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Commit 964c9dff authored by Alexander Popov's avatar Alexander Popov Committed by Kees Cook
Browse files

stackleak: Allow runtime disabling of kernel stack erasing 



Introduce CONFIG_STACKLEAK_RUNTIME_DISABLE option, which provides
'stack_erasing' sysctl. It can be used in runtime to control kernel
stack erasing for kernels built with CONFIG_GCC_PLUGIN_STACKLEAK.

Suggested-by: default avatarIngo Molnar <mingo@kernel.org>
Signed-off-by: default avatarAlexander Popov <alex.popov@linux.com>
Tested-by: default avatarLaura Abbott <labbott@redhat.com>
Signed-off-by: default avatarKees Cook <keescook@chromium.org>
parent ed535a2d

Documentation/sysctl/kernel.txt

+18 −0
Original line number Diff line number Diff line
@@ -89,6 +89,7 @@ show up in /proc/sys/kernel: 
- shmmni

- softlockup_all_cpu_backtrace

- soft_watchdog

- stack_erasing

- stop-a                      [ SPARC only ]

- sysrq                       ==> Documentation/admin-guide/sysrq.rst

- sysctl_writes_strict
@@ -987,6 +988,23 @@ detect a hard lockup condition. 


==============================================================



stack_erasing



This parameter can be used to control kernel stack erasing at the end

of syscalls for kernels built with CONFIG_GCC_PLUGIN_STACKLEAK.



That erasing reduces the information which kernel stack leak bugs

can reveal and blocks some uninitialized stack variable attacks.

The tradeoff is the performance impact: on a single CPU system kernel

compilation sees a 1% slowdown, other systems and workloads may vary.



  0: kernel stack erasing is disabled, STACKLEAK_METRICS are not updated.



  1: kernel stack erasing is enabled (default), it is performed before

     returning to the userspace at the end of syscalls.



==============================================================



tainted:



Non-zero if the kernel has been tainted. Numeric values, which can be

include/linux/stackleak.h

+6 −0
Original line number Diff line number Diff line
@@ -22,6 +22,12 @@ static inline void stackleak_task_init(struct task_struct *t) 
	t->prev_lowest_stack = t->lowest_stack;

# endif

}



#ifdef CONFIG_STACKLEAK_RUNTIME_DISABLE

int stack_erasing_sysctl(struct ctl_table *table, int write,

			void __user *buffer, size_t *lenp, loff_t *ppos);

#endif



#else /* !CONFIG_GCC_PLUGIN_STACKLEAK */

static inline void stackleak_task_init(struct task_struct *t) { }

#endif

kernel/stackleak.c

+38 −0
Original line number Diff line number Diff line
@@ -12,6 +12,41 @@ 


#include <linux/stackleak.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 __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;

}



#define skip_erasing()	static_branch_unlikely(&stack_erasing_bypass)

#else

#define skip_erasing()	false

#endif /* CONFIG_STACKLEAK_RUNTIME_DISABLE */



asmlinkage void stackleak_erase(void)

{

	/* It would be nice not to have 'kstack_ptr' and 'boundary' on stack */
@@ -20,6 +55,9 @@ asmlinkage void stackleak_erase(void) 
	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;

kernel/sysctl.c

+14 −1
Original line number Diff line number Diff line
@@ -91,7 +91,9 @@ 
#ifdef CONFIG_CHR_DEV_SG

#include <scsi/sg.h>

#endif



#ifdef CONFIG_STACKLEAK_RUNTIME_DISABLE

#include <linux/stackleak.h>

#endif

#ifdef CONFIG_LOCKUP_DETECTOR

#include <linux/nmi.h>

#endif
@@ -1232,6 +1234,17 @@ static struct ctl_table kern_table[] = { 
		.extra1		= &zero,

		.extra2		= &one,

	},

#endif

#ifdef CONFIG_STACKLEAK_RUNTIME_DISABLE

	{

		.procname	= "stack_erasing",

		.data		= NULL,

		.maxlen		= sizeof(int),

		.mode		= 0600,

		.proc_handler	= stack_erasing_sysctl,

		.extra1		= &zero,

		.extra2		= &one,

	},

#endif

	{ }

};

scripts/gcc-plugins/Kconfig

+8 −0
Original line number Diff line number Diff line
@@ -182,4 +182,12 @@ config STACKLEAK_METRICS 
	  can be useful for estimating the STACKLEAK performance impact for

	  your workloads.



config STACKLEAK_RUNTIME_DISABLE

	bool "Allow runtime disabling of kernel stack erasing"

	depends on GCC_PLUGIN_STACKLEAK

	help

	  This option provides 'stack_erasing' sysctl, which can be used in

	  runtime to control kernel stack erasing for kernels built with

	  CONFIG_GCC_PLUGIN_STACKLEAK.



endif