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Subject: Re: [PATCH 1/4] exec: Fix a deadlock in ptrace
To:     "Eric W. Biederman" <ebiederm@xmission.com>
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From:   Bernd Edlinger <bernd.edlinger@hotmail.de>
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On 3/10/20 4:13 PM, Eric W. Biederman wrote:
> Bernd Edlinger <bernd.edlinger@hotmail.de> writes:
> 
>> This fixes a deadlock in the tracer when tracing a multi-threaded
>> application that calls execve while more than one thread are running.
>>
>> I observed that when running strace on the gcc test suite, it always
>> blocks after a while, when expect calls execve, because other threads
>> have to be terminated.  They send ptrace events, but the strace is no
>> longer able to respond, since it is blocked in vm_access.
>>
>> The deadlock is always happening when strace needs to access the
>> tracees process mmap, while another thread in the tracee starts to
>> execve a child process, but that cannot continue until the
>> PTRACE_EVENT_EXIT is handled and the WIFEXITED event is received:
> 
> Overall this looks good.  Mind if I change the subject to:
> "exec: Fix a deadlock in strace" ?
> 

Sure, go ahead.

Thanks
Bernd.

> Eric
> 
> 
>>
>> strace          D    0 30614  30584 0x00000000
>> Call Trace:
>> __schedule+0x3ce/0x6e0
>> schedule+0x5c/0xd0
>> schedule_preempt_disabled+0x15/0x20
>> __mutex_lock.isra.13+0x1ec/0x520
>> __mutex_lock_killable_slowpath+0x13/0x20
>> mutex_lock_killable+0x28/0x30
>> mm_access+0x27/0xa0
>> process_vm_rw_core.isra.3+0xff/0x550
>> process_vm_rw+0xdd/0xf0
>> __x64_sys_process_vm_readv+0x31/0x40
>> do_syscall_64+0x64/0x220
>> entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>
>> expect          D    0 31933  30876 0x80004003
>> Call Trace:
>> __schedule+0x3ce/0x6e0
>> schedule+0x5c/0xd0
>> flush_old_exec+0xc4/0x770
>> load_elf_binary+0x35a/0x16c0
>> search_binary_handler+0x97/0x1d0
>> __do_execve_file.isra.40+0x5d4/0x8a0
>> __x64_sys_execve+0x49/0x60
>> do_syscall_64+0x64/0x220
>> entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>
>> This changes mm_access to use the new exec_update_mutex
>> instead of cred_guard_mutex.
>>
>> This patch is based on the following patch by Eric W. Biederman:
>> "[PATCH 0/5] Infrastructure to allow fixing exec deadlocks"
>> Link: https://lore.kernel.org/lkml/87v9ne5y4y.fsf_-_@x220.int.ebiederm.org/
>>
>> Signed-off-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
>> ---
>>  kernel/fork.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/fork.c b/kernel/fork.c
>> index c12595a..5720ff3 100644
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -1224,7 +1224,7 @@ struct mm_struct *mm_access(struct task_struct *task, unsigned int mode)
>>  	struct mm_struct *mm;
>>  	int err;
>>  
>> -	err =  mutex_lock_killable(&task->signal->cred_guard_mutex);
>> +	err =  mutex_lock_killable(&task->signal->exec_update_mutex);
>>  	if (err)
>>  		return ERR_PTR(err);
>>  
>> @@ -1234,7 +1234,7 @@ struct mm_struct *mm_access(struct task_struct *task, unsigned int mode)
>>  		mmput(mm);
>>  		mm = ERR_PTR(-EACCES);
>>  	}
>> -	mutex_unlock(&task->signal->cred_guard_mutex);
>> +	mutex_unlock(&task->signal->exec_update_mutex);
>>  
>>  	return mm;
>>  }