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