[PATCH 1/2] exec: Properly mark the point of no return

Eric W. Biederman
2020-03-05T21:17:53Z

Add a flag binfmt->unrecoverable to mark when execution has gotten to
the point where it is impossible to return to userspace with the
calling process unchanged.

While techinically this state starts as soon as de_thread starts
killing threads, the only return path at that point is if there is a
fatal signal pending. I have choosen instead to set unrecoverable
when the killing stops, and there are possibilities of failures other
than fatal signals. In particular it is possible for the allocation
of a new sighand structure to fail.

Setting unrecoverable at this point has the benefit that other actions
can be taken after the other threads are all dead, and the
unrecoverable flag can double as a flag that those actions have been
taken.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
fs/exec.c | 7 ++++---
include/linux/binfmts.h | 7 ++++++-
2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index db17be51b112..c243f9660d46 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1061,7 +1061,7 @@ static int exec_mmap(struct mm_struct *mm)
* disturbing other processes. (Other processes might share the signal
* table via the CLONE_SIGHAND option to clone().)
*/
-static int de_thread(struct task_struct *tsk)
+static int de_thread(struct linux_binprm *bprm, struct task_struct *tsk)
{
struct signal_struct *sig = tsk->signal;
struct sighand_struct *oldsighand = tsk->sighand;
@@ -1182,6 +1182,7 @@ static int de_thread(struct task_struct *tsk)
release_task(leader);
}

+ bprm->unrecoverable = true;
sig->group_exit_task = NULL;
sig->notify_count = 0;

@@ -1266,7 +1267,7 @@ int flush_old_exec(struct linux_binprm * bprm)
* Make sure we have a private signal table and that
* we are unassociated from the previous thread group.
*/
- retval = de_thread(current);
+ retval = de_thread(bprm, current);
if (retval)
goto out;

@@ -1664,7 +1665,7 @@ int search_binary_handler(struct linux_binprm *bprm)

read_lock(&binfmt_lock);
put_binfmt(fmt);
- if (retval < 0 && !bprm->mm) {
+ if (retval < 0 && bprm->unrecoverable) {
/* we got to flush_old_exec() and failed after it */
read_unlock(&binfmt_lock);
force_sigsegv(SIGSEGV);
diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
index b40fc633f3be..12263115ce7a 100644
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -44,7 +44,12 @@ struct linux_binprm {
* exec has happened. Used to sanitize execution environment
* and to set AT_SECURE auxv for glibc.
*/
- secureexec:1;
+ secureexec:1,
+ /*
+ * Set when changes have been made that prevent returning
+ * to userspace.
+ */
+ unrecoverable:1;
#ifdef __alpha__
unsigned int taso:1;
#endif
--
2.25.0