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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 <[email protected]>
- *
- * 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>
- #include <linux/init.h>
- static DEFINE_STATIC_KEY_FALSE(stack_erasing_bypass);
- #ifdef CONFIG_SYSCTL
- static int stack_erasing_sysctl(struct ctl_table *table, int write,
- void __user *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;
- }
- static struct ctl_table stackleak_sysctls[] = {
- {
- .procname = "stack_erasing",
- .data = NULL,
- .maxlen = sizeof(int),
- .mode = 0600,
- .proc_handler = stack_erasing_sysctl,
- .extra1 = SYSCTL_ZERO,
- .extra2 = SYSCTL_ONE,
- },
- {}
- };
- static int __init stackleak_sysctls_init(void)
- {
- register_sysctl_init("kernel", stackleak_sysctls);
- return 0;
- }
- late_initcall(stackleak_sysctls_init);
- #endif /* CONFIG_SYSCTL */
- #define skip_erasing() static_branch_unlikely(&stack_erasing_bypass)
- #else
- #define skip_erasing() false
- #endif /* CONFIG_STACKLEAK_RUNTIME_DISABLE */
- static __always_inline void __stackleak_erase(bool on_task_stack)
- {
- 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, erase_high;
- erase_low = stackleak_find_top_of_poison(task_stack_low,
- current->lowest_stack);
- #ifdef CONFIG_STACKLEAK_METRICS
- current->prev_lowest_stack = erase_low;
- #endif
- /*
- * Write poison to the task's stack between 'erase_low' and
- * 'erase_high'.
- *
- * If we're running on a different stack (e.g. an entry trampoline
- * stack) we can erase everything below the pt_regs at the top of the
- * task stack.
- *
- * If we're running on the task stack itself, we must not clobber any
- * stack used by this function and its caller. We assume that this
- * function has a fixed-size stack frame, and the current stack pointer
- * doesn't change while we write poison.
- */
- if (on_task_stack)
- erase_high = current_stack_pointer;
- else
- erase_high = task_stack_high;
- while (erase_low < erase_high) {
- *(unsigned long *)erase_low = STACKLEAK_POISON;
- erase_low += sizeof(unsigned long);
- }
- /* Reset the 'lowest_stack' value for the next syscall */
- current->lowest_stack = task_stack_high;
- }
- /*
- * Erase and poison the portion of the task stack used since the last erase.
- * Can be called from the task stack or an entry stack when the task stack is
- * no longer in use.
- */
- asmlinkage void noinstr stackleak_erase(void)
- {
- if (skip_erasing())
- return;
- __stackleak_erase(on_thread_stack());
- }
- /*
- * Erase and poison the portion of the task stack used since the last erase.
- * Can only be called from the task stack.
- */
- asmlinkage void noinstr stackleak_erase_on_task_stack(void)
- {
- if (skip_erasing())
- return;
- __stackleak_erase(true);
- }
- /*
- * Erase and poison the portion of the task stack used since the last erase.
- * Can only be called from a stack other than the task stack.
- */
- asmlinkage void noinstr stackleak_erase_off_task_stack(void)
- {
- if (skip_erasing())
- return;
- __stackleak_erase(false);
- }
- 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 >= stackleak_task_low_bound(current)) {
- current->lowest_stack = sp;
- }
- }
- EXPORT_SYMBOL(stackleak_track_stack);
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