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

[Sourabh Jain]

On 01/02/23 07:25, Michael Ellerman wrote:
> Sourabh Jain <sourabhjain at linux.ibm.com> writes:
>> On 30/01/23 07:17, Michael Ellerman wrote:
>>> 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: 2377c92e37fe ("powerpc/kexec_file: fix FDT size estimation for kdump kernel")
>>> Cc: stable at vger.kernel.org # v5.12+
>>> Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
>>> ---
>>>    arch/powerpc/kexec/file_load_64.c | 10 ++++++----
>>>    1 file changed, 6 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/arch/powerpc/kexec/file_load_64.c b/arch/powerpc/kexec/file_load_64.c
>>> index af8854f9eae3..3caee570e79b 100644
>>> --- a/arch/powerpc/kexec/file_load_64.c
>>> +++ b/arch/powerpc/kexec/file_load_64.c
>>> @@ -989,10 +989,12 @@ 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
>> I failed to replicate this issue.
>>
>> Qemu command used:
>> $ qemu-system-ppc64 -enable-kvm -smp 4,cores=2 -drive
>> file=my-image.qcow2 -nographic -m 2G
>>
>>
>> lsmem (inside guest):
>> RANGE                                 SIZE  STATE REMOVABLE BLOCK
>> 0x0000000000000000-0x000000007fffffff   2G online       yes 0-127
>>
>> Memory block size:        16M
>> Total online memory:       2G
>> Total offline memory:      0B
>>
>> Not sure what I am missing, but changes looks good to me.
> Hmm, interesting.
>
> Do you have /proc/device-tree/ibm,dynamic-reconfiguration-memory in the VM?

No I don't.

But if I add maxmem=x to qemu command line then ibm,dynamic-reconfiguration
DT node comes up.

> What version of qemu are you using? I think I tested with 6.2 and 7.1.

QEMU emulator version 7.2.50 (v7.2.0-904-g13356edb87)

I tried with QEMU emulator version 7.1.0 (v7.1.0), but not able to 
replicate it.

- Sourabh