zram: fix NULL pointer in comp_algorithm_show()

JIRA: https://issues.redhat.com/browse/RHEL-77764
CVE: CVE-2024-53222

commit f364cde
Author: Liu Shixin <liushixin2@huawei.com>
Date:   Fri Nov 8 18:01:47 2024 +0800

    zram: fix NULL pointer in comp_algorithm_show()

    LTP reported a NULL pointer dereference as followed:

     CPU: 7 UID: 0 PID: 5995 Comm: cat Kdump: loaded Not tainted 6.12.0-rc6+ #3
     Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
     pstate: 40400005 (nZcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
     pc : __pi_strcmp+0x24/0x140
     lr : zcomp_available_show+0x60/0x100 [zram]
     sp : ffff800088b93b90
     x29: ffff800088b93b90 x28: 0000000000000001 x27: 0000000000400cc0
     x26: 0000000000000ffe x25: ffff80007b3e2388 x24: 0000000000000000
     x23: ffff80007b3e2390 x22: ffff0004041a9000 x21: ffff80007b3e2900
     x20: 0000000000000000 x19: 0000000000000000 x18: 0000000000000000
     x17: 0000000000000000 x16: 0000000000000000 x15: 0000000000000000
     x14: 0000000000000000 x13: 0000000000000000 x12: 0000000000000000
     x11: 0000000000000000 x10: ffff80007b3e2900 x9 : ffff80007b3cb280
     x8 : 0101010101010101 x7 : 0000000000000000 x6 : 0000000000000000
     x5 : 0000000000000040 x4 : 0000000000000000 x3 : 00656c722d6f7a6c
     x2 : 0000000000000000 x1 : ffff80007b3e2900 x0 : 0000000000000000
     Call trace:
      __pi_strcmp+0x24/0x140
      comp_algorithm_show+0x40/0x70 [zram]
      dev_attr_show+0x28/0x80
      sysfs_kf_seq_show+0x90/0x140
      kernfs_seq_show+0x34/0x48
      seq_read_iter+0x1d4/0x4e8
      kernfs_fop_read_iter+0x40/0x58
      new_sync_read+0x9c/0x168
      vfs_read+0x1a8/0x1f8
      ksys_read+0x74/0x108
      __arm64_sys_read+0x24/0x38
      invoke_syscall+0x50/0x120
      el0_svc_common.constprop.0+0xc8/0xf0
      do_el0_svc+0x24/0x38
      el0_svc+0x38/0x138
      el0t_64_sync_handler+0xc0/0xc8
      el0t_64_sync+0x188/0x190

    The zram->comp_algs[ZRAM_PRIMARY_COMP] can be NULL in zram_add() if
    comp_algorithm_set() has not been called.  User can access the zram device
    by sysfs after device_add_disk(), so there is a time window to trigger the
    NULL pointer dereference.  Move it ahead device_add_disk() to make sure
    when user can access the zram device, it is ready.  comp_algorithm_set()
    is protected by zram->init_lock in other places and no such problem.

    Link: https://lkml.kernel.org/r/20241108100147.3776123-1-liushixin2@huawei.com
    Fixes: 7ac07a2 ("zram: preparation for multi-zcomp support")
    Signed-off-by: Liu Shixin <liushixin2@huawei.com>
    Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
    Cc: Jens Axboe <axboe@kernel.dk>
    Cc: Minchan Kim <minchan@kernel.org>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Signed-off-by: Ming Lei <ming.lei@redhat.com>

Author: Ming Lei

drivers/block/zram/zram_drv.c

@@ -2382,15 +2382,15 @@ static int zram_add(void)
 	zram->disk->private_data = zram;
 	snprintf(zram->disk->disk_name, 16, "zram%d", device_id);
 
+	zram_comp_params_reset(zram);
+	comp_algorithm_set(zram, ZRAM_PRIMARY_COMP, default_compressor);
+
 	/* Actual capacity set using sysfs (/sys/block/zram<id>/disksize */
 	set_capacity(zram->disk, 0);
 	ret = device_add_disk(NULL, zram->disk, zram_disk_groups);
 	if (ret)
 		goto out_cleanup_disk;
 
-	zram_comp_params_reset(zram);
-	comp_algorithm_set(zram, ZRAM_PRIMARY_COMP, default_compressor);
-
 	zram_debugfs_register(zram);
 	pr_info("Added device: %s\n", zram->disk->disk_name);
 	return device_id;