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>

2 files changed, 42 insertions, 38 deletions

diff --git a/fs/compat.c b/fs/compat.c
index 94502dab972a..6d6f98fe64a0 100644
--- a/fs/compat.c
+++ b/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 @@ out_file:
 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);
diff --git a/fs/exec.c b/fs/exec.c
index fb4f3cdda78c..172ceb6edde4 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1016,6 +1016,35 @@ out:
 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
  */
 void install_exec_creds(struct linux_binprm *bprm)
@@ -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 @@ out_file:
 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);