Re: [PATCH v2 1/4] zram: fix crashes due to use of cpu hotplug multistate

[Minchan Kim]

On Fri, Apr 23, 2021 at 01:11:05AM +0000, Luis Chamberlain wrote:
> Provide a simple state machine to fix races with driver exit where we
> remove the CPU multistate callbacks and re-initialization / creation of
> new per CPU instances which should be managed by these callbacks.
> 
> The zram driver makes use of cpu hotplug multistate support, whereby it
> associates a struct zcomp per CPU. Each struct zcomp represents a
> compression algorithm in charge of managing compression streams per CPU.
> Although a compiled zram driver only supports a fixed set of compression
> algorithms, each zram device gets a struct zcomp allocated per CPU. The
> "multi" in CPU hotplug multstate refers to these per cpu struct zcomp
> instances. Each of these will have the CPU hotplug callback called for
> it on CPU plug / unplug. The kernel's CPU hotplug multistate keeps a
> linked list of these different structures so that it will iterate over
> them on CPU transitions.
> 
> By default at driver initialization we will create just one zram device
> (num_devices=1) and a zcomp structure then set for the now default
> lzo-rle comrpession algorithm. At driver removal we first remove each
> zram device, and so we destroy the associated struct zcomp per CPU. But
> since we expose sysfs attributes to create new devices or reset / initialize
> existing zram devices, we can easily end up re-initializing a struct zcomp
> for a zram device before the exit routine of the module removes the cpu
> hotplug callback. When this happens the kernel's CPU hotplug will detect
> that at least one instance (struct zcomp for us) exists. This can happen
> in the following situation:
> 
> CPU 1                            CPU 2
> 
> class_unregister(...);
> idr_for_each(...);
> zram_debugfs_destroy();
>                                 disksize_store(...);
> idr_destroy(...);
> unregister_blkdev(...);
> cpuhp_remove_multi_state(...);
> 
> The warning comes up on cpuhp_remove_multi_state() when it sees that the
> state for CPUHP_ZCOMP_PREPARE does not have an empty instance linked list.
> In this case, that a struct zcom still exists, the driver allowed its
> creation per CPU even though we could have just freed them per CPU
> though a call on another CPU, and we are then later trying to remove the
> hotplug callback.
> 
> Fix all this by providing a zram initialization boolean
> protected the the shared in the driver zram_index_mutex,
> which we can use to annotate when sysfs attributes are
> safe to use or not -- once the driver is properly initialized.
> When the driver is going down we also are sure to not let
> userspace muck with attributes which may affect each per cpu
> struct zcomp.
> 
> This also fixes a series of possible memory leaks. The
> crashes and memory leaks can easily be caused by issuing
> the zram02.sh script from the LTP project [0] in a loop
> in two separate windows:
> 
>   cd testcases/kernel/device-drivers/zram
>   while true; do PATH=$PATH:$PWD:$PWD/../../../lib/ ./zram02.sh; done
> 
> You end up with a splat as follows:
> 
> kernel: zram: Removed device: zram0
> kernel: zram: Added device: zram0
> kernel: zram0: detected capacity change from 0 to 209715200
> kernel: Adding 104857596k swap on /dev/zram0.  Priority:-2 extents:1 across:104857596k SSFS
> kernel: zram0: detected capacitky change from 209715200 to 0
> kernel: zram0: detected capacity change from 0 to 209715200
> kernel: ------------[ cut here ]------------
> kernel: Error: Removing state 63 which has instances left.
> kernel: WARNING: CPU: 7 PID: 70457 at kernel/cpu.c:2069 __cpuhp_remove_state_cpuslocked+0xf9/0x100
> kernel: Modules linked in: zram(E-) zsmalloc(E) <etc>
> kernel: CPU: 7 PID: 70457 Comm: rmmod Tainted: G            E     5.12.0-rc1-next-20210304 #3
> kernel: Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
> kernel: RIP: 0010:__cpuhp_remove_state_cpuslocked+0xf9/0x100
> kernel: Code: <etc>
> kernel: RSP: 0018:ffffa800c139be98 EFLAGS: 00010282
> kernel: RAX: 0000000000000000 RBX: ffffffff9083db58 RCX: ffff9609f7dd86d8
> kernel: RDX: 00000000ffffffd8 RSI: 0000000000000027 RDI: ffff9609f7dd86d0
> kernel: RBP: 0000000000000000i R08: 0000000000000000 R09: ffffa800c139bcb8
> kernel: R10: ffffa800c139bcb0 R11: ffffffff908bea40 R12: 000000000000003f
> kernel: R13: 00000000000009d8 R14: 0000000000000000 R15: 0000000000000000
> kernel: FS: 00007f1b075a7540(0000) GS:ffff9609f7dc0000(0000) knlGS:0000000000000000
> kernel: CS:  0010 DS: 0000 ES 0000 CR0: 0000000080050033
> kernel: CR2: 00007f1b07610490 CR3: 00000001bd04e000 CR4: 0000000000350ee0
> kernel: Call Trace:
> kernel: __cpuhp_remove_state+0x2e/0x80
> kernel: __do_sys_delete_module+0x190/0x2a0
> kernel:  do_syscall_64+0x33/0x80
> kernel: entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> The "Error: Removing state 63 which has instances left" refers
> to the zram per CPU struc zcomp instances left.
> 
> [0] https://github.com/linux-test-project/ltp.git
> 
> Signed-off-by: Luis Chamberlain <mcgrof@xxxxxxxxxx>
Acked-by: Minchan Kim <minchan@xxxxxxxxxx>