powerpc/pseries: Whitelist dtl slub object for copying to userspace

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

powerpc/pseries: Whitelist dtl slub object for copying to userspace

Reading the dispatch trace log from /sys/kernel/debug/powerpc/dtl/cpu-*

results in a BUG() when the config CONFIG_HARDENED_USERCOPY is enabled as

shown below.

kernel BUG at mm/usercopy.c:102!

Oops: Exception in kernel mode, sig: 5 [#1]

LE PAGE_SIZE=64K MMU=Radix SMP NR_CPUS=2048 NUMA pSeries

Modules linked in: xfs libcrc32c dm_service_time sd_mod t10_pi sg ibmvfc

scsi_transport_fc ibmveth pseries_wdt dm_multipath dm_mirror dm_region_hash dm_log dm_mod fuse

CPU: 27 PID: 1815 Comm: python3 Not tainted 6.10.0-rc3 #85

Hardware name: IBM,9040-MRX POWER10 (raw) 0x800200 0xf000006 of:IBM,FW1060.00 (NM1060_042) hv:phyp pSeries

NIP: c0000000005d23d4 LR: c0000000005d23d0 CTR: 00000000006ee6f8

REGS: c000000120c078c0 TRAP: 0700 Not tainted (6.10.0-rc3)

MSR: 8000000000029033 <SF,EE,ME,IR,DR,RI,LE> CR: 2828220f XER: 0000000e

CFAR: c0000000001fdc80 IRQMASK: 0

[ ... GPRs omitted ... ]

NIP [c0000000005d23d4] usercopy_abort+0x78/0xb0

LR [c0000000005d23d0] usercopy_abort+0x74/0xb0

Call Trace:

usercopy_abort+0x74/0xb0 (unreliable)

__check_heap_object+0xf8/0x120

check_heap_object+0x218/0x240

__check_object_size+0x84/0x1a4

dtl_file_read+0x17c/0x2c4

full_proxy_read+0x8c/0x110

vfs_read+0xdc/0x3a0

ksys_read+0x84/0x144

system_call_exception+0x124/0x330

system_call_vectored_common+0x15c/0x2ec

--- interrupt: 3000 at 0x7fff81f3ab34

Commit 6d07d1cd300f ("usercopy: Restrict non-usercopy caches to size 0")

requires that only whitelisted areas in slab/slub objects can be copied to

userspace when usercopy hardening is enabled using CONFIG_HARDENED_USERCOPY.

Dtl contains hypervisor dispatch events which are expected to be read by

privileged users. Hence mark this safe for user access.

Specify useroffset=0 and usersize=DISPATCH_LOG_BYTES to whitelist the

entire object.