[PATCH] mm: bootmem: Check pfn_valid() before accessing struct page

[PATCH] mm: bootmem: Check pfn_valid() before accessing struct page

[Matt Fleming]

We need to check that a pfn is valid before handing it to pfn_to_page()
since on low memory systems with CONFIG_HIGHMEM=n it's possible that a
pfn may not have a corresponding struct page.

This is in fact the case for one of Alan's machines where some of the
EFI boot services pages live in highmem, and running a kernel without
CONFIG_HIGHMEM enabled results in the following oops,

 BUG: unable to handle kernel paging request at f7f1f080
 IP: [<c17fba96>] __free_pages_bootmem+0x5a/0xb8
 *pdpt = 0000000001887001 *pde = 0000000001984067 *pte = 000000000 0000000
 Oops: 0002 [#1] SMP

[...]

 Call Trace:
  [<c17feacc>] free_bootmem_late+0x2d/0x3d
  [<c17f1013>] efi_free_boot_services+0x48/0x5b
  [<c17ddc12>] start_kernel+0x3ad/0x3cf
  [<c17dd654>] ? set_init_arg+0x49/0x49
  [<c17dd380>] i386_start_kernel+0x12e/0x131

Reported-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>
---
 mm/bootmem.c   | 3 +++
 mm/nobootmem.c | 3 +++
 2 files changed, 6 insertions(+)

diff --git a/mm/bootmem.c b/mm/bootmem.c
index 90bd3507b413..406e9cb1d58c 100644
--- a/mm/bootmem.c
+++ b/mm/bootmem.c
@@ -164,6 +164,9 @@ void __init free_bootmem_late(unsigned long physaddr, unsigned long size)
 	end = PFN_DOWN(physaddr + size);
 
 	for (; cursor < end; cursor++) {
+		if (!pfn_valid(cursor))
+			continue;
+
 		__free_pages_bootmem(pfn_to_page(cursor), 0);
 		totalram_pages++;
 	}
diff --git a/mm/nobootmem.c b/mm/nobootmem.c
index 04a9d94333a5..afad246688ce 100644
--- a/mm/nobootmem.c
+++ b/mm/nobootmem.c
@@ -77,6 +77,9 @@ void __init free_bootmem_late(unsigned long addr, unsigned long size)
 	end = PFN_DOWN(addr + size);
 
 	for (; cursor < end; cursor++) {
+		if (!pfn_valid(cursor))
+			continue;
+
 		__free_pages_bootmem(pfn_to_page(cursor), 0);
 		totalram_pages++;
 	}
-- 
1.9.0

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Re: [PATCH] mm: bootmem: Check pfn_valid() before accessing struct page

[Dave Hansen]

On 05/27/2014 07:10 AM, Matt Fleming wrote:
> We need to check that a pfn is valid before handing it to pfn_to_page()
> since on low memory systems with CONFIG_HIGHMEM=n it's possible that a
> pfn may not have a corresponding struct page.
> 
> This is in fact the case for one of Alan's machines where some of the
> EFI boot services pages live in highmem, and running a kernel without
> CONFIG_HIGHMEM enabled results in the following oops
...
> diff --git a/mm/bootmem.c b/mm/bootmem.c
> index 90bd3507b413..406e9cb1d58c 100644
> --- a/mm/bootmem.c
> +++ b/mm/bootmem.c
> @@ -164,6 +164,9 @@ void __init free_bootmem_late(unsigned long physaddr, unsigned long size)
>  	end = PFN_DOWN(physaddr + size);
>  
>  	for (; cursor < end; cursor++) {
> +		if (!pfn_valid(cursor))
> +			continue;
> +
>  		__free_pages_bootmem(pfn_to_page(cursor), 0);
>  		totalram_pages++;
>  	}

I don't think this is quite right.  pfn_valid() tells us whether we have
a 'struct page' there or not.  *BUT*, it does not tell us whether it is
RAM that we can actually address and than can be freed in to the buddy
allocator.

I think sparsemem is where this matters.  Let's say mem= caused lowmem
to end in the middle of a section (or that 896MB wasn't
section-aligned).  Then someone calls free_bootmem_late() on an area
that is in the last section, but _above_ max_mapnr.  It'll be
pfn_valid(), we'll free it in to the buddy allocator, and we'll blam the
first time we try to write to a bogus vaddr after a phys_to_virt().

At a higher level, I don't like the idea of the bootmem code papering
over bugs when somebody calls in to it trying to _free_ stuff that's not
memory (as far as the kernel is concerned).

I think the right thing to do is to call in to the e820 code and see if
the range is E820_RAM before trying to bootmem-free it.

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Re: [PATCH] mm: bootmem: Check pfn_valid() before accessing struct page

[Fleming, Matt]

On 27 May 2014 19:45, Dave Hansen <dave.hansen@intel.com> wrote:
>
> I don't think this is quite right.  pfn_valid() tells us whether we have
> a 'struct page' there or not.  *BUT*, it does not tell us whether it is
> RAM that we can actually address and than can be freed in to the buddy
> allocator.
>
> I think sparsemem is where this matters.  Let's say mem= caused lowmem
> to end in the middle of a section (or that 896MB wasn't
> section-aligned).  Then someone calls free_bootmem_late() on an area
> that is in the last section, but _above_ max_mapnr.  It'll be
> pfn_valid(), we'll free it in to the buddy allocator, and we'll blam the
> first time we try to write to a bogus vaddr after a phys_to_virt().

Ah, the sparsemem case wasn't something I'd considered. Thanks Dave.

> At a higher level, I don't like the idea of the bootmem code papering
> over bugs when somebody calls in to it trying to _free_ stuff that's not
> memory (as far as the kernel is concerned).
>
> I think the right thing to do is to call in to the e820 code and see if
> the range is E820_RAM before trying to bootmem-free it.

OK, this makes sense. I'll try that approach and see if it also fixes
Alan's problem.

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