powerpc/rtas_flash: allow user copy to flash block cache objects

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

powerpc/rtas_flash: allow user copy to flash block cache objects

With hardened usercopy enabled (CONFIG_HARDENED_USERCOPY=y), using the

/proc/powerpc/rtas/firmware_update interface to prepare a system

firmware update yields a BUG():

kernel BUG at mm/usercopy.c:102!

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

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

Modules linked in:

CPU: 0 PID: 2232 Comm: dd Not tainted 6.5.0-rc3+ #2

Hardware name: IBM,8408-E8E POWER8E (raw) 0x4b0201 0xf000004 of:IBM,FW860.50 (SV860_146) hv:phyp pSeries

NIP: c0000000005991d0 LR: c0000000005991cc CTR: 0000000000000000

REGS: c0000000148c76a0 TRAP: 0700 Not tainted (6.5.0-rc3+)

MSR: 8000000000029033 <SF,EE,ME,IR,DR,RI,LE> CR: 24002242 XER: 0000000c

CFAR: c0000000001fbd34 IRQMASK: 0

[ ... GPRs omitted ... ]

NIP usercopy_abort+0xa0/0xb0

LR usercopy_abort+0x9c/0xb0

Call Trace:

usercopy_abort+0x9c/0xb0 (unreliable)

__check_heap_object+0x1b4/0x1d0

__check_object_size+0x2d0/0x380

rtas_flash_write+0xe4/0x250

proc_reg_write+0xfc/0x160

vfs_write+0xfc/0x4e0

ksys_write+0x90/0x160

system_call_exception+0x178/0x320

system_call_common+0x160/0x2c4

The blocks of the firmware image are copied directly from user memory

to objects allocated from flash_block_cache, so flash_block_cache must

be created using kmem_cache_create_usercopy() to mark it safe for user

access.

[mpe: Trim and indent oops]