AZL-55192

In the Linux kernel, the following vulnerability has been resolved:

zram: fix NULL pointer in compalgorithmshow()

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 : __pistrcmp+0x24/0x140 lr : zcompavailable_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: __pistrcmp+0x24/0x140 compalgorithm_show+0x40/0x70 [zram] devattrshow+0x28/0x80 sysfskfseqshow+0x90/0x140 kernfsseqshow+0x34/0x48 seqreaditer+0x1d4/0x4e8 kernfsfopreaditer+0x40/0x58 newsyncread+0x9c/0x168 vfsread+0x1a8/0x1f8 ksysread+0x74/0x108 _arm64sysread+0x24/0x38 invokesyscall+0x50/0x120 el0svccommon.constprop.0+0xc8/0xf0 doel0svc+0x24/0x38 el0svc+0x38/0x138 el0t64synchandler+0xc0/0xc8 el0t64sync+0x188/0x190

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