[PATCH] scatterlist: Validate page before calling PageSlab()

[Alan Mikhak]

From: Alan Mikhak <alan.mikhak@xxxxxxxxxx>

Modify sg_miter_stop() to validate the page pointer
before calling PageSlab(). This check prevents a crash
that will occur if PageSlab() gets called with a page
pointer that is not backed by page struct.

A virtual address obtained from ioremap() for a physical
address in PCI address space can be assigned to a
scatterlist segment using the public scatterlist API
as in the following example:

my_sg_set_page(struct scatterlist *sg,
               const void __iomem *ioaddr,
               size_t iosize)
{
	sg_set_page(sg,
		virt_to_page(ioaddr),
		(unsigned int)iosize,
		offset_in_page(ioaddr));
	sg_init_marker(sg, 1);
}

If the virtual address obtained from ioremap() is not
backed by a page struct, virt_to_page() returns an
invalid page pointer. However, sg_copy_buffer() can
correctly recover the original virtual address. Such
addresses can successfully be assigned to scatterlist
segments to transfer data across the PCI bus with
sg_copy_buffer() if it were not for the crash in
PageSlab() when called by sg_miter_stop().

Signed-off-by: Alan Mikhak <alan.mikhak@xxxxxxxxxx>
---
 lib/scatterlist.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/lib/scatterlist.c b/lib/scatterlist.c
index c2cf2c311b7d..f5c61cad40ba 100644
--- a/lib/scatterlist.c
+++ b/lib/scatterlist.c
@@ -807,6 +807,7 @@ void sg_miter_stop(struct sg_mapping_iter *miter)
 		miter->__remaining -= miter->consumed;
 
 		if ((miter->__flags & SG_MITER_TO_SG) &&
+		    pfn_valid(page_to_pfn(miter->page)) &&
 		    !PageSlab(miter->page))
 			flush_kernel_dcache_page(miter->page);
 
-- 
2.7.4