On Fri, Sep 23, 2016 at 1:40 PM, Jann Horn <jann@xxxxxxxxx> wrote:
> This ensures that self_privunit ("privilege unit locally unique ID")
> is only shared by processes that share the mm_struct and the signal_struct;
> not just spatially, but also temporally. In other words, if you do execve()
> or clone() without CLONE_THREAD, you get a new privunit that has never
> been used before.
>
> One reason for doing this is that it prevents an attacker from sending an
> arbitrary signal to a parent process after performing 2^32-1 execve()
> calls.
>
> The second reason for this is that it permits using the self_exec_luid in
> a later patch to check during a ptrace access whether subject and object
> are temporally and spatially equal for privilege checking purposes.
>
> The implementation of locally unique IDs is in sched.h and exec.c for now
> because those are the only users so far - if anything else wants to use
> them in the future, they can be moved elsewhere.
>
> changed in v2:
> - have 2^64 IDs per CPU instead of 2^64 shared ones (luid scheme,
> suggested by Andy Lutomirski)
> - take task_lock for reading in setup_new_exec() while bumping the LUID
>
> Signed-off-by: Jann Horn <jann@xxxxxxxxx>
> ---
> fs/exec.c | 41 +++++++++++++++++++++++++++++++++++++++--
> include/linux/sched.h | 17 +++++++++++++++--
> kernel/fork.c | 5 +++--
> kernel/signal.c | 5 ++++-
> 4 files changed, 61 insertions(+), 7 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index 84430ee..fcc11f0 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1281,6 +1281,34 @@ void would_dump(struct linux_binprm *bprm, struct file *file)
> }
> EXPORT_SYMBOL(would_dump);
>
> +static DEFINE_PER_CPU(u64, luid_counters);
> +
> +static int __init init_luid_counters(void)
> +{
> + unsigned int cpu;
> +
> + for_each_possible_cpu(cpu) {
> + /* value 0 is reserved for init */
> + per_cpu(luid_counters, cpu) = 1;
> + }
> +
> + return 0;
> +}
> +early_initcall(init_luid_counters);
How about static DEFINE_PER_CPU(u64, luid_counters) = 1? You could
optionally use local64_t instead, which would let you avoid needing to
think about preemption.
> +
> +/*
> + * Allocates a new LUID and writes the allocated LUID to @out.
> + * This function must not be called from IRQ context.
> + */
> +void fill_luid(struct luid *out)
> +{
> + preempt_disable();
> + out->count = raw_cpu_read(luid_counters);
> + raw_cpu_add(luid_counters, 1);
> + out->cpu = smp_processor_id();
> + preempt_enable();
> +}
> +
I would call this alloc_luid().
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