Re: [PATCH v8 11/12] zram: fix crashes with cpu hotplug multistate

Kees Cook
2021-10-05T20:55:39Z

On Mon, Sep 27, 2021 at 09:38:04AM -0700, 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
>
> disksize_store(...);
> class_unregister(...);
> idr_for_each(...);
> zram_debugfs_destroy();
>
> idr_destroy(...);
> unregister_blkdev(...);
> cpuhp_remove_multi_state(...);

So this is strictly separate from the sysfs/module unloading race?

-Kees

>
> 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 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. <etc>
> 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:<etc>
> 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 struct zcomp instances left.
>
> [0] https://github.com/linux-test-project/ltp.git
>
> Acked-by: Minchan Kim <minchan@kernel.org>
> Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
> ---
> drivers/block/zram/zram_drv.c | 63 ++++++++++++++++++++++++++++++-----
> 1 file changed, 55 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index f61910c65f0f..b26abcb955cc 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -44,6 +44,8 @@ static DEFINE_MUTEX(zram_index_mutex);
> static int zram_major;
> static const char *default_compressor = CONFIG_ZRAM_DEF_COMP;
>
> +static bool zram_up;
> +
> /* Module params (documentation at end) */
> static unsigned int num_devices = 1;
> /*
> @@ -1704,6 +1706,7 @@ static void zram_reset_device(struct zram *zram)
> comp = zram->comp;
> disksize = zram->disksize;
> zram->disksize = 0;
> + zram->comp = NULL;
>
> set_capacity_and_notify(zram->disk, 0);
> part_stat_set_all(zram->disk->part0, 0);
> @@ -1724,9 +1727,18 @@ static ssize_t disksize_store(struct device *dev,
> struct zram *zram = dev_to_zram(dev);
> int err;
>
> + mutex_lock(&zram_index_mutex);
> +
> + if (!zram_up) {
> + err = -ENODEV;
> + goto out;
> + }
> +
> disksize = memparse(buf, NULL);
> - if (!disksize)
> - return -EINVAL;
> + if (!disksize) {
> + err = -EINVAL;
> + goto out;
> + }
>
> down_write(&zram->init_lock);
> if (init_done(zram)) {
> @@ -1754,12 +1766,16 @@ static ssize_t disksize_store(struct device *dev,
> set_capacity_and_notify(zram->disk, zram->disksize >> SECTOR_SHIFT);
> up_write(&zram->init_lock);
>
> + mutex_unlock(&zram_index_mutex);
> +
> return len;
>
> out_free_meta:
> zram_meta_free(zram, disksize);
> out_unlock:
> up_write(&zram->init_lock);
> +out:
> + mutex_unlock(&zram_index_mutex);
> return err;
> }
>
> @@ -1775,8 +1791,17 @@ static ssize_t reset_store(struct device *dev,
> if (ret)
> return ret;
>
> - if (!do_reset)
> - return -EINVAL;
> + mutex_lock(&zram_index_mutex);
> +
> + if (!zram_up) {
> + len = -ENODEV;
> + goto out;
> + }
> +
> + if (!do_reset) {
> + len = -EINVAL;
> + goto out;
> + }
>
> zram = dev_to_zram(dev);
> bdev = zram->disk->part0;
> @@ -1785,7 +1810,8 @@ static ssize_t reset_store(struct device *dev,
> /* Do not reset an active device or claimed device */
> if (bdev->bd_openers || zram->claim) {
> mutex_unlock(&bdev->bd_disk->open_mutex);
> - return -EBUSY;
> + len = -EBUSY;
> + goto out;
> }
>
> /* From now on, anyone can't open /dev/zram[0-9] */
> @@ -1800,6 +1826,8 @@ static ssize_t reset_store(struct device *dev,
> zram->claim = false;
> mutex_unlock(&bdev->bd_disk->open_mutex);
>
> +out:
> + mutex_unlock(&zram_index_mutex);
> return len;
> }
>
> @@ -2010,6 +2038,10 @@ static ssize_t hot_add_show(struct class *class,
> int ret;
>
> mutex_lock(&zram_index_mutex);
> + if (!zram_up) {
> + mutex_unlock(&zram_index_mutex);
> + return -ENODEV;
> + }
> ret = zram_add();
> mutex_unlock(&zram_index_mutex);
>
> @@ -2037,6 +2069,11 @@ static ssize_t hot_remove_store(struct class *class,
>
> mutex_lock(&zram_index_mutex);
>
> + if (!zram_up) {
> + ret = -ENODEV;
> + goto out;
> + }
> +
> zram = idr_find(&zram_index_idr, dev_id);
> if (zram) {
> ret = zram_remove(zram);
> @@ -2046,6 +2083,7 @@ static ssize_t hot_remove_store(struct class *class,
> ret = -ENODEV;
> }
>
> +out:
> mutex_unlock(&zram_index_mutex);
> return ret ? ret : count;
> }
> @@ -2072,12 +2110,15 @@ static int zram_remove_cb(int id, void *ptr, void *data)
>
> static void destroy_devices(void)
> {
> + mutex_lock(&zram_index_mutex);
> + zram_up = false;
> class_unregister(&zram_control_class);
> idr_for_each(&zram_index_idr, &zram_remove_cb, NULL);
> zram_debugfs_destroy();
> idr_destroy(&zram_index_idr);
> unregister_blkdev(zram_major, "zram");
> cpuhp_remove_multi_state(CPUHP_ZCOMP_PREPARE);
> + mutex_unlock(&zram_index_mutex);
> }
>
> static int __init zram_init(void)
> @@ -2105,15 +2146,21 @@ static int __init zram_init(void)
> return -EBUSY;
> }
>
> + mutex_lock(&zram_index_mutex);
> +
> while (num_devices != 0) {
> - mutex_lock(&zram_index_mutex);
> ret = zram_add();
> - mutex_unlock(&zram_index_mutex);
> - if (ret < 0)
> + if (ret < 0) {
> + mutex_unlock(&zram_index_mutex);
> goto out_error;
> + }
> num_devices--;
> }
>
> + zram_up = true;
> +
> + mutex_unlock(&zram_index_mutex);
> +
> return 0;
>
> out_error:
> --
> 2.30.2
>

--
Kees Cook