[patch] mm: sparsemem memory_present() memory corruption fix

[Ingo Molnar]

finally found it ... the patch below solves the sparsemem crash and the 
testsystem boots up fine now:

  mars:~> uname -a
  Linux mars 2.6.25-rc9-sched-devel.git-x86-latest.git #985 SMP Wed Apr 16 
  01:37:37 CEST 2008 i686 i686 i386 GNU/Linux

yay! :-)

	Ingo

ps. anyone who can correctly guess the method with which i found the 
    exact place that corrupted memory will get a free beer next time we 
    meet :-)

------------------------->
Subject: mm: sparsemem memory_present() memory corruption fix
From: Ingo Molnar <mingo@xxxxxxx>
Date: Wed Apr 16 01:40:00 CEST 2008

fix memory corruption and crash on 32-bit x86 systems.

if a !PAE x86 kernel is booted on a 32-bit system with more than
4GB of RAM, then we call memory_present() with a start/end that
goes outside the scope of MAX_PHYSMEM_BITS.

that causes this loop to happily walk over the limit of the
sparse memory section map:

    for (pfn = start; pfn < end; pfn += PAGES_PER_SECTION) {
                unsigned long section = pfn_to_section_nr(pfn);
                struct mem_section *ms;

                sparse_index_init(section, nid);
                set_section_nid(section, nid);

                ms = __nr_to_section(section);
                if (!ms->section_mem_map)
                        ms->section_mem_map = sparse_encode_early_nid(nid) |

'ms' will be out of bounds and we'll corrupt a small amount of memory by
encoding the node ID. Depending on what that memory is, we might crash, 
misbehave or just not notice the bug.

the fix is to sanity check anything the architecture passes to sparsemem.

this bug seems to be rather old (as old as sparsemem support itself),
but the exact incarnation depended on random details like configs,
which made this bug more prominent in v2.6.25-to-be.

an additional enhancement might be to print a warning about ignored
or trimmed memory ranges.

Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
---
 mm/sparse.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

Index: linux/mm/sparse.c
===================================================================
--- linux.orig/mm/sparse.c
+++ linux/mm/sparse.c
@@ -149,8 +149,18 @@ static inline int sparse_early_nid(struc
 /* Record a memory area against a node. */
 void __init memory_present(int nid, unsigned long start, unsigned long end)
 {
+	unsigned long max_arch_pfn = 1ULL << (MAX_PHYSMEM_BITS-PAGE_SHIFT);
 	unsigned long pfn;
 
+	/*
+	 * Sanity checks - do not allow an architecture to pass
+	 * in larger pfns than the maximum scope of sparsemem:
+	 */
+	if (start >= max_arch_pfn)
+		return;
+	if (end >= max_arch_pfn)
+		end = max_arch_pfn;
+
 	start &= PAGE_SECTION_MASK;
 	for (pfn = start; pfn < end; pfn += PAGES_PER_SECTION) {
 		unsigned long section = pfn_to_section_nr(pfn);

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