On Tue, Apr 26, 2016 at 10:20 AM, Janis Danisevskis <jda...@google.com> wrote:
> The PR_DUMPABLE flag causes the pid related paths of the
> proc file system to be owned by ROOT. The implementation
> of pthread_set/getname_np however needs access to
> /proc/<pid>/task/<tid>/comm.
> If PR_DUMPABLE is false this implementation is locked out.
>
> This patch installs a special permission function for
> the file "comm" that grants read and write access to
> all threads of the same group regardless of the ownership
> of the inode. For all other threads the function falls back
> to the generic inode permission check.
>
> Signed-off-by: Janis Danisevskis <jda...@google.com>
Instead of a permissions function, perhaps this should be handled in
the open() of proc_pid_set_comm_operations (and the REG permissions
loosened)? I'm concerned there's a race here between the perm check
and the resulting open. I'd rather have the open doing the check to
eliminate the race.
-Kees
> ---
> fs/proc/base.c | 42 +++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 41 insertions(+), 1 deletion(-)
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index b1755b2..c8ceb3c8 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -3157,6 +3157,44 @@ int proc_pid_readdir(struct file *file, struct
> dir_context *ctx)
> }
>
> /*
> + * proc_tid_comm_permission is a special permission function exclusively
> + * used for the node /proc/<pid>/task/<tid>/comm.
> + * It bypasses generic permission checks in the case where a task of the same
> + * task group attempts to access the node.
> + * The rational behind this is that glibc and bionic access this node for
> + * cross thread naming (pthread_set/getname_np(!self)). However, if
> + * PR_SET_DUMPABLE gets set to 0 this node among others becomes uid=0 gid=0,
> + * which locks out the cross thread naming implementation.
> + * This function makes sure that the node is always accessible for members of
> + * same thread group.
> + */
> +static int proc_tid_comm_permission(struct inode *inode, int mask)
> +{
> + bool is_same_tgroup;
> + struct task_struct *task;
> +
> + task = get_proc_task(inode);
> + if (!task)
> + return -ESRCH;
> + is_same_tgroup = same_thread_group(current, task);
> + put_task_struct(task);
> +
> + if (likely(is_same_tgroup && !(mask & MAY_EXEC))) {
> + /* This file (/proc/<pid>/task/<tid>/comm) can always be
> + * read or written by the members of the corresponding
> + * thread group.
> + */
> + return 0;
> + }
> +
> + return generic_permission(inode, mask);
> +}
> +
> +static const struct inode_operations proc_tid_comm_inode_operations = {
> + .permission = proc_tid_comm_permission,
> +};
> +
> +/*
> * Tasks
> */
> static const struct pid_entry tid_base_stuff[] = {
> @@ -3174,7 +3212,9 @@ static const struct pid_entry tid_base_stuff[] = {
> #ifdef CONFIG_SCHED_DEBUG
> REG("sched", S_IRUGO|S_IWUSR, proc_pid_sched_operations),
> #endif
> - REG("comm", S_IRUGO|S_IWUSR, proc_pid_set_comm_operations),
> + NOD("comm", S_IFREG|S_IRUGO|S_IWUSR,
> + &proc_tid_comm_inode_operations,
> + &proc_pid_set_comm_operations, {}),
> #ifdef CONFIG_HAVE_ARCH_TRACEHOOK
> ONE("syscall", S_IRUSR, proc_pid_syscall),
> #endif
> --
> 2.8.0.rc3.226.g39d4020
>
--
Kees Cook
Chrome OS & Brillo Security
