Re: msync() bug

[Andrea Arcangeli <andrea@suse.de>]

On Mon, Jul 09, 2001 at 10:32:11PM +1000, Andrew Morton wrote:
> Abraham vd Merwe wrote:
> > 
> > Hi!
> > 
> > I was preparing some lecture last night and stumbled onto this bug. Maybe
> > some of you can shed some light on it.
> > 
> > Basically, I just memory map /dev/mem at 0xb8000 (text mode - yes I know you
> > shouldn't do this, but it was to illustrate something), reads 4k, changes it
> > writes it back.
> > 
> 
> The actual call trace is:
> 
> __set_page_dirty
> filemap_sync_pte
> filemap_sync_pte_range 
> filemap_sync_pmd_range 
> filemap_sync
> msync_interval
> sys_msync
> 
> We're crashing because __set_page_dirty dereferences page->mapping,
> but pages from a mmap() of /dev/mem seem to have a NULL ->mapping.
> 
> One of the very frustrating things about Linux kernel development
> is that the main source of tuition is merely the source code. You
> can stare at that for months (as I have) and still not have a firm
> grasp on the big-picture semantic *meaning* behind something as
> simple as a page having a null ->mapping.  Sigh.
> 
> So one is reduced to mimicry:
> 
> --- linux-2.4.7-pre3/mm/filemap.c	Wed Jul  4 18:21:32 2001
> +++ linux-akpm/mm/filemap.c	Mon Jul  9 22:22:46 2001
> @@ -1652,7 +1652,8 @@ static inline int filemap_sync_pte(pte_t
>  	if (pte_present(pte) && ptep_test_and_clear_dirty(ptep)) {
>  		struct page *page = pte_page(pte);
>  		flush_tlb_page(vma, address);
> -		set_page_dirty(page);
> +		if (page->mapping)
> +			set_page_dirty(page);
>  	}
>  	return 0;
>  }

Wrong fix, `page' is just garbage if some non memory was mapped in
userspace (like framebuffers or similar mmio regions were mapped etc..).

I fixed it right ages ago (also sumbitted to Linus but got not
integrated and I forgotten to resend):

	ftp://ftp.us.kernel.org/pub/linux/kernel/people/andrea/kernels/v2.4/2.4.7pre3aa1/00_msync-fb0-1

Please Linus include this time:

--- 2.4.5pre1aa3/mm/filemap.c.~1~	Fri May 11 02:08:28 2001
+++ 2.4.5pre1aa3/mm/filemap.c	Mon May 14 18:48:59 2001
@@ -1808,10 +1808,12 @@
 {
 	pte_t pte = *ptep;
 
-	if (pte_present(pte) && ptep_test_and_clear_dirty(ptep)) {
+	if (pte_present(pte)) {
 		struct page *page = pte_page(pte);
-		flush_tlb_page(vma, address);
-		set_page_dirty(page);
+		if (VALID_PAGE(page) && !PageReserved(page) && ptep_test_and_clear_dirty(ptep)) {
+			flush_tlb_page(vma, address);
+			set_page_dirty(page);
+		}
 	}
 	return 0;
 }

Andrea