Hi,
On 11/29/21 20:47, Alexander Mikhalitsyn wrote:
For 4.14.y:
Upstream commit 85b6d24646e4 ("shm: extend forced shm destroy to support objects from several IPC nses")
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@xxxxxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/20211109151501.4921-1-manfred@xxxxxxxxxxxxxxxx
Fixes: ab602f79915 ("shm: make exit_shm work proportional to task activity")
Co-developed-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
Signed-off-by: Alexander Mikhalitsyn <alexander.mikhalitsyn@xxxxxxxxxxxxx>
Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Cc: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Andrei Vagin <avagin@xxxxxxxxx>
Cc: Pavel Tikhomirov <ptikhomirov@xxxxxxxxxxxxx>
Cc: Vasily Averin <vvs@xxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/ipc_namespace.h | 15 +++
include/linux/sched/task.h | 2 +-
include/linux/shm.h | 13 ++-
ipc/shm.c | 176 +++++++++++++++++++++++++---------
4 files changed, 159 insertions(+), 47 deletions(-)
Tested ok: The crash is resolved, no observed degradations.
--
Manfred
