Re: BUG: /proc/kcore does not export direct-mapped memory on arm64 (and presumably some other architectures)

[Dave Anderson]

----- Original Message -----
> On 04/26/2018 02:16 PM, Kees Cook wrote:
> > On Thu, Apr 26, 2018 at 12:31 PM, Dave Anderson <anderson@xxxxxxxxxx>
> > wrote:
> >>
> >> While testing /proc/kcore as the live memory source for the crash utility,
> >> it fails on arm64.  The failure on arm64 occurs because only the
> >> vmalloc/module space segments are exported in PT_LOAD segments,
> >> and it's missing all of the PT_LOAD segments for the generic
> >> unity-mapped regions of physical memory, as well as their associated
> >> vmemmap sections.
> >>
> >> The mapping of unity-mapped RAM segments in fs/proc/kcore.c is
> >> architecture-neutral, and after debugging it, I found this as the
> >> problem.  For each chunk of physical memory, kcore_update_ram()
> >> calls walk_system_ram_range(), passing kclist_add_private() as a
> >> callback function to add the chunk to the kclist, and eventually
> >> leading to the creation of a PT_LOAD segment.
> >>
> >> kclist_add_private() does some verification of the memory region,
> >> but this one below is bogus for arm64:
> >>
> >>      static int
> >>      kclist_add_private(unsigned long pfn, unsigned long nr_pages, void
> >>      *arg)
> >>      {
> >>      ... [ cut ] ...
> >>              ent->addr = (unsigned long)__va((pfn << PAGE_SHIFT));
> >>      ... [ cut ] ...
> >>
> >>              /* Sanity check: Can happen in 32bit arch...maybe */
> >>              if (ent->addr < (unsigned long) __va(0))
> >>                      goto free_out;
> >>
> >> And that's because __va(0) is a bogus check for arm64.  It is checking
> >> whether the ent->addr value is less than the lowest possible unity-mapped
> >> address.  But "0" should not be used as a physical address on arm64; the
> >> lowest legitimate physical address for this __va() check would be the
> >> arm64
> >> PHYS_OFFSET, or memstart_addr:
> >>
> >> Here's the arm64 __va() and PHYS_OFFSET:
> >>
> >>    #define __va(x) ((void *)__phys_to_virt((phys_addr_t)(x)))
> >>    #define __phys_to_virt(x)       ((unsigned long)((x) - PHYS_OFFSET) |
> >>    PAGE_OFFSET)
> >>
> >>    extern s64                      memstart_addr;
> >>    /* PHYS_OFFSET - the physical address of the start of memory. */
> >>    #define PHYS_OFFSET             ({ VM_BUG_ON(memstart_addr & 1);
> >>    memstart_addr; })
> >>
> >> If PHYS_OFFSET/memstart_addr is anything other than 0 (it is 0x4000000000
> >> on my
> >> test system), the __va(0) calculation goes negative and creates a bogus,
> >> very
> >> large, virtual address.  And since the ent->addr virtual address is less
> >> than
> >> bogus __va(0) address, the test fails, and the memory chunk is rejected.
> >>
> >> Looking at the kernel sources, it seems that this would affect other
> >> architectures as well, i.e., the ones whose __va() is not a simple
> >> addition of the physical address with PAGE_OFFSET.
> >>
> >> Anyway, I don't know what the best approach for an architecture-neutral
> >> fix would be in this case.  So I figured I'd throw it out to you guys for
> >> some ideas.
> > 
> > I'm not as familiar with this code, but I've added Ard and Laura to CC
> > here, as this feels like something they'd be able to comment on. :)
> > 
> > -Kees
> > 
> 
> It seems backwards that we're converting a physical address to
> a virtual address and then validating that. I think checking against
> pfn_valid (to ensure there is a valid memmap entry)
> and then checking page_to_virt against virt_addr_valid to catch
> other cases (e.g. highmem or holes in the space) seems cleaner.

Hi Laura,

Thanks a lot for looking into this -- I couldn't find a maintainer for kcore.  

The patch looks good to me, as long as virt_addr_valid() will fail on 32-bit
arches when page_to_virt() creates an invalid address when it gets passed a
highmem-physical address.  

Thanks again,
  Dave


> Maybe something like:
> 
> diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
> index d1e82761de81..e64ecb9f2720 100644
> --- a/fs/proc/kcore.c
> +++ b/fs/proc/kcore.c
> @@ -209,25 +209,34 @@ kclist_add_private(unsigned long pfn, unsigned long
> nr_pages, void *arg)
>   {
>   	struct list_head *head = (struct list_head *)arg;
>   	struct kcore_list *ent;
> +	struct page *p;
> +
> +	if (!pfn_valid(pfn))
> +		return 1;
> +
> +	p = pfn_to_page(pfn);
> +	if (!memmap_valid_within(pfn, p, page_zone(p)))
> +		return 1;
>   
>   	ent = kmalloc(sizeof(*ent), GFP_KERNEL);
>   	if (!ent)
>   		return -ENOMEM;
> -	ent->addr = (unsigned long)__va((pfn << PAGE_SHIFT));
> +	ent->addr = (unsigned long)page_to_virt(p);
>   	ent->size = nr_pages << PAGE_SHIFT;
>   
> -	/* Sanity check: Can happen in 32bit arch...maybe */
> -	if (ent->addr < (unsigned long) __va(0))
> +	if (!virt_addr_valid(ent->addr))
>   		goto free_out;
>   
>   	/* cut not-mapped area. ....from ppc-32 code. */
>   	if (ULONG_MAX - ent->addr < ent->size)
>   		ent->size = ULONG_MAX - ent->addr;
>   
> -	/* cut when vmalloc() area is higher than direct-map area */
> -	if (VMALLOC_START > (unsigned long)__va(0)) {
> -		if (ent->addr > VMALLOC_START)
> -			goto free_out;
> +	/*
> +	 * We've already checked virt_addr_valid so we know this address
> +	 * is a valid pointer, therefore we can check against it to determine
> +	 * if we need to trim
> +	 */
> +	if (VMALLOC_START > ent->addr) {
>   		if (VMALLOC_START - ent->addr < ent->size)
>   			ent->size = VMALLOC_START - ent->addr;
>   	}
> 
>