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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
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Rahat Mahmood authored and shentubot committed Nov 1, 2018
1 parent b23cd33 commit 0e277a39c8b6f905e289b75e8ad0594e6b3562ca
Showing with 12 additions and 3 deletions.
  1. +11 −2 pkg/sentry/kernel/shm/shm.go
  2. +1 −1 pkg/sentry/syscalls/linux/sys_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
@@ -144,7 +144,7 @@ func Shmctl(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Syscal
return 0, nil, nil
case linux.SHM_LOCK, linux.SHM_UNLOCK:
// We currently do not support memmory locking anywhere.
// We currently do not support memory locking anywhere.
// mlock(2)/munlock(2) are currently stubbed out as no-ops so do the
// same here.
t.Kernel().EmitUnimplementedEvent(t)

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