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audit: only allow tasks to set their loginuid if it is -1

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At the moment we allow tasks to set their loginuid if they have
CAP_AUDIT_CONTROL.  In reality we want tasks to set the loginuid when they
log in and it be impossible to ever reset.  We had to make it mutable even
after it was once set (with the CAP) because on update and admin might have
to restart sshd.  Now sshd would get his loginuid and the next user which
logged in using ssh would not be able to set his loginuid.

Systemd has changed how userspace works and allowed us to make the kernel
work the way it should.  With systemd users (even admins) are not supposed
to restart services directly.  The system will restart the service for
them.  Thus since systemd is going to loginuid==-1, sshd would get -1, and
sshd would be allowed to set a new loginuid without special permissions.

If an admin in this system were to manually start an sshd he is inserting
himself into the system chain of trust and thus, logically, it's his
loginuid that should be used!  Since we have old systems I make this a
Kconfig option.

Signed-off-by: Eric Paris <eparis@redhat.com>
This commit is contained in:
Eric Paris 2012-01-03 14:23:08 -05:00 committed by Al Viro
parent 0a300be6d5
commit 633b454545
3 changed files with 24 additions and 4 deletions
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3
fs/proc/base.c
View File

@ -1197,9 +1197,6 @@ static ssize_t proc_loginuid_write(struct file * file, const char __user * buf,
ssize_t length;
uid_t loginuid;
if (!capable(CAP_AUDIT_CONTROL))
return -EPERM;
rcu_read_lock();
if (current != pid_task(proc_pid(inode), PIDTYPE_PID)) {
rcu_read_unlock();

14
init/Kconfig
View File

@ -372,6 +372,20 @@ config AUDIT_TREE
depends on AUDITSYSCALL
select FSNOTIFY
config AUDIT_LOGINUID_IMMUTABLE
bool "Make audit loginuid immutable"
depends on AUDIT
help
The config option toggles if a task setting it's loginuid requires
CAP_SYS_AUDITCONTROL or if that task should require no special permissions
but should instead only allow setting its loginuid if it was never
previously set. On systems which use systemd or a similar central
process to restart login services this should be set to true. On older
systems in which an admin would typically have to directly stop and
start processes this should be set to false. Setting this to true allows
one to drop potentially dangerous capabilites from the login tasks,
but may not be backwards compatible with older init systems.
source "kernel/irq/Kconfig"
menu "RCU Subsystem"

11
kernel/auditsc.c
View File

@ -2173,9 +2173,18 @@ static atomic_t session_id = ATOMIC_INIT(0);
int audit_set_loginuid(uid_t loginuid)
{
struct task_struct *task = current;
unsigned int sessionid = atomic_inc_return(&session_id);
struct audit_context *context = task->audit_context;
unsigned int sessionid;
#ifdef CONFIG_AUDIT_LOGINUID_IMMUTABLE
if (task->loginuid != -1)
return -EPERM;
#else /* CONFIG_AUDIT_LOGINUID_IMMUTABLE */
if (!capable(CAP_AUDIT_CONTROL))
return -EPERM;
#endif /* CONFIG_AUDIT_LOGINUID_IMMUTABLE */
sessionid = atomic_inc_return(&session_id);
if (context && context->in_syscall) {
struct audit_buffer *ab;