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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>
#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(void)
{
const unsigned long task_stack_low = stackleak_task_low_bound(current);
unsigned long erase_low = current->lowest_stack;
unsigned long erase_high;
unsigned int poison_count = 0;
const unsigned int depth = STACKLEAK_SEARCH_DEPTH / sizeof(unsigned long);
/* Search for the poison value in the kernel stack */
while (erase_low > task_stack_low && poison_count <= depth) {
if (*(unsigned long *)erase_low == STACKLEAK_POISON)
poison_count++;
else
poison_count = 0;
erase_low -= sizeof(unsigned long);
}
#ifdef CONFIG_STACKLEAK_METRICS
current->prev_lowest_stack = erase_low;
#endif
/*
* Now write the poison value to the kernel stack between 'erase_low'
* and 'erase_high'. We assume that the stack pointer doesn't change
* when we write poison.
*/
if (on_thread_stack())
erase_high = current_stack_pointer;
else
erase_high = current_top_of_stack();
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 = current_top_of_stack() - THREAD_SIZE/64;
}
asmlinkage void noinstr stackleak_erase(void)
{
if (skip_erasing())
return;
__stackleak_erase();
}
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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