x86/crash: fix crash_setup_memmap_entries() out-of-bounds access

[Dave Young]

Hi Mike,

Thanks for the patch! I suggest always cc kexec list for kexec/kdump
patches.
On 04/15/21 at 07:56pm, Mike Galbraith wrote:
> x86/crash: fix crash_setup_memmap_entries() KASAN vmalloc-out-of-bounds gripe
> 
> [   15.428011] BUG: KASAN: vmalloc-out-of-bounds in crash_setup_memmap_entries+0x17e/0x3a0
> [   15.428018] Write of size 8 at addr ffffc90000426008 by task kexec/1187
> 
> (gdb) list *crash_setup_memmap_entries+0x17e
> 0xffffffff8107cafe is in crash_setup_memmap_entries (arch/x86/kernel/crash.c:322).
> 317                                      unsigned long long mend)
> 318     {
> 319             unsigned long start, end;
> 320
> 321             cmem->ranges[0].start = mstart;
> 322             cmem->ranges[0].end = mend;
> 323             cmem->nr_ranges = 1;
> 324
> 325             /* Exclude elf header region */
> 326             start = image->arch.elf_load_addr;
> (gdb)
> 
> We're excluding two ranges, allocate the scratch space we need to do that.

I think 1 range should be fine, have you tested 1?

The code is just excluding the elf header space which will be loaded
first before anything else so I assume it will be just at the start of
the crashkernel resource region.  Thus [a b] after exclude the start
part will be [c b].  But I have not read the code for long time, maybe I
need to double check.

But anyway 2 would be good since the code is obscure we can easily miss
it in the future.  See how other people think.

> 
> Signed-off-by: Mike Galbraith <efault at gmx.de>
> ---
>  arch/x86/kernel/crash.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- a/arch/x86/kernel/crash.c
> +++ b/arch/x86/kernel/crash.c
> @@ -337,7 +337,7 @@ int crash_setup_memmap_entries(struct ki
>  	struct crash_memmap_data cmd;
>  	struct crash_mem *cmem;
> 
> -	cmem = vzalloc(sizeof(struct crash_mem));
> +	cmem = vzalloc(sizeof(struct crash_mem)+(2*sizeof(struct crash_mem_range)));

Thanks for the patch, can you try below?
vzalloc(struct_size(cmem, ranges, 2));


>  	if (!cmem)
>  		return -ENOMEM;
> 
> 

Thanks
Dave