On (24/11/08 18:01), Liu Shixin wrote:
> 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 explanation below is more than enough, I think this stack trace
doesn't really show anything new or interesting.
> 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.
>
> Fixes: 7ac07a26dea7 ("zram: preparation for multi-zcomp support")
So I think this fixes something much older, probably around e46b8a030d76d
time (2014).
> Signed-off-by: Liu Shixin <liushixin2@xxxxxxxxxx>
> ---
Reviewed-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
