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    Subject[PATCH] pid: change task_struct::pid to read-only
    Date
    task->pid is only ever assigned once (well ok, twice).  For system health and
    secure logging confidence, make it const to make it much more intentional when
    it is being changed.
    ---

    Peter, as you had suggested, does this approach work for you in terms of making
    task_struct::pid a lot more difficult to accidentally change to try to preserve
    its integrity?

    Is the use of memcpy() significantly different from *p = *q ?


    arch/x86/kernel/process.c | 2 +-
    fs/exec.c | 4 +++-
    include/linux/sched.h | 2 +-
    kernel/fork.c | 8 ++++++--
    4 files changed, 11 insertions(+), 5 deletions(-)

    diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
    index c83516b..4170026 100644
    --- a/arch/x86/kernel/process.c
    +++ b/arch/x86/kernel/process.c
    @@ -66,7 +66,7 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
    {
    int ret;

    - *dst = *src;
    + memcpy(dst, src, sizeof(struct task_struct));
    if (fpu_allocated(&src->thread.fpu)) {
    memset(&dst->thread.fpu, 0, sizeof(dst->thread.fpu));
    ret = fpu_alloc(&dst->thread.fpu);
    diff --git a/fs/exec.c b/fs/exec.c
    index 47d7edb..c8d0159 100644
    --- a/fs/exec.c
    +++ b/fs/exec.c
    @@ -908,6 +908,8 @@ static int de_thread(struct task_struct *tsk)
    */
    if (!thread_group_leader(tsk)) {
    struct task_struct *leader = tsk->group_leader;
    + /* tast_struct::pid is const pid_t, hence the ugly cast */
    + pid_t *pid_p = (pid_t*)&(tsk->pid);

    sig->notify_count = -1; /* for exit_notify() */
    for (;;) {
    @@ -950,7 +952,7 @@ static int de_thread(struct task_struct *tsk)
    * Note: The old leader also uses this pid until release_task
    * is called. Odd but simple and correct.
    */
    - tsk->pid = leader->pid;
    + *pid_p = leader->pid;
    change_pid(tsk, PIDTYPE_PID, task_pid(leader));
    transfer_pid(leader, tsk, PIDTYPE_PGID);
    transfer_pid(leader, tsk, PIDTYPE_SID);
    diff --git a/include/linux/sched.h b/include/linux/sched.h
    index d9ada71..45069c0 100644
    --- a/include/linux/sched.h
    +++ b/include/linux/sched.h
    @@ -1112,7 +1112,7 @@ struct task_struct {
    unsigned sched_reset_on_fork:1;
    unsigned sched_contributes_to_load:1;

    - pid_t pid;
    + const pid_t pid;
    pid_t tgid;

    #ifdef CONFIG_CC_STACKPROTECTOR
    diff --git a/kernel/fork.c b/kernel/fork.c
    index 086fe73..ec0683d 100644
    --- a/kernel/fork.c
    +++ b/kernel/fork.c
    @@ -286,7 +286,7 @@ void __init fork_init(unsigned long mempages)
    int __attribute__((weak)) arch_dup_task_struct(struct task_struct *dst,
    struct task_struct *src)
    {
    - *dst = *src;
    + memcpy(dst, src, sizeof(struct task_struct));
    return 0;
    }

    @@ -1137,6 +1137,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
    {
    int retval;
    struct task_struct *p;
    + pid_t *pid_p;

    if ((clone_flags & (CLONE_NEWNS|CLONE_FS)) == (CLONE_NEWNS|CLONE_FS))
    return ERR_PTR(-EINVAL);
    @@ -1392,7 +1393,10 @@ static struct task_struct *copy_process(unsigned long clone_flags,
    clear_all_latency_tracing(p);

    /* ok, now we should be set up.. */
    - p->pid = pid_nr(pid);
    +
    + /* tast_struct::pid is const pid_t, hence the ugly cast */
    + pid_p = (pid_t*)&(p->pid);
    + *pid_p = pid_nr(pid);
    if (clone_flags & CLONE_THREAD) {
    p->exit_signal = -1;
    p->group_leader = current->group_leader;
    --
    1.7.1


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