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?
