kernel/sys.c: fix the racy usage of task_lock(tsk->group_leader) in sys_prlimit64() paths

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: kernel/sys.c: fix the racy usage of task_lock(tsk->group_leader) in sys_prlimit64() paths The usage of task_lock(tsk->group_leader) in sys_prlimit64()->do_prlimit() path is very broken. sys_prlimit64() does get_task_struct(tsk) but this only protects task_struct itself. If tsk != current and tsk is not a leader, this process can exit/exec and task_lock(tsk->group_leader) may use the already freed task_struct. Another problem is that sys_prlimit64() can race with mt-exec which changes ->group_leader. In this case do_prlimit() may take the wrong lock, or (worse) ->group_leader may change between task_lock() and task_unlock(). Change sys_prlimit64() to take tasklist_lock when necessary. This is not nice, but I don't see a better fix for -stable.

References

• https://git.kernel.org/stable/c/1bc0d9315ef5296abb2c9fd840336255850ded18
• https://git.kernel.org/stable/c/132f827e7bac7373e1522e89709d70b43cae5342
• https://git.kernel.org/stable/c/19b45c84bd9fd42fa97ff80c6350d604cb871c75
• https://git.kernel.org/stable/c/6796412decd2d8de8ec708213bbc958fab72f143
• https://git.kernel.org/stable/c/a15f37a40145c986cdf289a4b88390f35efdecc4