_find_top_of_poison(task_stack_low,
+						 current->lowest_stack);
  #ifdef CONFIG_STACKLEAK_METRICS
  	current->prev_lowest_stack = erase_low;

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel