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

@@ -1485,20 +1485,15 @@ int compat_do_execve(char * filename,
 	if (!bprm)
 		goto out_files;
 
-	retval = -ERESTARTNOINTR;
-	if (mutex_lock_interruptible(&current->cred_guard_mutex))
+	retval = prepare_bprm_creds(bprm);
+	if (retval)
 		goto out_free;
-	current->in_execve = 1;
-
-	retval = -ENOMEM;
-	bprm->cred = prepare_exec_creds();
-	if (!bprm->cred)
-		goto out_unlock;
 
 	retval = check_unsafe_exec(bprm);
 	if (retval < 0)
-		goto out_unlock;
+		goto out_free;
 	clear_in_exec = retval;
+	current->in_execve = 1;
 
 	file = open_exec(filename);
 	retval = PTR_ERR(file);
@@ -1547,7 +1542,6 @@ int compat_do_execve(char * filename,
 	/* execve succeeded */
 	current->fs->in_exec = 0;
 	current->in_execve = 0;
-	mutex_unlock(&current->cred_guard_mutex);
 	acct_update_integrals(current);
 	free_bprm(bprm);
 	if (displaced)
@@ -1567,10 +1561,7 @@ int compat_do_execve(char * filename,
 out_unmark:
 	if (clear_in_exec)
 		current->fs->in_exec = 0;
-
-out_unlock:
 	current->in_execve = 0;
-	mutex_unlock(&current->cred_guard_mutex);
 
 out_free:
 	free_bprm(bprm);

fs/exec.c

@@ -1015,6 +1015,35 @@ int flush_old_exec(struct linux_binprm * bprm)
 
 EXPORT_SYMBOL(flush_old_exec);
 
+/*
+ * Prepare credentials and lock ->cred_guard_mutex.
+ * install_exec_creds() commits the new creds and drops the lock.
+ * Or, if exec fails before, free_bprm() should release ->cred and
+ * and unlock.
+ */
+int prepare_bprm_creds(struct linux_binprm *bprm)
+{
+	if (mutex_lock_interruptible(&current->cred_guard_mutex))
+		return -ERESTARTNOINTR;
+
+	bprm->cred = prepare_exec_creds();
+	if (likely(bprm->cred))
+		return 0;
+
+	mutex_unlock(&current->cred_guard_mutex);
+	return -ENOMEM;
+}
+
+void free_bprm(struct linux_binprm *bprm)
+{
+	free_arg_pages(bprm);
+	if (bprm->cred) {
+		mutex_unlock(&current->cred_guard_mutex);
+		abort_creds(bprm->cred);
+	}
+	kfree(bprm);
+}
+
 /*
  * install the new credentials for this executable
  */
@@ -1024,12 +1053,13 @@ void install_exec_creds(struct linux_binprm *bprm)
 
 	commit_creds(bprm->cred);
 	bprm->cred = NULL;
-
-	/* cred_guard_mutex must be held at least to this point to prevent
+	/*
+	 * cred_guard_mutex must be held at least to this point to prevent
 	 * ptrace_attach() from altering our determination of the task's
-	 * credentials; any time after this it may be unlocked */
-
+	 * credentials; any time after this it may be unlocked.
+	 */
 	security_bprm_committed_creds(bprm);
+	mutex_unlock(&current->cred_guard_mutex);
 }
 EXPORT_SYMBOL(install_exec_creds);
 
@@ -1246,14 +1276,6 @@ int search_binary_handler(struct linux_binprm *bprm,struct pt_regs *regs)
 
 EXPORT_SYMBOL(search_binary_handler);
 
-void free_bprm(struct linux_binprm *bprm)
-{
-	free_arg_pages(bprm);
-	if (bprm->cred)
-		abort_creds(bprm->cred);
-	kfree(bprm);
-}
-
 /*
  * sys_execve() executes a new program.
  */
@@ -1277,20 +1299,15 @@ int do_execve(char * filename,
 	if (!bprm)
 		goto out_files;
 
-	retval = -ERESTARTNOINTR;
-	if (mutex_lock_interruptible(&current->cred_guard_mutex))
+	retval = prepare_bprm_creds(bprm);
+	if (retval)
 		goto out_free;
-	current->in_execve = 1;
-
-	retval = -ENOMEM;
-	bprm->cred = prepare_exec_creds();
-	if (!bprm->cred)
-		goto out_unlock;
 
 	retval = check_unsafe_exec(bprm);
 	if (retval < 0)
-		goto out_unlock;
+		goto out_free;
 	clear_in_exec = retval;
+	current->in_execve = 1;
 
 	file = open_exec(filename);
 	retval = PTR_ERR(file);
@@ -1340,7 +1357,6 @@ int do_execve(char * filename,
 	/* execve succeeded */
 	current->fs->in_exec = 0;
 	current->in_execve = 0;
-	mutex_unlock(&current->cred_guard_mutex);
 	acct_update_integrals(current);
 	free_bprm(bprm);
 	if (displaced)
@@ -1360,10 +1376,7 @@ int do_execve(char * filename,
 out_unmark:
 	if (clear_in_exec)
 		current->fs->in_exec = 0;
-
-out_unlock:
 	current->in_execve = 0;
-	mutex_unlock(&current->cred_guard_mutex);
 
 out_free:
 	free_bprm(bprm);

include/linux/binfmts.h

@@ -117,6 +117,7 @@ extern int setup_arg_pages(struct linux_binprm * bprm,
 			   int executable_stack);
 extern int bprm_mm_init(struct linux_binprm *bprm);
 extern int copy_strings_kernel(int argc,char ** argv,struct linux_binprm *bprm);
+extern int prepare_bprm_creds(struct linux_binprm *bprm);
 extern void install_exec_creds(struct linux_binprm *bprm);
 extern void do_coredump(long signr, int exit_code, struct pt_regs *regs);
 extern int set_binfmt(struct linux_binfmt *new);