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

[Sourabh Jain <sourabhjain@linux.ibm.com>]

Hello Michael,

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@vger.kernel.org # v5.12+
> Signed-off-by: Michael Ellerman <mpe@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.

Thanks,
Sourabh Jain