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

[Laura Abbott]

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.
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;
 	}