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stackleak: clarify variable names
The logic within __stackleak_erase() can be a little hard to follow, as `boundary` switches from being the low bound to the high bound mid way through the function, and `kstack_ptr` is used to represent the start of the region to erase while `boundary` represents the end of the region to erase. Make this a little clearer by consistently using clearer variable names. The `boundary` variable is removed, the bounds of the region to erase are described by `erase_low` and `erase_high`, and bounds of the task stack are described by `task_stack_low` and `task_stack_high`. As the same time, remove the comment above the variables, since it is unclear whether it's intended as rationale, a complaint, or a TODO, and is more confusing than helpful. There should be no functional change as a result of this patch. 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/20220427173128.2603085-6-mark.rutland@arm.com
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@ -73,40 +73,38 @@ late_initcall(stackleak_sysctls_init);
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static __always_inline void __stackleak_erase(void)
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{
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const unsigned long task_stack_low = stackleak_task_low_bound(current);
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/* It would be nice not to have 'kstack_ptr' and 'boundary' on stack */
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unsigned long kstack_ptr = current->lowest_stack;
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unsigned long boundary = task_stack_low;
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unsigned long erase_low = current->lowest_stack;
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unsigned long erase_high;
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unsigned int poison_count = 0;
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const unsigned int depth = STACKLEAK_SEARCH_DEPTH / sizeof(unsigned long);
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/* Search for the poison value in the kernel stack */
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while (kstack_ptr > boundary && poison_count <= depth) {
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if (*(unsigned long *)kstack_ptr == STACKLEAK_POISON)
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while (erase_low > task_stack_low && poison_count <= depth) {
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if (*(unsigned long *)erase_low == STACKLEAK_POISON)
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poison_count++;
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else
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poison_count = 0;
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kstack_ptr -= sizeof(unsigned long);
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erase_low -= sizeof(unsigned long);
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}
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#ifdef CONFIG_STACKLEAK_METRICS
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current->prev_lowest_stack = kstack_ptr;
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current->prev_lowest_stack = erase_low;
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#endif
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/*
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* Now write the poison value to the kernel stack. Start from
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* 'kstack_ptr' and move up till the new 'boundary'. We assume that
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* the stack pointer doesn't change when we write poison.
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* Now write the poison value to the kernel stack between 'erase_low'
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* and 'erase_high'. We assume that the stack pointer doesn't change
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* when we write poison.
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*/
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if (on_thread_stack())
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boundary = current_stack_pointer;
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erase_high = current_stack_pointer;
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else
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boundary = current_top_of_stack();
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erase_high = current_top_of_stack();
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while (kstack_ptr < boundary) {
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*(unsigned long *)kstack_ptr = STACKLEAK_POISON;
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kstack_ptr += sizeof(unsigned long);
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while (erase_low < erase_high) {
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*(unsigned long *)erase_low = STACKLEAK_POISON;
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erase_low += sizeof(unsigned long);
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}
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/* Reset the 'lowest_stack' value for the next syscall */
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