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The stackleak_erase() code dynamically handles being on a task stack or another stack. In most cases, this is a fixed property of the caller, which the caller is aware of, as an architecture might always return using the task stack, or might always return using a trampoline stack. This patch adds stackleak_erase_on_task_stack() and stackleak_erase_off_task_stack() functions which callers can use to avoid on_thread_stack() check and associated redundant work when the calling stack is known. The existing stackleak_erase() is retained as a safe default. 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/20220427173128.2603085-13-mark.rutland@arm.com
169 lines
4.5 KiB
C
169 lines
4.5 KiB
C
// 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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#include <linux/init.h>
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static DEFINE_STATIC_KEY_FALSE(stack_erasing_bypass);
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#ifdef CONFIG_SYSCTL
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static int stack_erasing_sysctl(struct ctl_table *table, int write,
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void __user *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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static struct ctl_table stackleak_sysctls[] = {
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{
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.procname = "stack_erasing",
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.data = NULL,
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.maxlen = sizeof(int),
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.mode = 0600,
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.proc_handler = stack_erasing_sysctl,
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.extra1 = SYSCTL_ZERO,
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.extra2 = SYSCTL_ONE,
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},
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{}
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};
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static int __init stackleak_sysctls_init(void)
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{
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register_sysctl_init("kernel", stackleak_sysctls);
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return 0;
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}
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late_initcall(stackleak_sysctls_init);
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#endif /* CONFIG_SYSCTL */
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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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static __always_inline void __stackleak_erase(bool on_task_stack)
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{
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const unsigned long task_stack_low = stackleak_task_low_bound(current);
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const unsigned long task_stack_high = stackleak_task_high_bound(current);
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unsigned long erase_low, erase_high;
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erase_low = stackleak_find_top_of_poison(task_stack_low,
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current->lowest_stack);
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#ifdef CONFIG_STACKLEAK_METRICS
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current->prev_lowest_stack = erase_low;
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#endif
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/*
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* Write poison to the task's stack between 'erase_low' and
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* 'erase_high'.
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*
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* If we're running on a different stack (e.g. an entry trampoline
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* stack) we can erase everything below the pt_regs at the top of the
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* task stack.
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*
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* If we're running on the task stack itself, we must not clobber any
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* stack used by this function and its caller. We assume that this
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* function has a fixed-size stack frame, and the current stack pointer
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* doesn't change while we write poison.
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*/
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if (on_task_stack)
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erase_high = current_stack_pointer;
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else
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erase_high = task_stack_high;
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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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current->lowest_stack = task_stack_high;
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}
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/*
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* Erase and poison the portion of the task stack used since the last erase.
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* Can be called from the task stack or an entry stack when the task stack is
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* no longer in use.
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*/
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asmlinkage void noinstr stackleak_erase(void)
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{
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if (skip_erasing())
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return;
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__stackleak_erase(on_thread_stack());
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}
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/*
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* Erase and poison the portion of the task stack used since the last erase.
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* Can only be called from the task stack.
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*/
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asmlinkage void noinstr stackleak_erase_on_task_stack(void)
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{
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if (skip_erasing())
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return;
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__stackleak_erase(true);
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}
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/*
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* Erase and poison the portion of the task stack used since the last erase.
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* Can only be called from a stack other than the task stack.
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*/
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asmlinkage void noinstr stackleak_erase_off_task_stack(void)
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{
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if (skip_erasing())
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return;
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__stackleak_erase(false);
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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 >= stackleak_task_low_bound(current)) {
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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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