[PATCH][SRU][ARTFUL] UBUNTU:SAUCE: exec: fix lockup because retry loop may never exit

[Colin King]

From: Colin Ian King <colin.king at canonical.com>

BugLink: https://launchpad.net/bugs/1730717

My early fix for bug LP#1672819 could get stuck in an infinite
retry loop causing lockups.  Currently the retry is aggressively
spinning on a check and will never abort.  Relax the aggressive
retry by adding a small interruptible delay and limit the number
of retries.

I've analyzed the retries and 95% of the cases on an 8 CPU Xeon
host never hit the retry loop and less that 0.5% of retries are
ever more than 1 retry using the original reproducer program.
Considering that the reproducer is an extreeme case of forcing
the race condition I believe we now have a suitable balance of
non-aggressive CPU eating retries and a mechanism to bail out
after enough retries and avoid a lockup.

Admittedly this workaround is a bit of an ugly hack, but the
retry path is never executed for the majority of use cases and
only hits the retry/delay for the racy condition we hit with the
original bug.

Signed-off-by: Colin Ian King <colin.king at canonical.com>
---
 fs/exec.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 5c2c1ff3bd..0f47bb8486 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -62,6 +62,7 @@
 #include <linux/oom.h>
 #include <linux/compat.h>
 #include <linux/vmalloc.h>
+#include <linux/delay.h>
 
 #include <trace/events/fs.h>
 
@@ -1465,6 +1466,7 @@ static void check_unsafe_exec(struct linux_binprm *bprm)
 	struct task_struct *p = current, *t;
 	unsigned n_fs;
 	bool fs_recheck;
+	int count = 0;
 
 	if (p->ptrace)
 		bprm->unsafe |= LSM_UNSAFE_PTRACE;
@@ -1492,8 +1494,11 @@ static void check_unsafe_exec(struct linux_binprm *bprm)
 
 	if (p->fs->users > n_fs) {
 		if (fs_recheck) {
-			spin_unlock(&p->fs->lock);
-			goto recheck;
+			if (count++ < 20) {
+				spin_unlock(&p->fs->lock);
+				msleep_interruptible(1);
+				goto recheck;
+			}
 		}
 		bprm->unsafe |= LSM_UNSAFE_SHARE;
 	} else
-- 
2.14.1