| Message ID | 20200910202107.3799376-3-keescook@chromium.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Fork brute force attack mitigation (fbfam) | expand |
On Thu, Sep 10, 2020 at 01:21:03PM -0700, Kees Cook wrote: > From: John Wood <john.wood@gmx.com> > > Create a statistical data structure to hold all the necessary > information involve in a fork brute force attack. This info is a > timestamp for the first fork or execve and the number of crashes since > then. Moreover, due to this statitistical data will be shared between > different tasks, a reference counter it is necessary. > > For a correct management of an attack it is also necessary that all the > tasks hold statistical data. The same statistical data needs to be > shared between all the tasks that hold the same memory contents or in > other words, between all the tasks that have been forked without any > execve call. > > When a forked task calls the execve system call, the memory contents are > set with new values. So, in this scenario the parent's statistical data > no need to be share. Instead, a new statistical data structure must be > allocated to start a new cycle. > > The statistical data that every task holds needs to be clear when a task > exits. Due to this data is shared across multiples tasks, the reference > counter is useful to free the previous allocated data only when there > are not other pointers to the same data. Or in other words, when the > reference counter reaches zero. > > So, based in all the previous information add the api to manage all the > commented cases. > > Also, add to the struct task_struct a new field to point to the > statitistical data related to an attack. This way, all the tasks will > have access to this information. > > Signed-off-by: John Wood <john.wood@gmx.com> I think patch 1 should be merged into this one since the former doesn't really *do* anything. ;) > --- > include/fbfam/fbfam.h | 18 +++++ > include/linux/sched.h | 4 + > security/Makefile | 4 + > security/fbfam/Makefile | 2 + > security/fbfam/fbfam.c | 163 ++++++++++++++++++++++++++++++++++++++++ > 5 files changed, 191 insertions(+) > create mode 100644 include/fbfam/fbfam.h > create mode 100644 security/fbfam/Makefile > create mode 100644 security/fbfam/fbfam.c > > diff --git a/include/fbfam/fbfam.h b/include/fbfam/fbfam.h > new file mode 100644 > index 000000000000..b5b7d1127a52 > --- /dev/null > +++ b/include/fbfam/fbfam.h > @@ -0,0 +1,18 @@ > +/* SPDX-License-Identifier: GPL-2.0 */ > +#ifndef _FBFAM_H_ > +#define _FBFAM_H_ > + > +#include <linux/sched.h> > + > +#ifdef CONFIG_FBFAM > +int fbfam_fork(struct task_struct *child); > +int fbfam_execve(void); > +int fbfam_exit(void); > +#else > +static inline int fbfam_fork(struct task_struct *child) { return 0; } This appears to map well to LSM hook "task_alloc". > +static inline int fbfam_execve(void) { return 0; } This appears to map well to LSM hook "bprm_committing_creds". > +static inline int fbfam_exit(void) { return 0; } This appears to map well to LSM hook "task_free". > +#endif > + > +#endif /* _FBFAM_H_ */ > + > diff --git a/include/linux/sched.h b/include/linux/sched.h > index afe01e232935..00e1aa5e00cd 100644 > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -1315,6 +1315,10 @@ struct task_struct { > struct callback_head mce_kill_me; > #endif > > +#ifdef CONFIG_FBFAM > + struct fbfam_stats *fbfam_stats; > +#endif This could be part of the task_struct security blob used by LSMs. > + > /* > * New fields for task_struct should be added above here, so that > * they are included in the randomized portion of task_struct. > diff --git a/security/Makefile b/security/Makefile > index 3baf435de541..36dc4b536349 100644 > --- a/security/Makefile > +++ b/security/Makefile > @@ -36,3 +36,7 @@ obj-$(CONFIG_BPF_LSM) += bpf/ > # Object integrity file lists > subdir-$(CONFIG_INTEGRITY) += integrity > obj-$(CONFIG_INTEGRITY) += integrity/ > + > +# Object fbfam file lists > +subdir-$(CONFIG_FBFAM) += fbfam > +obj-$(CONFIG_FBFAM) += fbfam/ > diff --git a/security/fbfam/Makefile b/security/fbfam/Makefile > new file mode 100644 > index 000000000000..f4b9f0b19c44 > --- /dev/null > +++ b/security/fbfam/Makefile > @@ -0,0 +1,2 @@ > +# SPDX-License-Identifier: GPL-2.0 > +obj-$(CONFIG_FBFAM) += fbfam.o > diff --git a/security/fbfam/fbfam.c b/security/fbfam/fbfam.c > new file mode 100644 > index 000000000000..0387f95f6408 > --- /dev/null > +++ b/security/fbfam/fbfam.c > @@ -0,0 +1,163 @@ > +// SPDX-License-Identifier: GPL-2.0 > +#include <asm/current.h> > +#include <fbfam/fbfam.h> > +#include <linux/errno.h> > +#include <linux/gfp.h> > +#include <linux/jiffies.h> > +#include <linux/refcount.h> > +#include <linux/slab.h> > + > +/** > + * struct fbfam_stats - Fork brute force attack mitigation statistics. > + * @refc: Reference counter. > + * @faults: Number of crashes since jiffies. > + * @jiffies: First fork or execve timestamp. > + * > + * The purpose of this structure is to manage all the necessary information to > + * compute the crashing rate of an application. So, it holds a first fork or > + * execve timestamp and a number of crashes since then. This way the crashing > + * rate in milliseconds per fault can be compute when necessary with the > + * following formula: > + * > + * u64 delta_jiffies = get_jiffies_64() - fbfam_stats::jiffies; > + * u64 delta_time = jiffies64_to_msecs(delta_jiffies); > + * u64 crashing_rate = delta_time / (u64)fbfam_stats::faults; > + * > + * If the fbfam_stats::faults is zero, the above formula can't be used. In this > + * case, the crashing rate is zero. > + * > + * Moreover, since the same allocated structure will be used in every fork > + * since the first one or execve, it's also necessary a reference counter. > + */ > +struct fbfam_stats { > + refcount_t refc; > + unsigned int faults; > + u64 jiffies; > +}; > + > +/** > + * fbfam_new_stats() - Allocation of new statistics structure. > + * > + * If the allocation is successful the reference counter is set to one to > + * indicate that there will be one task that points to this structure. The > + * faults field is initialize to zero and the timestamp for this moment is set. > + * > + * Return: NULL if the allocation fails. A pointer to the new allocated > + * statistics structure if it success. > + */ > +static struct fbfam_stats *fbfam_new_stats(void) > +{ > + struct fbfam_stats *stats = kmalloc(sizeof(struct fbfam_stats), > + GFP_KERNEL); > + > + if (stats) { > + refcount_set(&stats->refc, 1); > + stats->faults = 0; > + stats->jiffies = get_jiffies_64(); > + } > + > + return stats; > +} > + > +/* > + * fbfam_fork() - Fork management. > + * @child: Pointer to the child task that will be created with the fork system > + * call. > + * > + * For a correct management of a fork brute force attack it is necessary that > + * all the tasks hold statistical data. The same statistical data needs to be > + * shared between all the tasks that hold the same memory contents or in other > + * words, between all the tasks that have been forked without any execve call. > + * > + * To ensure this, if the current task doesn't have statistical data when forks > + * (only possible in the first fork of the zero task), it is mandatory to > + * allocate a new one. This way, the child task always will share the statistics > + * with its parent. > + * > + * Return: -ENOMEN if the allocation of the new statistics structure fails. > + * Zero otherwise. > + */ > +int fbfam_fork(struct task_struct *child) > +{ > + struct fbfam_stats **stats = ¤t->fbfam_stats; > + > + if (!*stats) { > + *stats = fbfam_new_stats(); > + if (!*stats) > + return -ENOMEM; > + } > + > + refcount_inc(&(*stats)->refc); > + child->fbfam_stats = *stats; > + return 0; > +} > + > +/** > + * fbfam_execve() - Execve management. > + * > + * When a forked task calls the execve system call, the memory contents are set > + * with new values. So, in this scenario the parent's statistical data no need > + * to be share. Instead, a new statistical data structure must be allocated to > + * start a new cycle. This condition is detected when the statistics reference > + * counter holds a value greater than or equal to two (a fork always sets the > + * statistics reference counter to two since the parent and the child task are > + * sharing the same data). > + * > + * However, if the execve function is called immediately after another execve > + * call, althought the memory contents are reset, there is no need to allocate > + * a new statistical data structure. This is possible because at this moment > + * only one task (the task that calls the execve function) points to the data. > + * In this case, the previous allocation is used and only the faults and time > + * fields are reset. > + * > + * Return: -ENOMEN if the allocation of the new statistics structure fails. > + * -EFAULT if the current task doesn't have statistical data. Zero > + * otherwise. > + */ > +int fbfam_execve(void) > +{ > + struct fbfam_stats **stats = ¤t->fbfam_stats; > + > + if (!*stats) > + return -EFAULT; > + > + if (!refcount_dec_not_one(&(*stats)->refc)) { > + /* execve call after an execve call */ > + (*stats)->faults = 0; > + (*stats)->jiffies = get_jiffies_64(); > + return 0; > + } > + > + /* execve call after a fork call */ > + *stats = fbfam_new_stats(); > + if (!*stats) > + return -ENOMEM; > + > + return 0; > +} > + > +/** > + * fbfam_exit() - Exit management. > + * > + * The statistical data that every task holds needs to be clear when a task > + * exits. Due to this data is shared across multiples tasks, the reference > + * counter is useful to free the previous allocated data only when there are > + * not other pointers to the same data. Or in other words, when the reference > + * counter reaches zero. > + * > + * Return: -EFAULT if the current task doesn't have statistical data. Zero > + * otherwise. > + */ > +int fbfam_exit(void) > +{ > + struct fbfam_stats *stats = current->fbfam_stats; > + > + if (!stats) > + return -EFAULT; > + > + if (refcount_dec_and_test(&stats->refc)) > + kfree(stats); > + > + return 0; > +} > + > -- > 2.25.1 > Jann mentions some concerns about races, and I'd agree: this doesn't feel right to me, but I've not had a chance to study it yet. I'm concerned about the lifetime management of the stats vs the task hierarchy.
diff --git a/include/fbfam/fbfam.h b/include/fbfam/fbfam.h new file mode 100644 index 000000000000..b5b7d1127a52 --- /dev/null +++ b/include/fbfam/fbfam.h @@ -0,0 +1,18 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +#ifndef _FBFAM_H_ +#define _FBFAM_H_ + +#include <linux/sched.h> + +#ifdef CONFIG_FBFAM +int fbfam_fork(struct task_struct *child); +int fbfam_execve(void); +int fbfam_exit(void); +#else +static inline int fbfam_fork(struct task_struct *child) { return 0; } +static inline int fbfam_execve(void) { return 0; } +static inline int fbfam_exit(void) { return 0; } +#endif + +#endif /* _FBFAM_H_ */ + diff --git a/include/linux/sched.h b/include/linux/sched.h index afe01e232935..00e1aa5e00cd 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1315,6 +1315,10 @@ struct task_struct { struct callback_head mce_kill_me; #endif +#ifdef CONFIG_FBFAM + struct fbfam_stats *fbfam_stats; +#endif + /* * New fields for task_struct should be added above here, so that * they are included in the randomized portion of task_struct. diff --git a/security/Makefile b/security/Makefile index 3baf435de541..36dc4b536349 100644 --- a/security/Makefile +++ b/security/Makefile @@ -36,3 +36,7 @@ obj-$(CONFIG_BPF_LSM) += bpf/ # Object integrity file lists subdir-$(CONFIG_INTEGRITY) += integrity obj-$(CONFIG_INTEGRITY) += integrity/ + +# Object fbfam file lists +subdir-$(CONFIG_FBFAM) += fbfam +obj-$(CONFIG_FBFAM) += fbfam/ diff --git a/security/fbfam/Makefile b/security/fbfam/Makefile new file mode 100644 index 000000000000..f4b9f0b19c44 --- /dev/null +++ b/security/fbfam/Makefile @@ -0,0 +1,2 @@ +# SPDX-License-Identifier: GPL-2.0 +obj-$(CONFIG_FBFAM) += fbfam.o diff --git a/security/fbfam/fbfam.c b/security/fbfam/fbfam.c new file mode 100644 index 000000000000..0387f95f6408 --- /dev/null +++ b/security/fbfam/fbfam.c @@ -0,0 +1,163 @@ +// SPDX-License-Identifier: GPL-2.0 +#include <asm/current.h> +#include <fbfam/fbfam.h> +#include <linux/errno.h> +#include <linux/gfp.h> +#include <linux/jiffies.h> +#include <linux/refcount.h> +#include <linux/slab.h> + +/** + * struct fbfam_stats - Fork brute force attack mitigation statistics. + * @refc: Reference counter. + * @faults: Number of crashes since jiffies. + * @jiffies: First fork or execve timestamp. + * + * The purpose of this structure is to manage all the necessary information to + * compute the crashing rate of an application. So, it holds a first fork or + * execve timestamp and a number of crashes since then. This way the crashing + * rate in milliseconds per fault can be compute when necessary with the + * following formula: + * + * u64 delta_jiffies = get_jiffies_64() - fbfam_stats::jiffies; + * u64 delta_time = jiffies64_to_msecs(delta_jiffies); + * u64 crashing_rate = delta_time / (u64)fbfam_stats::faults; + * + * If the fbfam_stats::faults is zero, the above formula can't be used. In this + * case, the crashing rate is zero. + * + * Moreover, since the same allocated structure will be used in every fork + * since the first one or execve, it's also necessary a reference counter. + */ +struct fbfam_stats { + refcount_t refc; + unsigned int faults; + u64 jiffies; +}; + +/** + * fbfam_new_stats() - Allocation of new statistics structure. + * + * If the allocation is successful the reference counter is set to one to + * indicate that there will be one task that points to this structure. The + * faults field is initialize to zero and the timestamp for this moment is set. + * + * Return: NULL if the allocation fails. A pointer to the new allocated + * statistics structure if it success. + */ +static struct fbfam_stats *fbfam_new_stats(void) +{ + struct fbfam_stats *stats = kmalloc(sizeof(struct fbfam_stats), + GFP_KERNEL); + + if (stats) { + refcount_set(&stats->refc, 1); + stats->faults = 0; + stats->jiffies = get_jiffies_64(); + } + + return stats; +} + +/* + * fbfam_fork() - Fork management. + * @child: Pointer to the child task that will be created with the fork system + * call. + * + * For a correct management of a fork brute force attack it is necessary that + * all the tasks hold statistical data. The same statistical data needs to be + * shared between all the tasks that hold the same memory contents or in other + * words, between all the tasks that have been forked without any execve call. + * + * To ensure this, if the current task doesn't have statistical data when forks + * (only possible in the first fork of the zero task), it is mandatory to + * allocate a new one. This way, the child task always will share the statistics + * with its parent. + * + * Return: -ENOMEN if the allocation of the new statistics structure fails. + * Zero otherwise. + */ +int fbfam_fork(struct task_struct *child) +{ + struct fbfam_stats **stats = ¤t->fbfam_stats; + + if (!*stats) { + *stats = fbfam_new_stats(); + if (!*stats) + return -ENOMEM; + } + + refcount_inc(&(*stats)->refc); + child->fbfam_stats = *stats; + return 0; +} + +/** + * fbfam_execve() - Execve management. + * + * When a forked task calls the execve system call, the memory contents are set + * with new values. So, in this scenario the parent's statistical data no need + * to be share. Instead, a new statistical data structure must be allocated to + * start a new cycle. This condition is detected when the statistics reference + * counter holds a value greater than or equal to two (a fork always sets the + * statistics reference counter to two since the parent and the child task are + * sharing the same data). + * + * However, if the execve function is called immediately after another execve + * call, althought the memory contents are reset, there is no need to allocate + * a new statistical data structure. This is possible because at this moment + * only one task (the task that calls the execve function) points to the data. + * In this case, the previous allocation is used and only the faults and time + * fields are reset. + * + * Return: -ENOMEN if the allocation of the new statistics structure fails. + * -EFAULT if the current task doesn't have statistical data. Zero + * otherwise. + */ +int fbfam_execve(void) +{ + struct fbfam_stats **stats = ¤t->fbfam_stats; + + if (!*stats) + return -EFAULT; + + if (!refcount_dec_not_one(&(*stats)->refc)) { + /* execve call after an execve call */ + (*stats)->faults = 0; + (*stats)->jiffies = get_jiffies_64(); + return 0; + } + + /* execve call after a fork call */ + *stats = fbfam_new_stats(); + if (!*stats) + return -ENOMEM; + + return 0; +} + +/** + * fbfam_exit() - Exit management. + * + * The statistical data that every task holds needs to be clear when a task + * exits. Due to this data is shared across multiples tasks, the reference + * counter is useful to free the previous allocated data only when there are + * not other pointers to the same data. Or in other words, when the reference + * counter reaches zero. + * + * Return: -EFAULT if the current task doesn't have statistical data. Zero + * otherwise. + */ +int fbfam_exit(void) +{ + struct fbfam_stats *stats = current->fbfam_stats; + + if (!stats) + return -EFAULT; + + if (refcount_dec_and_test(&stats->refc)) + kfree(stats); + + return 0; +} +