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Subject[PATCH 2/4] 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
---
fs/exec.c | 3 +
include/linux/binfmts.h | 1
include/linux/sysctl.h | 1
kernel/audit.c | 16 +++++++++
kernel/audit.h | 1
kernel/auditsc.c | 82 ++++++++++++++++++++++++++++++++----------------
kernel/sysctl.c | 11 ++++++
7 files changed, 89 insertions(+), 26 deletions(-)

Index: linux-2.6-2/kernel/auditsc.c
===================================================================
--- linux-2.6-2.orig/kernel/auditsc.c 2007-06-05 09:51:53.000000000 +0200
+++ linux-2.6-2/kernel/auditsc.c 2007-06-05 10:03:31.000000000 +0200
@@ -156,7 +156,7 @@ 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 {
@@ -834,6 +834,47 @@ static int audit_log_pid_context(struct
return rc;
}

+static void audit_log_execve_info(struct audit_buffer *ab,
+ struct audit_aux_data_execve *axi)
+{
+ int i;
+ long len;
+ const char __user *p = (const char __user *)axi->mm->arg_start;
+
+ if (axi->mm != current->mm)
+ return; /* execve failed, no additional info */
+
+ for (i = 0; i < axi->argc; i++, p += len) {
+ long ret;
+ char *tmp;
+
+ len = strnlen_user(p, MAX_ARG_PAGES*PAGE_SIZE);
+ /*
+ * We just created this mm, if we can't find the strings
+ * we just copied in something is _very_ wrong.
+ */
+ BUG_ON(!len);
+
+ tmp = kmalloc(len, GFP_KERNEL);
+ if (!tmp) {
+ audit_panic("out of memory for argv string\n");
+ break;
+ }
+
+ ret = copy_from_user(tmp, p, len);
+ /*
+ * There is no reason for this copy to be short.
+ */
+ BUG_ON(ret);
+
+ audit_log_format(ab, "a%d=", i);
+ audit_log_untrustedstring(ab, tmp);
+ audit_log_format(ab, "\n");
+
+ kfree(tmp);
+ }
+}
+
static void audit_log_exit(struct audit_context *context, struct task_struct *tsk)
{
int i, call_panic = 0;
@@ -974,13 +1016,7 @@ static void audit_log_exit(struct audit_

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: {
@@ -1824,32 +1860,31 @@ int __audit_ipc_set_perm(unsigned long q
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;
Index: linux-2.6-2/fs/exec.c
===================================================================
--- linux-2.6-2.orig/fs/exec.c 2007-06-05 09:51:42.000000000 +0200
+++ linux-2.6-2/fs/exec.c 2007-06-05 10:03:11.000000000 +0200
@@ -1154,6 +1154,7 @@ int do_execve(char * filename,
{
struct linux_binprm *bprm;
struct file *file;
+ unsigned long tmp;
int retval;
int i;

@@ -1208,9 +1209,11 @@ int do_execve(char * filename,
if (retval < 0)
goto out;

+ tmp = bprm->p;
retval = copy_strings(bprm->argc, argv, bprm);
if (retval < 0)
goto out;
+ bprm->argv_len = tmp - bprm->p;

retval = search_binary_handler(bprm,regs);
if (retval >= 0) {
Index: linux-2.6-2/include/linux/binfmts.h
===================================================================
--- linux-2.6-2.orig/include/linux/binfmts.h 2007-06-05 09:51:44.000000000 +0200
+++ linux-2.6-2/include/linux/binfmts.h 2007-06-05 10:03:11.000000000 +0200
@@ -40,6 +40,7 @@ struct linux_binprm{
unsigned interp_flags;
unsigned interp_data;
unsigned long loader, exec;
+ unsigned long argv_len;
};

#define BINPRM_FLAGS_ENFORCE_NONDUMP_BIT 0
Index: linux-2.6-2/kernel/sysctl.c
===================================================================
--- linux-2.6-2.orig/kernel/sysctl.c 2007-06-05 09:51:53.000000000 +0200
+++ linux-2.6-2/kernel/sysctl.c 2007-06-05 10:04:05.000000000 +0200
@@ -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;
@@ -615,6 +616,16 @@ static ctl_table kern_table[] = {
.proc_handler = &proc_dointvec,
},
#endif
+#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 = 0 }
};
--

-
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