exec: do not sleep in TASK_TRACED under ->cred_guard_mutex
authorOleg Nesterov <oleg@redhat.com>
Sat, 5 Sep 2009 18:17:13 +0000 (11:17 -0700)
committerLinus Torvalds <torvalds@linux-foundation.org>
Sat, 5 Sep 2009 18:30:42 +0000 (11:30 -0700)
commita2a8474c3fff88d8dd52d05cb450563fb26fd26c
tree455415da039a77627f47ed19318bb9c8aa17de4d
parentdd5d241ea955006122d76af88af87de73fec25b4
exec: do not sleep in TASK_TRACED under ->cred_guard_mutex

Tom Horsley reports that his debugger hangs when it tries to read
/proc/pid_of_tracee/maps, this happens since

"mm_for_maps: take ->cred_guard_mutex to fix the race with exec"
04b836cbf19e885f8366bccb2e4b0474346c02d

commit in 2.6.31.

But the root of the problem lies in the fact that do_execve() path calls
tracehook_report_exec() which can stop if the tracer sets PT_TRACE_EXEC.

The tracee must not sleep in TASK_TRACED holding this mutex.  Even if we
remove ->cred_guard_mutex from mm_for_maps() and proc_pid_attr_write(),
another task doing PTRACE_ATTACH should not hang until it is killed or the
tracee resumes.

With this patch do_execve() does not use ->cred_guard_mutex directly and
we do not hold it throughout, instead:

- introduce prepare_bprm_creds() helper, it locks the mutex
  and calls prepare_exec_creds() to initialize bprm->cred.

- install_exec_creds() drops the mutex after commit_creds(),
  and thus before tracehook_report_exec()->ptrace_stop().

  or, if exec fails,

  free_bprm() drops this mutex when bprm->cred != NULL which
  indicates install_exec_creds() was not called.

Reported-by: Tom Horsley <tom.horsley@att.net>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: David Howells <dhowells@redhat.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: James Morris <jmorris@namei.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
fs/compat.c
fs/exec.c
include/linux/binfmts.h