stackleak: rework stack low bound handling

In stackleak_task_init(), stackleak_track_stack(), and __stackleak_erase(), we open-code skipping the STACK_END_MAGIC at the bottom of the stack. Each case is implemented slightly differently, and only the __stackleak_erase() case is commented. In stackleak_task_init() and stackleak_track_stack() we unconditionally add sizeof(unsigned long) to the lowest stack address. In stackleak_task_init() we use end_of_stack() for this, and in stackleak_track_stack() we use task_stack_page(). In __stackleak_erase() we handle this by detecting if `kstack_ptr` has hit the stack end boundary, and if so, conditionally moving it above the magic. This patch adds a new stackleak_task_low_bound() helper which is used in all three cases, which unconditionally adds sizeof(unsigned long) to the lowest address on the task stack, with commentary as to why. This uses end_of_stack() as stackleak_task_init() did prior to this patch, as this is consistent with the code in kernel/fork.c which initializes the STACK_END_MAGIC value. In __stackleak_erase() we no longer need to check whether we've spilled into the STACK_END_MAGIC value, as stackleak_track_stack() ensures that `current->lowest_stack` stops immediately above this, and similarly the poison scan will stop immediately above this. For stackleak_task_init() and stackleak_track_stack() this results in no change to code generation. For __stackleak_erase() the generated assembly is slightly simpler and shorter.

Mark Rutland committed 4 years ago

Commit 9ec79841 parent ac7838b2 files · +18 −11Browse files

Showing the changes to kernel (1 of 2 files). Show all

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kernel/stackleak.c+4−10

@@ -72,9 +72,11 @@ late_initcall(stackleak_sysctls_init); · 71 unmodified lines
72	72
73	73	static __always_inline void __stackleak_erase(void)
74	74	{
	75	+ const unsigned long task_stack_low = stackleak_task_low_bound(current);
	76	+
75	77	/* It would be nice not to have 'kstack_ptr' and 'boundary' on stack */
76	78	unsigned long kstack_ptr = current->lowest_stack;
77		− unsigned long boundary = (unsigned long)end_of_stack(current);
	79	+ unsigned long boundary = task_stack_low;
78	80	unsigned int poison_count = 0;
79	81	const unsigned int depth = STACKLEAK_SEARCH_DEPTH / sizeof(unsigned long);
80	82
@@ -88,13 +90,6 @@ static __always_inline void __stackleak_erase(void) · 7 unmodified lines
88	90	kstack_ptr -= sizeof(unsigned long);
89	91	}
90	92
91		− /*
92		− * One 'long int' at the bottom of the thread stack is reserved and
93		− * should not be poisoned (see CONFIG_SCHED_STACK_END_CHECK=y).
94		− */
95		− if (kstack_ptr == boundary)
96		− kstack_ptr += sizeof(unsigned long);
97		−
98	93	#ifdef CONFIG_STACKLEAK_METRICS
99	94	current->prev_lowest_stack = kstack_ptr;
100	95	#endif
@@ -140,8 +135,7 @@ void __used __no_caller_saved_registers noinstr stackleak_track_stack(void) · 39 unmodified lines
140	135	/* 'lowest_stack' should be aligned on the register width boundary */
141	136	sp = ALIGN(sp, sizeof(unsigned long));
142	137	if (sp < current->lowest_stack &&
143		− sp >= (unsigned long)task_stack_page(current) +
144		− sizeof(unsigned long)) {
	138	+ sp >= stackleak_task_low_bound(current)) {
145	139	current->lowest_stack = sp;
146	140	}
147	141	}