Re: [PATCH 1/4] exec: Fix a deadlock in ptrace

[Kees Cook]

On Tue, Mar 10, 2020 at 02:43:41PM +0100, Bernd Edlinger wrote:
> 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:
> 
> 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_-_@xxxxxxxxxxxxxxxxxxxxx/
> 
> Signed-off-by: Bernd Edlinger <bernd.edlinger@xxxxxxxxxx>

Cool, yes, on top of the new infrastructure this looks correct to me --
the new mutex wraps mm changes and mm_access() is looking at *drum roll*
the mm! :)

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> ---
>  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;
>  }
> -- 
> 1.9.1

-- 
Kees Cook