// SPDX-License-Identifier: GPL-2.0
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/*
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* This code fills the used part of the kernel stack with a poison value
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* before returning to userspace. It's part of the STACKLEAK feature
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* ported from grsecurity/PaX.
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*
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* Author: Alexander Popov <alex.popov@linux.com>
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*
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* STACKLEAK reduces the information which kernel stack leak bugs can
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* reveal and blocks some uninitialized stack variable attacks.
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*/
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#include <linux/stackleak.h>
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#include <linux/kprobes.h>
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#ifdef CONFIG_STACKLEAK_RUNTIME_DISABLE
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#include <linux/jump_label.h>
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#include <linux/sysctl.h>
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static DEFINE_STATIC_KEY_FALSE(stack_erasing_bypass);
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int stack_erasing_sysctl(struct ctl_table *table, int write,
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void *buffer, size_t *lenp, loff_t *ppos)
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{
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int ret = 0;
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int state = !static_branch_unlikely(&stack_erasing_bypass);
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int prev_state = state;
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table->data = &state;
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table->maxlen = sizeof(int);
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ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
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state = !!state;
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if (ret || !write || state == prev_state)
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return ret;
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if (state)
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static_branch_disable(&stack_erasing_bypass);
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else
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static_branch_enable(&stack_erasing_bypass);
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pr_warn("stackleak: kernel stack erasing is %s\n",
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state ? "enabled" : "disabled");
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return ret;
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}
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#define skip_erasing() static_branch_unlikely(&stack_erasing_bypass)
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#else
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#define skip_erasing() false
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#endif /* CONFIG_STACKLEAK_RUNTIME_DISABLE */
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asmlinkage void noinstr stackleak_erase(void)
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{
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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 = (unsigned long)end_of_stack(current);
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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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if (skip_erasing())
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return;
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/* Check that 'lowest_stack' value is sane */
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if (unlikely(kstack_ptr - boundary >= THREAD_SIZE))
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kstack_ptr = boundary;
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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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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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}
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/*
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* One 'long int' at the bottom of the thread stack is reserved and
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* should not be poisoned (see CONFIG_SCHED_STACK_END_CHECK=y).
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*/
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if (kstack_ptr == boundary)
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kstack_ptr += sizeof(unsigned long);
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#ifdef CONFIG_STACKLEAK_METRICS
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current->prev_lowest_stack = kstack_ptr;
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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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*/
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if (on_thread_stack())
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boundary = current_stack_pointer;
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else
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boundary = 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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}
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/* Reset the 'lowest_stack' value for the next syscall */
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current->lowest_stack = current_top_of_stack() - THREAD_SIZE/64;
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}
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void __used __no_caller_saved_registers noinstr stackleak_track_stack(void)
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{
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unsigned long sp = current_stack_pointer;
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/*
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* Having CONFIG_STACKLEAK_TRACK_MIN_SIZE larger than
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* STACKLEAK_SEARCH_DEPTH makes the poison search in
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* stackleak_erase() unreliable. Let's prevent that.
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*/
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BUILD_BUG_ON(CONFIG_STACKLEAK_TRACK_MIN_SIZE > STACKLEAK_SEARCH_DEPTH);
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/* 'lowest_stack' should be aligned on the register width boundary */
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sp = ALIGN(sp, sizeof(unsigned long));
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if (sp < current->lowest_stack &&
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sp >= (unsigned long)task_stack_page(current) +
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sizeof(unsigned long)) {
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current->lowest_stack = sp;
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}
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}
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EXPORT_SYMBOL(stackleak_track_stack);
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