Re: [PATCHv4] exec: Fix a deadlock in ptrace

[Christian Brauner]

On Mon, Mar 02, 2020 at 06:26:47PM -0800, Kees Cook wrote:
> On Mon, Mar 02, 2020 at 10:18:07PM +0000, 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
> > 
> > The proposed solution is to take the cred_guard_mutex only
> > in a critical section at the beginning, and at the end of the
> > execve function, and let PTRACE_ATTACH fail with EAGAIN while
> > execve is not complete, but other functions like vm_access are
> > allowed to complete normally.
> 
> Sorry to be bummer, but I don't think this will work. A few more things
> during the exec process depend on cred_guard_mutex being held.
> 
> If I'm reading this patch correctly, this changes the lifetime of the
> cred_guard_mutex lock to be:
> 	- during prepare_bprm_creds()
> 	- from flush_old_exec() through install_exec_creds()
> Before, cred_guard_mutex was held from prepare_bprm_creds() through
> install_exec_creds().
> 
> That means, for example, that check_unsafe_exec()'s documented invariant
> is violated:
>     /*
>      * determine how safe it is to execute the proposed program
>      * - the caller must hold ->cred_guard_mutex to protect against
>      *   PTRACE_ATTACH or seccomp thread-sync
>      */
>     static void check_unsafe_exec(struct linux_binprm *bprm) ...
> which is looking at no_new_privs as well as other details, and making
> decisions about the bprm state from the current state.
> 
> I think it also means that the potentially multiple invocations
> of bprm_fill_uid() (via prepare_binprm() via binfmt_script.c and
> binfmt_misc.c) would be changing bprm->cred details (uid, gid) without
> a lock (another place where current's no_new_privs is evaluated).
> 
> Related, it also means that cred_guard_mutex is unheld for every
> invocation of search_binary_handler() (which can loop via the previously
> mentioned binfmt_script.c and binfmt_misc.c), if any of them have hidden
> dependencies on cred_guard_mutex. (Thought I only see bprm_fill_uid()
> currently.)

So one issue I see with having to reacquire the cred_guard_mutex might
be that this would allow tasks holding the cred_guard_mutex to block a
killed exec'ing task from exiting, right?