On 3/13/20 10:13 AM, Kirill Tkhai wrote:

Despite this should fix the problem, this looks like a broken puzzle.

We can't use bprm->cred as an identifier whether the mutex was locked or not.
We can check for bprm->cred in regard to cred_guard_mutex, because of there is
strong rule: "cred_guard_mutex is becomes locked together with bprm->cred assignment
(see prepare_bprm_creds()), and it becomes unlocked together with bprm->cred zeroing".
Take attention on modularity of all this: there is no dependencies between anything else.

In regard to newly introduced exec_update_mutex, your fix and source patch way look like
an obfuscation. The mutex becomes deadly glued to unrelated bprm->cred and bprm->mm,
and this introduces the problems in the future modifications and support of all involved
entities. If someone wants to move some functions in relation to each other, there will
be a pain, and this person will have to go again the same dependencies and bug way,
Eric stepped on in the original patch.

Okay, yes, valid points you make, thanks.
I just wanted to understand what was exactly wrong with this patch,
since the failure mode looked a lot like it was failing because of
something clobbering the data unexpectedly.


So I have posted a few updated patch for the failed one here:

[PATCH v3 5/5] exec: Add a exec_update_mutex to replace cred_guard_mutex
[PATCH] pidfd: Use new infrastructure to fix deadlocks in execve

which replaces these:
[PATCH v2 5/5] exec: Add a exec_update_mutex to replace cred_guard_mutex
https://lore.kernel.org/lkml/87zhcq4jdj.fsf_-_@x220.int.ebiederm.org/ (local)

[PATCH] pidfd: Stop taking cred_guard_mutex 
https://lore.kernel.org/lkml/87wo7svy96.fsf_-_@x220.int.ebiederm.org/ (local)


and a new patch series to fix deadlock in ptrace_attach and update doc:
[PATCH 0/2] exec: Fix dead-lock in de_thread with ptrace_attach
[PATCH 1/2] exec: Fix dead-lock in de_thread with ptrace_attach
[PATCH 2/2] doc: Update documentation of ->exec_*_mutex


Other patches needed, still valid:

[PATCH v2 1/5] exec: Only compute current once in flush_old_exec
https://lore.kernel.org/lkml/87pndm5y3l.fsf_-_@x220.int.ebiederm.org/ (local)

[PATCH v2 2/5] exec: Factor unshare_sighand out of de_thread and call it separately
https://lore.kernel.org/lkml/87k13u5y26.fsf_-_@x220.int.ebiederm.org/ (local)

[PATCH v2 4/5] exec: Move exec_mmap right after de_thread in flush_old_exec
https://lore.kernel.org/lkml/875zfe5xzb.fsf_-_@x220.int.ebiederm.org/ (local)

[PATCH 1/4] exec: Fix a deadlock in ptrace
https://lore.kernel.org/lkml/AM6PR03MB517033EAD25BED15CC84E17DE4FF0@AM6PR03MB5170.eurprd03.prod.outlook.com/ (local)

[PATCH 2/4] selftests/ptrace: add test cases for dead-locks
https://lore.kernel.org/lkml/AM6PR03MB51703199741A2C27A78980FFE4FF0@AM6PR03MB5170.eurprd03.prod.outlook.com/ (local)

[PATCH 3/4] mm: docs: Fix a comment in process_vm_rw_core
https://lore.kernel.org/lkml/AM6PR03MB5170ED6D4D216EEEEF400136E4FF0@AM6PR03MB5170.eurprd03.prod.outlook.com/ (local)

[PATCH 4/4] kernel: doc: remove outdated comment cred.c
https://lore.kernel.org/lkml/AM6PR03MB517039DB07AB641C194FEA57E4FF0@AM6PR03MB5170.eurprd03.prod.outlook.com/ (local)

[PATCH 1/4] kernel/kcmp.c: Use new infrastructure to fix deadlocks in execve
https://lore.kernel.org/lkml/AM6PR03MB517057A2269C3A4FB287B76EE4FF0@AM6PR03MB5170.eurprd03.prod.outlook.com/ (local)

[PATCH 2/4] proc: Use new infrastructure to fix deadlocks in execve
https://lore.kernel.org/lkml/AM6PR03MB51705D211EC8E7EA270627B1E4FF0@AM6PR03MB5170.eurprd03.prod.outlook.com/ (local)

[PATCH 3/4] proc: io_accounting: Use new infrastructure to fix deadlocks in execve
https://lore.kernel.org/lkml/AM6PR03MB5170BD2476E35068E182EFA4E4FF0@AM6PR03MB5170.eurprd03.prod.outlook.com/ (local)

[PATCH 4/4] perf: Use new infrastructure to fix deadlocks in execve
https://lore.kernel.org/lkml/AM6PR03MB517035DEEDB9C8699CB6B34EE4FF0@AM6PR03MB5170.eurprd03.prod.outlook.com/ (local)


I think most of the existing patches are already approved, but if
there are still change requests, please let me know.


Thanks
Bernd.