powerpc/kexec_file: Fix division by zero in extra size estimation

In kexec_extra_fdt_size_ppc64() there's logic to estimate how much
extra space will be needed in the device tree for some memory related
properties.

That logic uses the size of RAM divided by drmem_lmb_size() to do the
estimation. However drmem_lmb_size() can be zero if the machine has no
hotpluggable memory configured, which is the case when booting with qemu
and no maxmem=x parameter is passed (the default).

The division by zero is reported by UBSAN, and can also lead to an
overflow and a warning from kvmalloc, and kdump kernel loading fails:

  WARNING: CPU: 0 PID: 133 at mm/util.c:596 kvmalloc_node+0x15c/0x160
  Modules linked in:
  CPU: 0 PID: 133 Comm: kexec Not tainted 6.2.0-rc5-03455-g07358bd97810 #223
  Hardware name: IBM pSeries (emulated by qemu) POWER9 (raw) 0x4e1200 0xf000005 of:SLOF,git-dd0dca pSeries
  NIP:  c00000000041ff4c LR: c00000000041fe58 CTR: 0000000000000000
  REGS: c0000000096ef750 TRAP: 0700   Not tainted  (6.2.0-rc5-03455-g07358bd97810)
  MSR:  800000000282b033 <SF,VEC,VSX,EE,FP,ME,IR,DR,RI,LE>  CR: 24248242  XER: 2004011e
  CFAR: c00000000041fed0 IRQMASK: 0
  ...
  NIP kvmalloc_node+0x15c/0x160
  LR  kvmalloc_node+0x68/0x160
  Call Trace:
    kvmalloc_node+0x68/0x160 (unreliable)
    of_kexec_alloc_and_setup_fdt+0xb8/0x7d0
    elf64_load+0x25c/0x4a0
    kexec_image_load_default+0x58/0x80
    sys_kexec_file_load+0x5c0/0x920
    system_call_exception+0x128/0x330
    system_call_vectored_common+0x15c/0x2ec

To fix it, skip the calculation if drmem_lmb_size() is zero.

Fixes: 2377c92e ("powerpc/kexec_file: fix FDT size estimation for kdump kernel")
Cc: stable@vger.kernel.org # v5.12+
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230130014707.541110-1-mpe@ellerman.id.au

arch/powerpc/kexec/file_load_64.c

@@ -989,10 +989,13 @@ unsigned int kexec_extra_fdt_size_ppc64(struct kimage *image)
 	 * linux,drconf-usable-memory properties. Get an approximate on the
 	 * number of usable memory entries and use for FDT size estimation.
 	 */
-	usm_entries = ((memblock_end_of_DRAM() / drmem_lmb_size()) +
-		       (2 * (resource_size(&crashk_res) / drmem_lmb_size())));
-
-	extra_size = (unsigned int)(usm_entries * sizeof(u64));
+	if (drmem_lmb_size()) {
+		usm_entries = ((memblock_end_of_DRAM() / drmem_lmb_size()) +
+			       (2 * (resource_size(&crashk_res) / drmem_lmb_size())));
+		extra_size = (unsigned int)(usm_entries * sizeof(u64));
+	} else {
+		extra_size = 0;
+	}
 
 	/*
 	 * Get the number of CPU nodes in the current DT. This allows to