[PATCH 5/5] keep /dev/kmem from triggering BUG_ON() with DEBUG_VIRTUAL

[Dave Hansen]

/dev/kmem can essentially be used to make the kernel call __pa()
on any address.  But, with DEBUG_VIRTUAL, we are now ensuring
that when __pa() is only called on addresses for which its
results are correct.  For everything else, we BUG_ON().

What this means is that a hapless /dev/kmem user can
unintentionally make the kernel BUG_ON().

This adds a function, is_kernel_linear_vaddr() to the /dev/kmem
code to ensure that __pa() will not BUG_ON().  It will, instead,
return an error up to the user.

Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
---

 linux.git-davehans/arch/x86/mm/pageattr.c  |   19 +++++++++++++++++++
 linux.git-davehans/drivers/char/mem.c      |    8 ++++++++
 linux.git-davehans/include/linux/mmdebug.h |    2 ++
 3 files changed, 29 insertions(+)

diff -puN arch/x86/mm/pageattr.c~make-devmem-work-on-highmem3b arch/x86/mm/pageattr.c
--- linux.git/arch/x86/mm/pageattr.c~make-devmem-work-on-highmem3b	2013-04-10 16:23:45.780087708 -0700
+++ linux.git-davehans/arch/x86/mm/pageattr.c	2013-04-10 16:23:45.786087714 -0700
@@ -406,6 +406,25 @@ phys_addr_t slow_virt_to_phys(void *virt
 }
 EXPORT_SYMBOL_GPL(slow_virt_to_phys);
 
+#ifdef CONFIG_DEBUG_VIRTUAL
+int is_kernel_linear_vaddr(void *kernel_vaddr)
+{
+	int err;
+	unsigned long pte_walk_paddr;
+	unsigned long linear_paddr = __pa_nodebug(kernel_vaddr);
+
+	err = kernel_lookup_vaddr(kernel_vaddr, &pte_walk_paddr);
+	/* if there is no pte, it can not be in the normal linear map */
+	if (err)
+		return 0;
+	/* is there is a pte which does not match the linear layout? */
+	if (pte_walk_paddr != linear_paddr)
+		return 0;
+
+	return 1;
+}
+#endif /* CONFIG_DEBUG_VIRTUAL */
+
 /*
  * Set the new pmd in all the pgds we know about:
  */
diff -puN drivers/char/mem.c~make-devmem-work-on-highmem3b drivers/char/mem.c
--- linux.git/drivers/char/mem.c~make-devmem-work-on-highmem3b	2013-04-10 16:23:45.781087709 -0700
+++ linux.git-davehans/drivers/char/mem.c	2013-04-10 16:23:45.787087715 -0700
@@ -357,6 +357,14 @@ static int mmap_kmem(struct file *file,
 	if (!pfn_valid(pfn))
 		return -EIO;
 
+	/*
+	 * the __pa() calculation below only makes sense if the kernel's
+	 * pagetables are set up in the way that __pa() expects.  Otherwise,
+	 * we are effectively mapping random memory in to place.
+	 */
+	if (!is_kernel_linear_vaddr(kernel_vaddr))
+		return -EIO;
+
 	/* Turn a kernel-virtual address into a physical page frame */
 	pfn = __pa(kernel_vaddr) >> PAGE_SHIFT;
 
diff -puN include/linux/mmdebug.h~make-devmem-work-on-highmem3b include/linux/mmdebug.h
--- linux.git/include/linux/mmdebug.h~make-devmem-work-on-highmem3b	2013-04-10 16:23:45.783087711 -0700
+++ linux.git-davehans/include/linux/mmdebug.h	2013-04-10 16:23:45.787087715 -0700
@@ -9,8 +9,10 @@
 
 #ifdef CONFIG_DEBUG_VIRTUAL
 #define VIRTUAL_BUG_ON(cond) BUG_ON(cond)
+extern int is_kernel_linear_vaddr(void *kernel_vaddr);
 #else
 #define VIRTUAL_BUG_ON(cond) do { } while (0)
+static inline int is_kernel_linear_vaddr(void *kernel_vaddr) { return 1; }
 #endif
 
 #endif
_
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