powerpc/pseries/memhp: Fix access beyond end of drmem array

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

powerpc/pseries/memhp: Fix access beyond end of drmem array

dlpar_memory_remove_by_index() may access beyond the bounds of the

drmem lmb array when the LMB lookup fails to match an entry with the

given DRC index. When the search fails, the cursor is left pointing to

&drmem_info->lmbs[drmem_info->n_lmbs], which is one element past the

last valid entry in the array. The debug message at the end of the

function then dereferences this pointer:

pr_debug("Failed to hot-remove memory at %llx\n",

lmb->base_addr);

This was found by inspection and confirmed with KASAN:

pseries-hotplug-mem: Attempting to hot-remove LMB, drc index 1234

==================================================================

BUG: KASAN: slab-out-of-bounds in dlpar_memory+0x298/0x1658

Read of size 8 at addr c000000364e97fd0 by task bash/949

dump_stack_lvl+0xa4/0xfc (unreliable)

print_report+0x214/0x63c

kasan_report+0x140/0x2e0

__asan_load8+0xa8/0xe0

dlpar_memory+0x298/0x1658

handle_dlpar_errorlog+0x130/0x1d0

dlpar_store+0x18c/0x3e0

kobj_attr_store+0x68/0xa0

sysfs_kf_write+0xc4/0x110

kernfs_fop_write_iter+0x26c/0x390

vfs_write+0x2d4/0x4e0

ksys_write+0xac/0x1a0

system_call_exception+0x268/0x530

system_call_vectored_common+0x15c/0x2ec

Allocated by task 1:

kasan_save_stack+0x48/0x80

kasan_set_track+0x34/0x50

kasan_save_alloc_info+0x34/0x50

__kasan_kmalloc+0xd0/0x120

__kmalloc+0x8c/0x320

kmalloc_array.constprop.0+0x48/0x5c

drmem_init+0x2a0/0x41c

do_one_initcall+0xe0/0x5c0

kernel_init_freeable+0x4ec/0x5a0

kernel_init+0x30/0x1e0

ret_from_kernel_user_thread+0x14/0x1c

The buggy address belongs to the object at c000000364e80000

which belongs to the cache kmalloc-128k of size 131072

The buggy address is located 0 bytes to the right of

allocated 98256-byte region [c000000364e80000, c000000364e97fd0)

==================================================================

pseries-hotplug-mem: Failed to hot-remove memory at 0

Log failed lookups with a separate message and dereference the

cursor only when it points to a valid entry.