Commit 3e74fabd authored Mar 20, 2020 by Bernd Edlinger Committed by Eric W. Biederman Mar 25, 2020

exec: Fix a deadlock in strace

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_-_@x220.int.ebiederm.org/



Signed-off-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

parent eea96732

kernel/fork.c

+2 −2

Original line number	Diff line number	Diff line
		@@ -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;
		}

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