On Fri, 6 Apr 2012 13:44:43 -0700 Kees Cook <keescook@xxxxxxxxxxxx> wrote: > On Fri, Apr 6, 2012 at 1:23 PM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > On Thu, 29 Mar 2012 15:01:53 -0500 > > Will Drewry <wad@xxxxxxxxxxxx> wrote: > > > >> [This patch depends on luto@xxxxxxx's no_new_privs patch: > >> https://lkml.org/lkml/2012/1/30/264 > >> included in this series for ease of consumption. > >> ] > >> > >> This patch adds support for seccomp mode 2. Mode 2 introduces the > >> ability for unprivileged processes to install system call filtering > >> policy expressed in terms of a Berkeley Packet Filter (BPF) program. > >> This program will be evaluated in the kernel for each system call > >> the task makes and computes a result based on data in the format > >> of struct seccomp_data. > >> ... > >> +static void seccomp_filter_log_failure(int syscall) > >> +{ > >> + int compat = 0; > >> +#ifdef CONFIG_COMPAT > >> + compat = is_compat_task(); > >> +#endif > > > > hm, I'm surprised that we don't have a zero-returning implementation of > > is_compat_task() when CONFIG_COMPAT=n. Seems silly. Blames Arnd. > > There is I can't find it. The definition in include/linux/compat.h is inside #ifdef CONFIG_COMPAT. > >> +static long seccomp_attach_filter(struct sock_fprog *fprog) > >> +{ > >> + struct seccomp_filter *filter; > >> + unsigned long fp_size = fprog->len * sizeof(struct sock_filter); > >> + unsigned long total_insns = fprog->len; > >> + long ret; > >> + > >> + if (fprog->len == 0 || fprog->len > BPF_MAXINSNS) > >> + return -EINVAL; > >> + > >> + for (filter = current->seccomp.filter; filter; filter = filter->prev) > >> + total_insns += filter->len + 4; /* include a 4 instr penalty */ > > > > So tasks don't share filters? We copy them by value at fork? Do we do > > this at vfork() too? > > The filter chain is shared (and refcounted). So what's the locking rule for accessing and modifying that singly-linked list? > ... > >> +/* put_seccomp_filter - decrements the ref count of tsk->seccomp.filter */ > >> +void put_seccomp_filter(struct task_struct *tsk) > >> +{ > >> + struct seccomp_filter *orig = tsk->seccomp.filter; > >> + /* Clean up single-reference branches iteratively. */ > >> + while (orig && atomic_dec_and_test(&orig->usage)) { > >> + struct seccomp_filter *freeme = orig; > >> + orig = orig->prev; > >> + kfree(freeme); > >> + } > >> +} > > > > So if one of the filters in the list has an elevated refcount, we bail > > out on the remainder of the list. Seems odd. > > This so that every filter in the list doesn't need to have their > refcount raised. As long as the counting up matching the counting > down, it's fine. This allows for process trees branching the filter > list at different times still being safe. IIUC, this code was based on > how namespace refcounting is handled. I spent some time proving to > myself that it was correctly refcounted a while back. More eyes is > better, of course. :) Please ensure that future readers of this code have a description of how it is supposed to work. -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
