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    Subject[PATCH v2 3/5] ptrace: make PTRACE_SEIZE set ptrace options specified in 'data' parameter
    Date
    This can be used to close a few corner cases in strace where we get
    unwanted racy behavior after attach, but before we have a chance
    to set options (the notorious post-execve SIGTRAP comes to mind),
    and removes the need to track "did we set opts for this task" state
    in strace internals.

    While we are at it:

    Make it possible to extend SEIZE in the future with more functionality
    by passing non-zero 'addr' parameter.
    To that end, error out if 'addr' is non-zero.
    PTRACE_ATTACH did not (and still does not) have such check,
    and users (strace) do pass garbage there... let's avoid repeating
    this mistake with SEIZE.

    Set all task->ptrace bits in one operation - before this change,
    we were adding PT_SEIZED and PT_PTRACE_CAP with task->ptrace |= BIT ops.
    This was probably ok (not a bug), but let's be on a safer side.

    Changes in v2: update ptrace_attach() call in compat_sys_ptrace() too.

    Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
    Acked-by: Tejun Heo <tj@kernel.org>
    ---
    kernel/ptrace.c | 31 +++++++++++++++++++++----------
    1 files changed, 21 insertions(+), 10 deletions(-)

    diff --git a/kernel/ptrace.c b/kernel/ptrace.c
    index 9acd07a..4661c5b 100644
    --- a/kernel/ptrace.c
    +++ b/kernel/ptrace.c
    @@ -231,6 +231,7 @@ bool ptrace_may_access(struct task_struct *task, unsigned int mode)
    }

    static int ptrace_attach(struct task_struct *task, long request,
    + unsigned long addr,
    unsigned long flags)
    {
    bool seize = (request == PTRACE_SEIZE);
    @@ -238,19 +239,29 @@ static int ptrace_attach(struct task_struct *task, long request,

    /*
    * SEIZE will enable new ptrace behaviors which will be implemented
    - * gradually. SEIZE_DEVEL is used to prevent applications
    + * gradually. SEIZE_DEVEL bit is used to prevent applications
    * expecting full SEIZE behaviors trapping on kernel commits which
    * are still in the process of implementing them.
    *
    * Only test programs for new ptrace behaviors being implemented
    * should set SEIZE_DEVEL. If unset, SEIZE will fail with -EIO.
    *
    - * Once SEIZE behaviors are completely implemented, this flag and
    - * the following test will be removed.
    + * Once SEIZE behaviors are completely implemented, this flag
    + * will be removed.
    */
    retval = -EIO;
    - if (seize && !(flags & PTRACE_SEIZE_DEVEL))
    - goto out;
    + if (seize) {
    + if (addr != 0)
    + goto out;
    + if (!(flags & PTRACE_SEIZE_DEVEL))
    + goto out;
    + flags &= ~(unsigned long)PTRACE_SEIZE_DEVEL;
    + if (flags & ~(unsigned long)PTRACE_O_MASK)
    + goto out;
    + flags = PT_PTRACED | PT_SEIZED | (flags << PT_OPT_FLAG_SHIFT);
    + } else {
    + flags = PT_PTRACED;
    + }

    audit_ptrace(task);

    @@ -282,11 +293,11 @@ static int ptrace_attach(struct task_struct *task, long request,
    if (task->ptrace)
    goto unlock_tasklist;

    - task->ptrace = PT_PTRACED;
    if (seize)
    - task->ptrace |= PT_SEIZED;
    + flags |= PT_SEIZED;
    if (ns_capable(task_user_ns(task), CAP_SYS_PTRACE))
    - task->ptrace |= PT_PTRACE_CAP;
    + flags |= PT_PTRACE_CAP;
    + task->ptrace = flags;

    __ptrace_link(task, current);

    @@ -879,7 +890,7 @@ SYSCALL_DEFINE4(ptrace, long, request, long, pid, unsigned long, addr,
    }

    if (request == PTRACE_ATTACH || request == PTRACE_SEIZE) {
    - ret = ptrace_attach(child, request, data);
    + ret = ptrace_attach(child, request, addr, data);
    /*
    * Some architectures need to do book-keeping after
    * a ptrace attach.
    @@ -1022,7 +1033,7 @@ asmlinkage long compat_sys_ptrace(compat_long_t request, compat_long_t pid,
    }

    if (request == PTRACE_ATTACH || request == PTRACE_SEIZE) {
    - ret = ptrace_attach(child, request, data);
    + ret = ptrace_attach(child, request, addr, data);
    /*
    * Some architectures need to do book-keeping after
    * a ptrace attach.
    --
    1.7.7.6


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