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SubjectRe: [PATCH] signals: check_kill_permission: don't check creds if same_thread_group()
On 05/18, David Howells wrote:
>
> Oleg Nesterov <oleg@redhat.com> wrote:
>
> > Also, move "cred = current_cred()" down to avoid calling get_current()
> > twice.
>
> I don't see what you mean by this. same_thread_group() doesn't call
> current_cred(), so why this change?

Yes, but both current_cred() and same_thread_group(current, t) call
get_current(), and gcc doesn't cache the result because we call
audit_signal_info() in between.

In fact, initially I was going to send the patch below, but then
decided to make a more simple change.

Oleg.

--- x/kernel/signal.c
+++ x/kernel/signal.c
@@ -642,8 +642,6 @@ static inline bool si_fromuser(const str
static int check_kill_permission(int sig, struct siginfo *info,
struct task_struct *t)
{
- const struct cred *cred = current_cred(), *tcred;
- struct pid *sid;
int error;

if (!valid_signal(sig))
@@ -656,23 +654,29 @@ static int check_kill_permission(int sig
if (error)
return error;

- tcred = __task_cred(t);
- if ((cred->euid ^ tcred->suid) &&
- (cred->euid ^ tcred->uid) &&
- (cred->uid ^ tcred->suid) &&
- (cred->uid ^ tcred->uid) &&
- !capable(CAP_KILL)) {
- switch (sig) {
- case SIGCONT:
- sid = task_session(t);
- /*
- * We don't return the error if sid == NULL. The
- * task was unhashed, the caller must notice this.
- */
- if (!sid || sid == task_session(current))
- break;
- default:
- return -EPERM;
+ if (!same_thread_group(current, t)) {
+ const struct cred *cred = current_cred();
+ const struct cred *tcred = __task_cred(t);
+ struct pid *sid;
+
+ if ((cred->euid ^ tcred->suid) &&
+ (cred->euid ^ tcred->uid) &&
+ (cred->uid ^ tcred->suid) &&
+ (cred->uid ^ tcred->uid) &&
+ !capable(CAP_KILL)) {
+ switch (sig) {
+ case SIGCONT:
+ sid = task_session(t);
+ /*
+ * We don't return the error if sid == NULL.
+ * The task was unhashed, the caller must
+ * notice this.
+ */
+ if (!sid || sid == task_session(current))
+ break;
+ default:
+ return -EPERM;
+ }
}
}



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