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Subject[patch 18/39] remap_file_pages protection support: add VM_FAULT_SIGSEGV
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From: Ingo Molnar <mingo@elte.hu>

Since with remap_file_pages w/prot we may put PROT_NONE on a single PTE rather
than a VMA, we must handle that inside handle_mm_fault.

This value must be handled in the arch-specific fault handlers, and this
change must be ported to every arch on the world; now the new support is not
in a separate syscall, so this *must* be done unless we want stability /
security issues (the *BUG()* for unknown return values of handle_mm_fault() is
triggerable from userspace calling remap_file_pages, and on other archs, we
have VM_FAULT_OOM which is worse). However, I've alleviated this need via the
previous "safety net" patch.

This patch includes the arch-specific part for i386.

Note, however, that _proper_ support is more intrusive; we can allow a write
on a readonly VMA, but the arch fault handler currently stops that; it should
test VM_NONUNIFORM instead and call handle_mm_fault() in case it's set. And it
will have to do on its own all protection checks. This is in the following
patches.

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
---

linux-2.6.git-paolo/arch/i386/mm/fault.c | 2 ++
linux-2.6.git-paolo/include/linux/mm.h | 9 +++++----
linux-2.6.git-paolo/mm/memory.c | 12 ++++++++++++
3 files changed, 19 insertions(+), 4 deletions(-)

diff -puN arch/i386/mm/fault.c~rfp-add-vm_fault_sigsegv arch/i386/mm/fault.c
--- linux-2.6.git/arch/i386/mm/fault.c~rfp-add-vm_fault_sigsegv 2005-08-11 14:19:57.000000000 +0200
+++ linux-2.6.git-paolo/arch/i386/mm/fault.c 2005-08-11 14:19:58.000000000 +0200
@@ -351,6 +351,8 @@ good_area:
goto do_sigbus;
case VM_FAULT_OOM:
goto out_of_memory;
+ case VM_FAULT_SIGSEGV:
+ goto bad_area;
default:
BUG();
}
diff -puN include/linux/mm.h~rfp-add-vm_fault_sigsegv include/linux/mm.h
--- linux-2.6.git/include/linux/mm.h~rfp-add-vm_fault_sigsegv 2005-08-11 14:19:58.000000000 +0200
+++ linux-2.6.git-paolo/include/linux/mm.h 2005-08-11 14:19:58.000000000 +0200
@@ -632,10 +632,11 @@ static inline int page_mapped(struct pag
* Used to decide whether a process gets delivered SIGBUS or
* just gets major/minor fault counters bumped up.
*/
-#define VM_FAULT_OOM (-1)
-#define VM_FAULT_SIGBUS 0
-#define VM_FAULT_MINOR 1
-#define VM_FAULT_MAJOR 2
+#define VM_FAULT_OOM (-1)
+#define VM_FAULT_SIGBUS 0
+#define VM_FAULT_MINOR 1
+#define VM_FAULT_MAJOR 2
+#define VM_FAULT_SIGSEGV 3

#define offset_in_page(p) ((unsigned long)(p) & ~PAGE_MASK)

diff -puN mm/memory.c~rfp-add-vm_fault_sigsegv mm/memory.c
--- linux-2.6.git/mm/memory.c~rfp-add-vm_fault_sigsegv 2005-08-11 14:19:58.000000000 +0200
+++ linux-2.6.git-paolo/mm/memory.c 2005-08-11 14:19:58.000000000 +0200
@@ -1995,6 +1995,18 @@ static inline int handle_pte_fault(struc
return do_swap_page(mm, vma, address, pte, pmd, entry, write_access);
}

+ /*
+ * Generate a SIGSEGV if a PROT_NONE page is accessed; this is handled
+ * in arch-specific code if the whole VMA has PROT_NONE, and here if
+ * just this pte has PROT_NONE (which can be done only with
+ * remap_file_pages).
+ */
+ if (pgprot_val(pte_to_pgprot(entry)) == pgprot_val(__P000)) {
+ pte_unmap(pte);
+ spin_unlock(&mm->page_table_lock);
+ return VM_FAULT_SIGSEGV;
+ }
+
if (write_access) {
if (!pte_write(entry))
return do_wp_page(mm, vma, address, pte, pmd, entry);
_
-
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