linux-stable/include/linux/stackleak.h
Mark Rutland 77cf2b6dee stackleak: rework poison scanning
Currently we over-estimate the region of stack which must be erased.

To determine the region to be erased, we scan downwards 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.

* 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.

  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.

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.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Alexander Popov <alex.popov@linux.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220427173128.2603085-8-mark.rutland@arm.com
2022-05-08 01:33:08 -07:00

83 lines
2.1 KiB
C

/* SPDX-License-Identifier: GPL-2.0 */
#ifndef _LINUX_STACKLEAK_H
#define _LINUX_STACKLEAK_H
#include <linux/sched.h>
#include <linux/sched/task_stack.h>
/*
* Check that the poison value points to the unused hole in the
* virtual memory map for your platform.
*/
#define STACKLEAK_POISON -0xBEEF
#define STACKLEAK_SEARCH_DEPTH 128
#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
#include <asm/stacktrace.h>
/*
* The lowest address on tsk's stack which we can plausibly erase.
*/
static __always_inline unsigned long
stackleak_task_low_bound(const struct task_struct *tsk)
{
/*
* The lowest unsigned long on the task stack contains STACK_END_MAGIC,
* which we must not corrupt.
*/
return (unsigned long)end_of_stack(tsk) + sizeof(unsigned long);
}
/*
* The address immediately after the highest address on tsk's stack which we
* can plausibly erase.
*/
static __always_inline unsigned long
stackleak_task_high_bound(const struct task_struct *tsk)
{
/*
* The task's pt_regs lives at the top of the task stack and will be
* overwritten by exception entry, so there's no need to erase them.
*/
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);
# ifdef CONFIG_STACKLEAK_METRICS
t->prev_lowest_stack = t->lowest_stack;
# endif
}
#else /* !CONFIG_GCC_PLUGIN_STACKLEAK */
static inline void stackleak_task_init(struct task_struct *t) { }
#endif
#endif