Re: [PATCH v7 3/9] seccomp: introduce writer locking
From: Oleg Nesterov <oleg@redhat.com>
Date: 2014-06-24 18:32:32
Also in:
linux-arch, linux-arm-kernel, linux-mips, lkml
I am puzzled by the usage of smp_load_acquire(), On 06/23, Kees Cook wrote:
quoted hunk ↗ jump to hunk
static u32 seccomp_run_filters(int syscall) { - struct seccomp_filter *f; + struct seccomp_filter *f = smp_load_acquire(¤t->seccomp.filter); struct seccomp_data sd; u32 ret = SECCOMP_RET_ALLOW; /* Ensure unexpected behavior doesn't result in failing open. */ - if (WARN_ON(current->seccomp.filter == NULL)) + if (WARN_ON(f == NULL)) return SECCOMP_RET_KILL; populate_seccomp_data(&sd);@@ -186,9 +186,8 @@ static u32 seccomp_run_filters(int syscall) * All filters in the list are evaluated and the lowest BPF return * value always takes priority (ignoring the DATA). */ - for (f = current->seccomp.filter; f; f = f->prev) { + for (; f; f = smp_load_acquire(&f->prev)) { u32 cur_ret = SK_RUN_FILTER(f->prog, (void *)&sd); - if ((cur_ret & SECCOMP_RET_ACTION) < (ret & SECCOMP_RET_ACTION)) ret = cur_ret;
OK, in this case the 1st one is probably fine, altgough it is not clear to me why it is better than read_barrier_depends(). But why do we need a 2nd one inside the loop? And if we actually need it (I don't think so) then why it is safe to use f->prog without load_acquire ?
void get_seccomp_filter(struct task_struct *tsk)
{
- struct seccomp_filter *orig = tsk->seccomp.filter;
+ struct seccomp_filter *orig = smp_load_acquire(&tsk->seccomp.filter);
if (!orig)
return;This one looks unneeded. First of all, afaics atomic_inc() should work correctly without any barriers, otherwise it is buggy. But even this doesn't matter. With this changes get_seccomp_filter() must be called under ->siglock, it can't race with add-filter and thus tsk->seccomp.filter should be stable.
quoted hunk ↗ jump to hunk
/* Reference count is bounded by the number of total processes. */@@ -361,7 +364,7 @@ void put_seccomp_filter(struct task_struct *tsk) /* Clean up single-reference branches iteratively. */ while (orig && atomic_dec_and_test(&orig->usage)) { struct seccomp_filter *freeme = orig; - orig = orig->prev; + orig = smp_load_acquire(&orig->prev); seccomp_filter_free(freeme); }
This one looks unneeded too. And note that this patch does not add smp_load_acquire() to read tsk->seccomp.filter. atomic_dec_and_test() adds mb(), we do not need more barriers to access ->prev ? Oleg.