Re: [PATCH v3] kexec: fix out of the ELF headers buffer issue in syscall kexec_file_load()

[Dave Young]

On 09/29/15 at 08:58pm, Lee, Chun-Yi wrote:
> This patch modified the code in fill_up_crash_elf_data by using
> walk_system_ram_res instead of walk_system_ram_range to count the max
> number of crash memory ranges. That's because the walk_system_ram_range
> filters out small memory regions that are resided in the same page, but
> walk_system_ram_res does not.
> 
> The oringial issue is page fault error that sometimes happened on big machines
> when preparing ELF headers:
> 
> [  305.291522] BUG: unable to handle kernel paging request at ffffc90613fc9000
> [  305.299621] IP: [<ffffffff8103d645>] prepare_elf64_ram_headers_callback+0x165/0x260
> [  305.308300] PGD e000032067 PUD 6dcbec54067 PMD 9dc9bdeb067 PTE 0
> [  305.315393] Oops: 0002 [#1] SMP
> [...snip]
> [  305.420953] task: ffff8e1c01ced600 ti: ffff8e1c03ec2000 task.ti: ffff8e1c03ec2000
> [  305.429292] RIP: 0010:[<ffffffff8103d645>]  [<ffffffff8103d645>] prepare_elf64_ra
> m_headers_callback+0x165/0x260
> [...snip]
> 
> After tracing prepare_elf64_headers and prepare_elf64_ram_headers_callback,
> the code uses walk_system_ram_res to fill-in crash memory regions information
> to program header, so it counts those small memory regions that are resided in
> a page area. But, when kernel was using walk_system_ram_range in
> fill_up_crash_elf_data to count the number of crash memory regions, it filters
> out small regions. I printed those small memory regions, for example:
> 
> kexec: Get nr_ram ranges. vaddr=0xffff880077592258 paddr=0x77592258, sz=0xdc0
> 
> Base on the code in walk_system_ram_range, this memory region will be filtered
> out:
> 
> pfn = (0x77592258 + 0x1000 - 1) >> 12 = 0x77593
> end_pfn = (0x77592258 + 0xfc0 -1 + 1) >> 12 = 0x77593
> end_pfn - pfn = 0x77593 - 0x77593 = 0  <=== if (end_pfn > pfn) is FALSE
> 
> So, the max_nr_ranges that's counted by kernel doesn't include small memory
> regions. That causes the page fault issue happened in later code path for
> preparing EFL headers.
> 
> This issus is not easy to reproduce on small machines that don't have too
> many CPUs because the allocated page aligned ELF buffer has more free space
> to cover those small memory regions' PT_LOAD headers.
> 
> v3:
> Changed the declaration of nr_ranges to be unsigned int*
> 
> v2:
> To simplify the patch description, removed some things about CPU number to
> avoid confusing patch reviewer.
> 
> Signed-off-by: Lee, Chun-Yi <jlee@xxxxxxxx>
> ---
>  arch/x86/kernel/crash.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
> index e068d66..74ca2fe 100644
> --- a/arch/x86/kernel/crash.c
> +++ b/arch/x86/kernel/crash.c
> @@ -185,10 +185,9 @@ void native_machine_crash_shutdown(struct pt_regs *regs)
>  }
>  
>  #ifdef CONFIG_KEXEC_FILE
> -static int get_nr_ram_ranges_callback(unsigned long start_pfn,
> -				unsigned long nr_pfn, void *arg)
> +static int get_nr_ram_ranges_callback(u64 start, u64 end, void *arg)
>  {
> -	int *nr_ranges = arg;
> +	unsigned int *nr_ranges = arg;
>  
>  	(*nr_ranges)++;
>  	return 0;
> @@ -214,7 +213,7 @@ static void fill_up_crash_elf_data(struct crash_elf_data *ced,
>  
>  	ced->image = image;
>  
> -	walk_system_ram_range(0, -1, &nr_ranges,
> +	walk_system_ram_res(0, -1, &nr_ranges,
>  				get_nr_ram_ranges_callback);
>  
>  	ced->max_nr_ranges = nr_ranges;

Acked-by: Dave Young <dyoung@xxxxxxxxxx>

Thanks
Dave
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