[PATCH v2 1/2] sched/core: Propagate pointer hash errors from PR_SCHED_CORE_GET

[Hui Su <sh_def@163.com>]

PR_SCHED_CORE_GET hashes the internal core scheduling cookie before
returning an opaque identifier to userspace.

ptr_to_hashval() returns -EBUSY until the random key used for pointer
hashing has been initialized. However, PR_SCHED_CORE_GET currently
ignores that return value.

Since id is initialized to zero, a failed hash is therefore reported
as a successful GET with cookie 0. Cookie 0 represents the default
core scheduling cookie, so a task which already has a core scheduling
cookie can temporarily appear to have the default cookie during early
boot.

Propagate errors from ptr_to_hashval() instead. In particular, this
allows userspace to retry PR_SCHED_CORE_GET when pointer hashing is
temporarily unavailable.

An early-boot x86_64 QEMU reproducer observed the following before the
fix:

  CREATE ret=0 errno=0 (Success)
  GET ret=0 errno=0 (Success) cookie=0x0000000000000000

With this fix, the same GET correctly returned:

  GET ret=-1 errno=16 (Device or resource busy)

The full cs_prctl_test also failed seven cookie assertions on the
unfixed kernel after PR_SCHED_CORE_CREATE succeeded but subsequent GETs
continued to report cookie 0.

Fixes: 7ac592aa35a6 ("sched: prctl() core-scheduling interface")
Cc: stable@vger.kernel.org
Signed-off-by: Hui Su <sh_def@163.com>
---
v2:
- No changes.

 kernel/sched/core_sched.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core_sched.c b/kernel/sched/core_sched.c
index 43e0bde3038e..f119a2973ca8 100644
--- a/kernel/sched/core_sched.c
+++ b/kernel/sched/core_sched.c
@@ -177,8 +177,9 @@ int sched_core_share_pid(unsigned int cmd, pid_t pid, enum pid_type type,
 		}
 		cookie = sched_core_clone_cookie(task);
 		if (cookie) {
-			/* XXX improve ? */
-			ptr_to_hashval((void *)cookie, &id);
+			err = ptr_to_hashval((void *)cookie, &id);
+			if (err)
+				goto out;
 		}
 		err = put_user(id, (u64 __user *)uaddr);
 		goto out;
-- 
2.55.0