linux-kernel - [PATCH 6.6.y] zram: fix NULL pointer in comp_algorithm

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Message-ID: <20250311003949.3927527-1-jianqi.ren.cn@windriver.com>
Date: Tue, 11 Mar 2025 08:39:49 +0800
From: <jianqi.ren.cn@...driver.com>
To: <stable@...r.kernel.org>, <liushixin2@...wei.com>
CC: <linux-kernel@...r.kernel.org>, <gregkh@...uxfoundation.org>,
        <patches@...ts.linux.dev>, <minchan@...nel.org>,
        <senozhatsky@...omium.org>, <axboe@...nel.dk>,
        <akpm@...ux-foundation.org>, <linux-block@...r.kernel.org>
Subject: [PATCH 6.6.y] zram: fix NULL pointer in comp_algorithm_show()

From: Liu Shixin <liushixin2@...wei.com>

[ Upstream commit f364cdeb38938f9d03061682b8ff3779dd1730e5 ]

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: 7ac07a26dea7 ("zram: preparation for multi-zcomp support")
Signed-off-by: Liu Shixin <liushixin2@...wei.com>
Reviewed-by: Sergey Senozhatsky <senozhatsky@...omium.org>
Cc: Jens Axboe <axboe@...nel.dk>
Cc: Minchan Kim <minchan@...nel.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
[This fix does not backport zram_comp_params_reset which was introduced after
 v6.6, in commit f2bac7ad187d ("zram: introduce zcomp_params structure")]
Signed-off-by: Jianqi Ren <jianqi.ren.cn@...driver.com>
Signed-off-by: He Zhe <zhe.he@...driver.com>
---
Verified the build test
---
 drivers/block/zram/zram_drv.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index b73038ad86f7..44cf0e51d7db 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -2247,6 +2247,8 @@ static int zram_add(void)
 	zram->disk->private_data = zram;
 	snprintf(zram->disk->disk_name, 16, "zram%d", device_id);
 
+	comp_algorithm_set(zram, ZRAM_PRIMARY_COMP, default_compressor);
+
 	/* Actual capacity set using sysfs (/sys/block/zram<id>/disksize */
 	set_capacity(zram->disk, 0);
 	/* zram devices sort of resembles non-rotational disks */
@@ -2281,8 +2283,6 @@ static int zram_add(void)
 	if (ret)
 		goto out_cleanup_disk;
 
-	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;
-- 
2.25.1