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85b6d24646
Currently, the exit_shm() function not designed to work properly when
task->sysvshm.shm_clist holds shm objects from different IPC namespaces.
This is a real pain when sysctl kernel.shm_rmid_forced = 1, because it
leads to use-after-free (reproducer exists).
This is an attempt to fix the problem by extending exit_shm mechanism to
handle shm's destroy from several IPC ns'es.
To achieve that we do several things:
1. add a namespace (non-refcounted) pointer to the struct shmid_kernel
2. during new shm object creation (newseg()/shmget syscall) we
initialize this pointer by current task IPC ns
3. exit_shm() fully reworked such that it traverses over all shp's in
task->sysvshm.shm_clist and gets IPC namespace not from current task
as it was before but from shp's object itself, then call
shm_destroy(shp, ns).
Note: We need to be really careful here, because as it was said before
(1), our pointer to IPC ns non-refcnt'ed. To be on the safe side we
using special helper get_ipc_ns_not_zero() which allows to get IPC ns
refcounter only if IPC ns not in the "state of destruction".
Q/A
Q: Why can we access shp->ns memory using non-refcounted pointer?
A: Because shp object lifetime is always shorther than IPC namespace
lifetime, so, if we get shp object from the task->sysvshm.shm_clist
while holding task_lock(task) nobody can steal our namespace.
Q: Does this patch change semantics of unshare/setns/clone syscalls?
A: No. It's just fixes non-covered case when process may leave IPC
namespace without getting task->sysvshm.shm_clist list cleaned up.
Link: https://lkml.kernel.org/r/67bb03e5-f79c-1815-e2bf-949c67047418@colorfullife.com
Link: https://lkml.kernel.org/r/20211109151501.4921-1-manfred@colorfullife.com
Fixes:
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|---|---|---|
| .. | ||
| autogroup.h | ||
| clock.h | ||
| coredump.h | ||
| cpufreq.h | ||
| cputime.h | ||
| deadline.h | ||
| debug.h | ||
| hotplug.h | ||
| idle.h | ||
| init.h | ||
| isolation.h | ||
| jobctl.h | ||
| loadavg.h | ||
| mm.h | ||
| nohz.h | ||
| numa_balancing.h | ||
| prio.h | ||
| rt.h | ||
| sd_flags.h | ||
| signal.h | ||
| smt.h | ||
| stat.h | ||
| sysctl.h | ||
| task_stack.h | ||
| task.h | ||
| topology.h | ||
| types.h | ||
| user.h | ||
| wake_q.h | ||
| xacct.h | ||