On Thu, 1 Jul 2021 at 23:41, Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote: > > Marco Elver <elver@xxxxxxxxxx> writes: > > > If perf_event_open() is called with another task as target and > > perf_event_attr::sigtrap is set, and the target task's user does not > > match the calling user, also require the CAP_KILL capability. > > > > Otherwise, with the CAP_PERFMON capability alone it would be possible > > for a user to send SIGTRAP signals via perf events to another user's > > tasks. This could potentially result in those tasks being terminated if > > they cannot handle SIGTRAP signals. > > > > Note: The check complements the existing capability check, but is not > > supposed to supersede the ptrace_may_access() check. At a high level we > > now have: > > > > capable of CAP_PERFMON and (CAP_KILL if sigtrap) > > OR > > ptrace_may_access() // also checks for same thread-group and uid > > Is there anyway we could have a comment that makes the required > capability checks clear? > > Basically I see an inlined version of kill_ok_by_cred being implemented > without the comments on why the various pieces make sense. I'll add more comments. It probably also makes sense to factor the code here into its own helper. > Certainly ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS) should not > be a check to allow writing/changing a task. It needs to be > PTRACE_MODE_ATTACH_REALCREDS, like /proc/self/mem uses. So if attr.sigtrap the checked ptrace mode needs to switch to PTRACE_MODE_ATTACH_REALCREDS. Otherwise, it is possible to send a signal if only read-ptrace permissions are granted. Is my assumption here correct? > Now in practice I think your patch probably has the proper checks in > place for sending a signal but it is far from clear. Thanks, -- Marco
