From 0e277a39c8b6f905e289b75e8ad0594e6b3562ca Mon Sep 17 00:00:00 2001 From: Rahat Mahmood Date: Thu, 1 Nov 2018 15:53:25 -0700 Subject: Prevent premature destruction of shm segments. Shm segments can be marked for lazy destruction via shmctl(IPC_RMID), which destroys a segment once it is no longer attached to any processes. We were unconditionally decrementing the segment refcount on shmctl(IPC_RMID) which allowed a user to force a segment to be destroyed by repeatedly calling shmctl(IPC_RMID), with outstanding memory maps to the segment. This is problematic because the memory released by a segment destroyed this way can be reused by a different process while remaining accessible by the process with outstanding maps to the segment. PiperOrigin-RevId: 219713660 Change-Id: I443ab838322b4fb418ed87b2722c3413ead21845 --- pkg/sentry/kernel/shm/shm.go | 13 +++++++++++-- 1 file changed, 11 insertions(+), 2 deletions(-) (limited to 'pkg/sentry/kernel/shm') diff --git a/pkg/sentry/kernel/shm/shm.go b/pkg/sentry/kernel/shm/shm.go index 2feffe612..f760f5f76 100644 --- a/pkg/sentry/kernel/shm/shm.go +++ b/pkg/sentry/kernel/shm/shm.go @@ -575,10 +575,19 @@ func (s *Shm) destroy() { func (s *Shm) MarkDestroyed() { s.mu.Lock() defer s.mu.Unlock() + // Prevent the segment from being found in the registry. s.key = linux.IPC_PRIVATE - s.pendingDestruction = true - s.DecRef() + + // Only drop the segment's self-reference once, when destruction is + // requested. Otherwise, repeated calls shmctl(IPC_RMID) would force a + // segment to be destroyed prematurely, potentially with active maps to the + // segment's address range. Remaining references are dropped when the + // segment is detached or unmaped. + if !s.pendingDestruction { + s.pendingDestruction = true + s.DecRef() + } } // checkOwnership verifies whether a segment may be accessed by ctx as an -- cgit v1.2.3