Re: [PATCH v2 5/5] exec: Add a exec_update_mutex to replace cred_guard_mutex

[Bernd Edlinger]

On 3/9/20 7:36 PM, Eric W. Biederman wrote:
> 
> My rewritten change description reads as follows:
> 
>     exec: Add a exec_update_mutex to replace cred_guard_mutex

is this "an" exec_update_mutex?

>     
>     The cred_guard_mutex is problematic as it is held over possibly
>     indefinite waits for userspace.  The possilbe indefinite waits for
>     userspace that I have identified are: The cred_guard_mutex is held in
>     PTRACE_EVENT_EXIT waiting for the tracer.  The cred_guard_mutex is
>     held over "put_user(0, tsk->clear_child_tid)" in exit_mm().  The
>     cred_guard_mutex is held over "get_user(futex_offset, ...")  in
>     exit_robust_list.  The cred_guard_mutex held over copy_strings.
>     
>     The functions get_user and put_user can trigger a page fault which can
>     potentially wait indefinitely in the case of userfaultfd or if
>     userspace implements part of the page fault path.
>     
>     In any of those cases the userspace process that the kernel is waiting
>     for might userspace might make a different system call that winds up
                ^-------------^
                      ^- remove this
>     taking the cred_guard_mutex and result in deadlock.
>     
>     Holding a mutex over any of those possibly indefinite waits for
>     userspace does not appear necessary.  Add exec_update_mutex that will
>     just cover updating the process during exec where the permissions and
>     the objects pointed to by the task struct may be out of sync.
>     
>     The plan is to switch the users of cred_guard_mutex to
>     exec_udpate_mutex one by one.  This lets us move forward while still

            ^-- typo: update

>     being careful and not introducing any regressions.
>     
>     Link: https://lore.kernel.org/lkml/20160921152946.GA24210@xxxxxxxxxxxxxx/
>     Link: https://lore.kernel.org/lkml/AM6PR03MB5170B06F3A2B75EFB98D071AE4E60@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
>     Link: https://lore.kernel.org/linux-fsdevel/20161102181806.GB1112@xxxxxxxxxx/
>     Link: https://lore.kernel.org/lkml/20160923095031.GA14923@xxxxxxxxxx/
>     Link: https://lore.kernel.org/lkml/20170213141452.GA30203@xxxxxxxxxx/
>     Ref: 45c1a159b85b ("Add PTRACE_O_TRACEVFORKDONE and PTRACE_O_TRACEEXIT facilities.")
>     Ref: 456f17cd1a28 ("[PATCH] user-vm-unlock-2.5.31-A2")
>     Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> 
> Does that sound better?
> 

almost done.

> Eric
>