From 964c9dff0091893a9a74a88edf984c6da0b779f7 Mon Sep 17 00:00:00 2001 From: Alexander Popov Date: Fri, 17 Aug 2018 01:17:03 +0300 Subject: 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: Ingo Molnar Signed-off-by: Alexander Popov Tested-by: Laura Abbott Signed-off-by: Kees Cook --- Documentation/sysctl/kernel.txt | 18 ++++++++++++++++++ include/linux/stackleak.h | 6 ++++++ kernel/stackleak.c | 38 ++++++++++++++++++++++++++++++++++++++ kernel/sysctl.c | 15 ++++++++++++++- scripts/gcc-plugins/Kconfig | 8 ++++++++ 5 files changed, 84 insertions(+), 1 deletion(-) diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt index 37a679501ddc..1b8775298cf7 100644 --- a/Documentation/sysctl/kernel.txt +++ b/Documentation/sysctl/kernel.txt @@ -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 diff --git a/include/linux/stackleak.h b/include/linux/stackleak.h index b911b973d328..3d5c3271a9a8 100644 --- a/include/linux/stackleak.h +++ b/include/linux/stackleak.h @@ -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 diff --git a/kernel/stackleak.c b/kernel/stackleak.c index f66239572c89..e42892926244 100644 --- a/kernel/stackleak.c +++ b/kernel/stackleak.c @@ -12,6 +12,41 @@ #include +#ifdef CONFIG_STACKLEAK_RUNTIME_DISABLE +#include +#include + +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; diff --git a/kernel/sysctl.c b/kernel/sysctl.c index cc02050fd0c4..3ae223f7b5df 100644 --- a/kernel/sysctl.c +++ b/kernel/sysctl.c @@ -91,7 +91,9 @@ #ifdef CONFIG_CHR_DEV_SG #include #endif - +#ifdef CONFIG_STACKLEAK_RUNTIME_DISABLE +#include +#endif #ifdef CONFIG_LOCKUP_DETECTOR #include #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 { } }; diff --git a/scripts/gcc-plugins/Kconfig b/scripts/gcc-plugins/Kconfig index b0a015ef5268..0d5c799688f0 100644 --- a/scripts/gcc-plugins/Kconfig +++ b/scripts/gcc-plugins/Kconfig @@ -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 -- cgit v1.2.3