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

Bernd Edlinger
2020-03-09T18:47:41Z

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@dhcp22.suse.cz/
> Link: https://lore.kernel.org/lkml/AM6PR03MB5170B06F3A2B75EFB98D071AE4E60@AM6PR03MB5170.eurprd03.prod.outlook.com/
> Link: https://lore.kernel.org/linux-fsdevel/20161102181806.GB1112@redhat.com/
> Link: https://lore.kernel.org/lkml/20160923095031.GA14923@redhat.com/
> Link: https://lore.kernel.org/lkml/20170213141452.GA30203@redhat.com/
> 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@xmission.com>
>
> Does that sound better?
>

almost done.

> Eric
>