Re: [PATCH, for 2.6.29] ptrace: fix the usage of ptrace_fork()

[Oleg Nesterov <oleg@redhat.com>]

On 02/09, Oleg Nesterov wrote:
>
> I noticed by pure accident we have ptrace_fork() and friends. This was
> added by "x86, bts: add fork and exit handling", commit
> bf53de907dfdaac178c92d774aae7370d7b97d20

Hmm. Looks like we have more problems here...

"x86, bts: memory accounting", commit c5dee6177f4bd2095aab7d9be9f6ebdddd6deee9.

PTRACE_BTS_CONFIG allocates ->bts_buffer via alloc_locked_buffer()
which updates mm->total_vm/locked_vm.

ptrace_detach() does free_locked_buffer() which "restores" mm->xxx_vm.

But if the tracer exits we are doing __ptrace_unlink()->ptrace_bts_untrace()
which uses a plain kfree(), in that case we don't update mm->xxx_vm ?

Note that the exiting tracer can have sub-threads, so the whole process
does not necessary dies.

Or, the tracer can reap a zombie tracee without PTRACE_DETACH, in that
case we don't update ->mm too.

Oh, and afaics ptrace_detach()->ptrace_bts_detach() can race with the
tracer's sub-thread which does do_wait()->release_task() (if the tracee
was killed before detach takes tasklist), the kernel can crash in this
case.

Unless I missed something, This all looks rather wrong, and I wasn't
aware about these changes :(

Oleg.