audit: rework execve audit

The purpose of audit_bprm() is to log the argv array to a userspace daemon at
the end of the execve system call.  Since user-space hasn't had time to run,
this array is still in pristine state on the process' stack; so no need to
copy it, we can just grab it from there.

In order to minimize the damage to audit_log_*() copy each string into a
temporary kernel buffer first.

Currently the audit code requires that the full argument vector fits in a
single packet.  So currently it does clip the argv size to a (sysctl) limit,
but only when execve auditing is enabled.

If the audit protocol gets extended to allow for multiple packets this check
can be removed.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ollie Wild <aaw@google.com>
Cc: <linux-audit@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 74 insertions, 21 deletions

diff --git a/kernel/auditsc.c b/kernel/auditsc.c
index b7640a5f382a..535586fc498b 100644
--- a/kernel/auditsc.c
+++ b/kernel/auditsc.c
@@ -153,7 +153,7 @@ struct audit_aux_data_execve {
         struct audit_aux_data   d;
         int argc;
         int envc;
-        char mem[0];
+        struct mm_struct *mm;
 };
 
 struct audit_aux_data_socketcall {
@@ -831,6 +831,55 @@ static int audit_log_pid_context(struct audit_context *context, pid_t pid,
         return rc;
 }
 
+static void audit_log_execve_info(struct audit_buffer *ab,
+                struct audit_aux_data_execve *axi)
+{
+        int i;
+        long len, ret;
+        const char __user *p = (const char __user *)axi->mm->arg_start;
+        char *buf;
+
+        if (axi->mm != current->mm)
+                return; /* execve failed, no additional info */
+
+        for (i = 0; i < axi->argc; i++, p += len) {
+                len = strnlen_user(p, MAX_ARG_PAGES*PAGE_SIZE);
+                /*
+                 * We just created this mm, if we can't find the strings
+                 * we just copied into it something is _very_ wrong. Similar
+                 * for strings that are too long, we should not have created
+                 * any.
+                 */
+                if (!len || len > MAX_ARG_STRLEN) {
+                        WARN_ON(1);
+                        send_sig(SIGKILL, current, 0);
+                }
+
+                buf = kmalloc(len, GFP_KERNEL);
+                if (!buf) {
+                        audit_panic("out of memory for argv string\n");
+                        break;
+                }
+
+                ret = copy_from_user(buf, p, len);
+                /*
+                 * There is no reason for this copy to be short. We just
+                 * copied them here, and the mm hasn't been exposed to user-
+                 * space yet.
+                 */
+                if (!ret) {
+                        WARN_ON(1);
+                        send_sig(SIGKILL, current, 0);
+                }
+
+                audit_log_format(ab, "a%d=", i);
+                audit_log_untrustedstring(ab, buf);
+                audit_log_format(ab, "\n");
+
+                kfree(buf);
+        }
+}
+
 static void audit_log_exit(struct audit_context *context, struct task_struct *tsk)
 {
         int i, call_panic = 0;
@@ -971,13 +1020,7 @@ static void audit_log_exit(struct audit_context *context, struct task_struct *ts
 
                 case AUDIT_EXECVE: {
                         struct audit_aux_data_execve *axi = (void *)aux;
-                        int i;
-                        const char *p;
-                        for (i = 0, p = axi->mem; i < axi->argc; i++) {
-                                audit_log_format(ab, "a%d=", i);
-                                p = audit_log_untrustedstring(ab, p);
-                                audit_log_format(ab, "\n");
-                        }
+                        audit_log_execve_info(ab, axi);
                         break; }
 
                 case AUDIT_SOCKETCALL: {
@@ -1821,32 +1864,31 @@ int __audit_ipc_set_perm(unsigned long qbytes, uid_t uid, gid_t gid, mode_t mode
         return 0;
 }
 
+int audit_argv_kb = 32;
+
 int audit_bprm(struct linux_binprm *bprm)
 {
         struct audit_aux_data_execve *ax;
         struct audit_context *context = current->audit_context;
-        unsigned long p, next;
-        void *to;
 
         if (likely(!audit_enabled || !context || context->dummy))
                 return 0;
 
-        ax = kmalloc(sizeof(*ax) + PAGE_SIZE * MAX_ARG_PAGES - bprm->p,
-                                GFP_KERNEL);
+        /*
+         * Even though the stack code doesn't limit the arg+env size any more,
+         * the audit code requires that _all_ arguments be logged in a single
+         * netlink skb. Hence cap it :-(
+         */
+        if (bprm->argv_len > (audit_argv_kb << 10))
+                return -E2BIG;
+
+        ax = kmalloc(sizeof(*ax), GFP_KERNEL);
         if (!ax)
                 return -ENOMEM;
 
         ax->argc = bprm->argc;
         ax->envc = bprm->envc;
-        for (p = bprm->p, to = ax->mem; p < MAX_ARG_PAGES*PAGE_SIZE; p = next) {
-                struct page *page = bprm->page[p / PAGE_SIZE];
-                void *kaddr = kmap(page);
-                next = (p + PAGE_SIZE) & ~(PAGE_SIZE - 1);
-                memcpy(to, kaddr + (p & (PAGE_SIZE - 1)), next - p);
-                to += next - p;
-                kunmap(page);
-        }
-
+        ax->mm = bprm->mm;
         ax->d.type = AUDIT_EXECVE;
         ax->d.next = context->aux;
         context->aux = (void *)ax;
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 3ed4912bf183..8db41764e2a1 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -78,6 +78,7 @@ extern int percpu_pagelist_fraction;
 extern int compat_log;
 extern int maps_protect;
 extern int sysctl_stat_interval;
+extern int audit_argv_kb;
 
 /* this is needed for the proc_dointvec_minmax for [fs_]overflow UID and GID */
 static int maxolduid = 65535;
@@ -306,6 +307,16 @@ static ctl_table kern_table[] = {
                 .mode           = 0644,
                 .proc_handler   = &proc_dointvec,
         },
+#ifdef CONFIG_AUDITSYSCALL
+        {
+                .ctl_name       = CTL_UNNUMBERED,
+                .procname       = "audit_argv_kb",
+                .data           = &audit_argv_kb,
+                .maxlen         = sizeof(int),
+                .mode           = 0644,
+                .proc_handler   = &proc_dointvec,
+        },
+#endif
         {
                 .ctl_name       = KERN_CORE_PATTERN,
                 .procname       = "core_pattern",