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Subject[PATCH v3] 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>
---
include/linux/binfmts.h | 3 +-
fs/exec.c | 63 ++++++++++++++++++++++++++++--------------------
fs/compat.c | 17 +++---------
3 files changed, 44 insertions(+), 39 deletions(-)
--- WAIT/include/linux/binfmts.h~CRED_MUTEX_EEEVENT_EXEC 2009-07-24 19:02:19.000000000 +0200
+++ WAIT/include/linux/binfmts.h 2009-09-03 17:37:04.000000000 +0200
@@ -117,9 +117,10 @@ extern int setup_arg_pages(struct linux_
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 void install_exec_creds(struct linux_binprm *bprm);
extern void do_coredump(long signr, int exit_code, struct pt_regs *regs);
extern void set_binfmt(struct linux_binfmt *new);
+extern int prepare_bprm_creds(struct linux_binprm *bprm);
+extern void install_exec_creds(struct linux_binprm *bprm);
extern void free_bprm(struct linux_binprm *);

#endif /* __KERNEL__ */
--- WAIT/fs/exec.c~CRED_MUTEX_EEEVENT_EXEC 2009-09-03 15:31:08.000000000 +0200
+++ WAIT/fs/exec.c 2009-09-04 19:09:14.000000000 +0200
@@ -1018,6 +1018,35 @@ 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)
@@ -1026,12 +1055,13 @@ void install_exec_creds(struct linux_bin

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);

@@ -1248,14 +1278,6 @@ int search_binary_handler(struct linux_b

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.
*/
@@ -1279,20 +1301,15 @@ 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);
@@ -1342,7 +1359,6 @@ 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)
@@ -1362,10 +1378,7 @@ 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);
--- WAIT/fs/compat.c~CRED_MUTEX_EEEVENT_EXEC 2009-09-03 15:31:08.000000000 +0200
+++ WAIT/fs/compat.c 2009-09-03 16:42:17.000000000 +0200
@@ -1486,20 +1486,15 @@ 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);
@@ -1548,7 +1543,6 @@ 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)
@@ -1568,10 +1562,7 @@ 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);


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