From 77cf2b6dee6680536a3109d894f1b1ccda3fc5bf Mon Sep 17 00:00:00 2001 From: Mark Rutland Date: Wed, 27 Apr 2022 18:31:22 +0100 Subject: [PATCH] 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 Cc: Alexander Popov Cc: Andrew Morton Cc: Andy Lutomirski Cc: Kees Cook Signed-off-by: Kees Cook Link: https://lore.kernel.org/r/20220427173128.2603085-8-mark.rutland@arm.com --- include/linux/stackleak.h | 26 ++++++++++++++++++++++++++ kernel/stackleak.c | 16 +++------------- 2 files changed, 29 insertions(+), 13 deletions(-) diff --git a/include/linux/stackleak.h b/include/linux/stackleak.h index 467661aeb413..c36e7a3b45e7 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 ba346d46218f..afd54b8e10b8 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); + unsigned long erase_low, erase_high; - /* 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); - } + erase_low = stackleak_find_top_of_poison(task_stack_low, + current->lowest_stack); #ifdef CONFIG_STACKLEAK_METRICS current->prev_lowest_stack = erase_low;