Detect a TOCTOU race condition attack on the filesystem by checking if
the effective syscall (i.e. LSM hooks) see the same files as previously
checked by the seccomp filter.
Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
Cc: David Drysdale <drysdale@xxxxxxxxxx>
Cc: James Morris <james.l.morris@xxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Paul Moore <pmoore@xxxxxxxxxx>
Cc: Serge E. Hallyn <serge@xxxxxxxxxx>
Cc: Will Drewry <wad@xxxxxxxxxxxx>
---
include/linux/seccomp.h | 27 ++++++++
kernel/fork.c | 2 +
kernel/seccomp.c | 103 +++++++++++++++++++++++++++-
security/seccomp/checker_fs.c | 153 ++++++++++++++++++++++++++++++++++++++++++
security/seccomp/checker_fs.h | 18 +++++
security/seccomp/lsm.c | 48 +++++++++++++
6 files changed, 350 insertions(+), 1 deletion(-)
create mode 100644 security/seccomp/checker_fs.h
diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
index 0c5468f78945..8ea63813ca64 100644
--- a/include/linux/seccomp.h
+++ b/include/linux/seccomp.h
@@ -13,7 +13,9 @@
struct seccomp_filter;
struct seccomp_filter_checker_group;
+struct seccomp_argeval_checked;
struct seccomp_argeval_cache;
+struct seccomp_argeval_syscall;
/**
* struct seccomp - the state of a seccomp'ed process
@@ -38,7 +40,9 @@ struct seccomp {
struct seccomp_filter_checker_group *checker_group;
/* syscall-lifetime data */
+ struct seccomp_argeval_checked *arg_checked;
struct seccomp_argeval_cache *arg_cache;
+ struct seccomp_argeval_syscall *orig_syscall;
#endif
};
@@ -153,6 +157,14 @@ struct seccomp_argeval_cache_fs {
u64 hash_len;
};
+struct seccomp_argeval_history {
+ /* @checker point to current.seccomp->checker_group->checkers[] */
+ struct seccomp_filter_checker *checker;
+ u8 asked;
+ u8 result;
+ struct seccomp_argeval_history *next;
+};
+
/**
* struct seccomp_argeval_cache_entry
*
@@ -178,6 +190,13 @@ struct seccomp_argeval_cache {
struct seccomp_argeval_cache *next;
};
+/* Use get_argrule_checker() */
+struct seccomp_argeval_checked {
+ u32 check;
+ struct seccomp_argeval_history *history;
+ struct seccomp_argeval_checked *next;
+};
+
void put_seccomp_filter_checker(struct seccomp_filter_checker *);
u8 seccomp_argrule_path(const u8(*)[6], const u64(*)[6], u8,
@@ -186,6 +205,14 @@ u8 seccomp_argrule_path(const u8(*)[6], const u64(*)[6], u8,
long seccomp_set_argcheck_fs(const struct seccomp_checker *,
struct seccomp_filter_checker *);
+/* Need to save syscall properties to be able to properly recheck the filters
+ * even if the syscall and its arguments has been tampered by a tracer process.
+ */
+struct seccomp_argeval_syscall {
+ int nr;
+ u64 args[6];
+};
+
#endif /* CONFIG_SECURITY_SECCOMP */
#else /* CONFIG_SECCOMP_FILTER */
diff --git a/kernel/fork.c b/kernel/fork.c
index b8155ebdd308..f41912acd755 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -361,7 +361,9 @@ static struct task_struct *dup_task_struct(struct task_struct *orig)
tsk->seccomp.filter = NULL;
#ifdef CONFIG_SECURITY_SECCOMP
tsk->seccomp.checker_group = NULL;
+ tsk->seccomp.arg_checked = NULL;
tsk->seccomp.arg_cache = NULL;
+ tsk->seccomp.orig_syscall = NULL;
#endif /* CONFIG_SECURITY_SECCOMP */
#endif /* CONFIG_SECCOMP */
diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index a8a6ba31ecc4..735b7caf4e06 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -286,6 +286,40 @@ struct syscall_argdesc *syscall_nr_to_argdesc(int nr)
return &(*seccomp_sa)[nr];
}
+/* Return a new empty history entry for the check type or NULL if ENOMEM */
+static struct seccomp_argeval_history *new_argeval_history(u32 check)
+{
+ struct seccomp_argeval_checked **arg_checked;
+ struct seccomp_argeval_history **history;
+ bool found = false;
+
+ /* Find the check type */
+ arg_checked = ¤t->seccomp.arg_checked;
+ while (*arg_checked) {
+ if ((*arg_checked)->check == check) {
+ found = true;
+ break;
+ }
+ arg_checked = &(*arg_checked)->next;
+ }
+ if (!found) {
+ *arg_checked = kmalloc(sizeof(**arg_checked), GFP_KERNEL);
+ if (!*arg_checked)
+ return NULL;
+ (*arg_checked)->check = check;
+ (*arg_checked)->history = NULL;
+ (*arg_checked)->next = NULL;
+ }
+
+ /* Append to history */
+ history = &(*arg_checked)->history;
+ while (*history)
+ history = &(*history)->next;
+ *history = kzalloc(sizeof(**history), GFP_KERNEL);
+
+ return *history;
+}
+
/* Return the argument group address that match the group ID, or NULL
* otherwise.
*/
@@ -299,6 +333,7 @@ static struct seccomp_filter_checker_group *seccomp_update_argrule_data(
const struct syscall_argdesc *argdesc;
struct seccomp_filter_checker *checker;
seccomp_argrule_t *engine;
+ struct seccomp_argeval_history *history;
const u8 group_id = ret_data & SECCOMP_RET_CHECKER_GROUP;
const u8 to_check = (ret_data & SECCOMP_RET_ARG_MATCHES) >> 8;
@@ -327,6 +362,17 @@ static struct seccomp_filter_checker_group *seccomp_update_argrule_data(
if (engine) {
match = (*engine)(&argdesc->args, &sd->args, to_check, checker);
+ /* Append the results to be able to replay the checks */
+ history = new_argeval_history(checker->check);
+ if (!history) {
+ /* XXX: return -ENOMEM somehow? */
+ break;
+ }
+ history->checker = checker;
+ history->asked = to_check;
+ history->result = match;
+
+ /* Store the matches after the history record */
for (j = 0; j < 6; j++) {
sd->arg_matches[j] |=
((BIT_ULL(j) & match) >> j) << i;
@@ -375,6 +421,39 @@ void flush_seccomp_cache(struct task_struct *tsk)
free_seccomp_argeval_cache(tsk->seccomp.arg_cache);
tsk->seccomp.arg_cache = NULL;
}
+
+static void free_seccomp_argeval_history(struct seccomp_argeval_history *history)
+{
+ struct seccomp_argeval_history *walker = history;
+
+ while (walker) {
+ struct seccomp_argeval_history *freeme = walker;
+
+ /* Must not free history->checker owned by
+ * current.seccomp->checker_group->checkers[]
+ */
+ walker = walker->next;
+ kfree(freeme);
+ }
+}
+
+static void free_seccomp_argeval_checked(struct seccomp_argeval_checked *checked)
+{
+ struct seccomp_argeval_checked *walker = checked;
+
+ while (walker) {
+ struct seccomp_argeval_checked *freeme = walker;
+
+ free_seccomp_argeval_history(walker->history);
+ walker = walker->next;
+ kfree(freeme);
+ }
+}
+
+static inline void free_seccomp_argeval_syscall(struct seccomp_argeval_syscall *orig_syscall)
+{
+ kfree(orig_syscall);
+}
#endif /* CONFIG_SECURITY_SECCOMP */
static void put_seccomp_filter(struct task_struct *tsk);
@@ -405,7 +484,27 @@ static u32 seccomp_run_filters(struct seccomp_data *sd)
sd = &sd_local;
}
#ifdef CONFIG_SECURITY_SECCOMP
- /* Cleanup old (syscall-lifetime) cache */
+ /* Backup the current syscall and its arguments (used by the filters),
+ * to not be misled in the LSM checks by a potential ptrace setregs
+ * command.
+ */
+ if (!current->seccomp.orig_syscall) {
+ current->seccomp.orig_syscall =
+ kmalloc(sizeof(*current->seccomp.orig_syscall), GFP_KERNEL);
+ if (!current->seccomp.orig_syscall)
+ return SECCOMP_RET_KILL;
+ }
+ current->seccomp.orig_syscall->nr = sd->nr;
+ current->seccomp.orig_syscall->args[0] = sd->args[0];
+ current->seccomp.orig_syscall->args[1] = sd->args[1];
+ current->seccomp.orig_syscall->args[2] = sd->args[2];
+ current->seccomp.orig_syscall->args[3] = sd->args[3];
+ current->seccomp.orig_syscall->args[4] = sd->args[4];
+ current->seccomp.orig_syscall->args[5] = sd->args[5];
+
+ /* Cleanup old (syscall-lifetime) history and cache */
+ free_seccomp_argeval_checked(current->seccomp.arg_checked);
+ current->seccomp.arg_checked = NULL;
flush_seccomp_cache(current);
#endif
@@ -786,7 +885,9 @@ void put_seccomp(struct task_struct *tsk)
put_seccomp_filter(tsk);
#ifdef CONFIG_SECURITY_SECCOMP
/* Free in that order because of referenced checkers */
+ free_seccomp_argeval_checked(tsk->seccomp.arg_checked);
free_seccomp_argeval_cache(tsk->seccomp.arg_cache);
+ free_seccomp_argeval_syscall(tsk->seccomp.orig_syscall);
put_seccomp_checker_group(tsk->seccomp.checker_group);
#endif
}
diff --git a/security/seccomp/checker_fs.c b/security/seccomp/checker_fs.c
index 0a5ec3a204e7..994d889b0334 100644
--- a/security/seccomp/checker_fs.c
+++ b/security/seccomp/checker_fs.c
@@ -8,6 +8,7 @@
* published by the Free Software Foundation.
*/
+#include <asm/syscall.h> /* syscall_get_nr() */
#include <linux/bitops.h> /* BIT() */
#include <linux/compat.h>
#include <linux/namei.h> /* user_lpath() */
@@ -17,6 +18,8 @@
#include <linux/syscalls.h> /* __SACT__CONST_CHAR_PTR */
#include <linux/uaccess.h> /* copy_from_user() */
+#include "checker_fs.h"
+
#ifdef CONFIG_COMPAT
/* struct seccomp_object_path */
struct compat_seccomp_object_path {
@@ -330,6 +333,156 @@ out:
return ret;
}
+/* argeval_find_args_file - return a bitmask of the syscall arguments matching
+ * a struct file and that have changed since the filter checks
+ *
+ * To match a file with a syscall argument, we get its path and deduce the
+ * corresponding user address (uptr). Then, if a match is found, the file's
+ * inode must match the cached inode, otherwise the access is denied if a
+ * second filter check doesn't match exactly the first one. This ensure the
+ * seccomp filter results are still the sames but a tracer process can still
+ * change the tracee syscall arguments.
+ *
+ * If the syscall take multiple paths and the same address is used but only one
+ * argument is checked by the filter, the inode will be checked for all paths
+ * with this same address, detecting a TOCTOU for all of them even if they were
+ * not evaluated by the filter.
+ */
+static u8 argeval_find_args_file(const struct file *file)
+{
+ const struct syscall_argdesc *argdesc;
+ struct dentry *dentry;
+ u8 result = 0;
+ struct seccomp_argeval_cache *arg_cache = current->seccomp.arg_cache;
+
+ if (unlikely(!file)) {
+ WARN_ON(1);
+ return 0;
+ }
+
+ /* Create the argument mask matching the uptr.
+ * The syscall arguments may have been changed by a tracer.
+ */
+ argdesc = syscall_nr_to_argdesc(syscall_get_nr(current,
+ task_pt_regs(current)));
+ if (unlikely(!argdesc)) {
+ WARN_ON(1);
+ return 0;
+ }
+ dentry = file->f_path.dentry;
+
+ /* Look in the cache for this path */
+ for (; arg_cache; arg_cache = arg_cache->next) {
+ struct seccomp_argeval_cache_entry *entry = arg_cache->entry;
+
+ switch (arg_cache->type) {
+ case SECCOMP_OBJTYPE_PATH:
+ /* Ignore the mount point to not be fooled by a
+ * malicious one. Only look for a previously
+ * seen dentry.
+ */
+ for (; entry; entry = entry->next) {
+ /* Check for the same filename/argument.
+ * If the hash and the length are the same
+ * but the path is different we treat it
+ * as a race-condition.
+ */
+ if (entry->fs.hash_len !=
+ dentry->d_name.hash_len)
+ continue;
+ /* Ignore exact match (i.e. pointed file didn't
+ * change)
+ */
+ if (entry->fs.path
+ && entry->fs.path->dentry == dentry)
+ continue;
+ /* TODO: Add process info/audit */
+ pr_warn("seccomp: TOCTOU race-condition detected!\n");
+ result |= entry->args;
+ }
+ break;
+ default:
+ WARN_ON(1);
+ }
+ }
+ return result;
+}
+
+/**
+ * argeval_history_recheck_file - recheck with the seccomp filters if needed
+ */
+static bool argeval_history_recheck_file(const struct seccomp_argeval_history
+ *history, seccomp_argrule_t *engine,
+ const struct syscall_argdesc *argdesc,
+ const u64(*args)[6], u8 arg_matches)
+{
+ /* Flush the cache to not rely on the first seccomp filter check
+ * results
+ */
+ flush_seccomp_cache(current);
+
+ while (history) {
+ /* Only check the changed arguments */
+ if (history->asked & arg_matches) {
+ u8 match = (*engine)(&argdesc->args, args,
+ history->asked, history->checker);
+
+ if (match != history->result)
+ return true;
+ }
+ history = history->next;
+ }
+ return false;
+}
+
+int seccomp_check_file(const struct file *file)
+{
+ int result = -EPERM;
+ const struct syscall_argdesc *argdesc;
+ struct seccomp_argeval_syscall *orig_syscall;
+ struct seccomp_argeval_checked *arg_checked;
+ seccomp_argrule_t *engine;
+ u8 arg_matches;
+
+ /* @file may be null (e.g. security_mmap_file) */
+ if (!file)
+ return 0;
+ /* Early check to exit quickly if no history */
+ arg_checked = current->seccomp.arg_checked;
+ if (!arg_checked)
+ return 0;
+ orig_syscall = current->seccomp.orig_syscall;
+ if (unlikely(!orig_syscall)) {
+ WARN_ON(1);
+ return 0;
+ }
+ /* Check if anything changed from the cache */
+ arg_matches = argeval_find_args_file(file);
+ if (!arg_matches)
+ return 0;
+ /* The syscall may have been changed by the tracer process */
+ argdesc = syscall_nr_to_argdesc(orig_syscall->nr);
+ if (!argdesc) {
+ WARN_ON(1);
+ goto out;
+ }
+ do {
+ engine = get_argrule_checker(arg_checked->check);
+ /* The syscall arguments may have been changed by the tracer
+ * process
+ */
+ /* FIXME: Adapt the checker to "struct file *" to avoid races */
+ result =
+ argeval_history_recheck_file(arg_checked->history, engine,
+ argdesc, &orig_syscall->args,
+ arg_matches) ? -EPERM : 0;
+ arg_checked = arg_checked->next;
+ } while (arg_checked);
+
+out:
+ return result;
+}
+
static long set_argtype_path(const struct seccomp_checker *user_checker,
struct seccomp_filter_checker *kernel_checker)
{
diff --git a/security/seccomp/checker_fs.h b/security/seccomp/checker_fs.h
new file mode 100644
index 000000000000..7ac102b510ec
--- /dev/null
+++ b/security/seccomp/checker_fs.h
@@ -0,0 +1,18 @@
+/*
+ * Seccomp Linux Security Module - File System Checkers
+ *
+ * Copyright (C) 2016 Mickaël Salaün <mic@xxxxxxxxxxx>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2, as
+ * published by the Free Software Foundation.
+ */
+
+#ifndef _SECURITY_SECCOMP_CHECKER_FS_H
+#define _SECURITY_SECCOMP_CHECKER_FS_H
+
+#include <linux/fs.h>
+
+int seccomp_check_file(const struct file *);
+
+#endif /* _SECURITY_SECCOMP_CHECKER_FS_H */
diff --git a/security/seccomp/lsm.c b/security/seccomp/lsm.c
index 93c881724341..7bde63505dbd 100644
--- a/security/seccomp/lsm.c
+++ b/security/seccomp/lsm.c
@@ -10,9 +10,13 @@
#include <asm/syscall.h> /* sys_call_table */
#include <linux/compat.h>
+#include <linux/cred.h>
+#include <linux/fs.h>
+#include <linux/lsm_hooks.h>
#include <linux/slab.h> /* kcalloc() */
#include <linux/syscalls.h> /* syscall_argdesc */
+#include "checker_fs.h"
#include "lsm.h"
/* TODO: Remove the need for CONFIG_SYSFS dependency */
@@ -22,6 +26,49 @@ struct syscall_argdesc (*seccomp_syscalls_argdesc)[] = NULL;
struct syscall_argdesc (*compat_seccomp_syscalls_argdesc)[] = NULL;
#endif /* CONFIG_COMPAT */
+#define SECCOMP_HOOK(CHECK, NAME, ...) \
+ static inline int seccomp_hook_##NAME(__VA_ARGS__) \
+ { \
+ return seccomp_check_##CHECK(CHECK); \
+ }
+
+#define SECCOMP_HOOK_INIT(NAME) LSM_HOOK_INIT(NAME, seccomp_hook_##NAME)
+
+/* TODO: file_set_fowner, file_alloc_security? */
+
+SECCOMP_HOOK(file, binder_transfer_file, struct task_struct *from, struct task_struct *to, struct file *file)
+SECCOMP_HOOK(file, file_permission, struct file *file, int mask)
+SECCOMP_HOOK(file, file_ioctl, struct file *file, unsigned int cmd, unsigned long arg)
+SECCOMP_HOOK(file, mmap_file, struct file *file, unsigned long reqprot, unsigned long prot, unsigned long flags)
+SECCOMP_HOOK(file, file_lock, struct file *file, unsigned int cmd)
+SECCOMP_HOOK(file, file_fcntl, struct file *file, unsigned int cmd, unsigned long arg)
+SECCOMP_HOOK(file, file_receive, struct file *file)
+SECCOMP_HOOK(file, file_open, struct file *file, const struct cred *cred)
+SECCOMP_HOOK(file, kernel_fw_from_file, struct file *file, char *buf, size_t size)
+SECCOMP_HOOK(file, kernel_module_from_file, struct file *file)
+
+/* TODO: Add hooks with:
+ * - struct dentry *
+ * - struct path *
+ * - struct inode *
+ * ...
+ */
+
+
+static struct security_hook_list seccomp_hooks[] = {
+ SECCOMP_HOOK_INIT(binder_transfer_file),
+ SECCOMP_HOOK_INIT(file_permission),
+ SECCOMP_HOOK_INIT(file_ioctl),
+ SECCOMP_HOOK_INIT(mmap_file),
+ SECCOMP_HOOK_INIT(file_lock),
+ SECCOMP_HOOK_INIT(file_fcntl),
+ SECCOMP_HOOK_INIT(file_receive),
+ SECCOMP_HOOK_INIT(file_open),
+ SECCOMP_HOOK_INIT(kernel_fw_from_file),
+ SECCOMP_HOOK_INIT(kernel_module_from_file),
+};
+
+
static const struct syscall_argdesc *__init
find_syscall_argdesc(const struct syscall_argdesc *start,
const struct syscall_argdesc *stop, const void *addr)
@@ -84,4 +131,5 @@ void __init seccomp_init(void)
{
pr_info("seccomp: Becoming ready for sandboxing\n");
init_argdesc();
+ security_add_hooks(seccomp_hooks, ARRAY_SIZE(seccomp_hooks));
}
--
2.8.0.rc3
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