Re: [PATCH] ALSA: core: fix unsigned int pages overflow when comapred

[Takashi Iwai]

On Thu, 19 Jul 2018 08:08:06 +0200,
Zhang, Jun wrote:
> 
> Hello, Takashi
> 
> I think use our patch, it's NOT possible that the returned size is over sgbuf->tblsize.
> 
> In function snd_malloc_sgbuf_pages, 
> 
> Pages is align page,
> sgbuf->tblsize is align 32*page,
> chunk is align 2^n*page,
> 
> in our panic case, pages = 123, tlbsize = 128,  
> 1st loop trunk = 32
> 2nd loop trunk = 32
> 3rd loop trunk = 32
> 4th loop trunk = 16
> 5th loop trunk = 16
> So in 5th loop pages-trunk = -5, which make dead loop. 

Looking at the code again, yeah, you are right, that won't happen.

And now it becomes clear: the fundamental problem is that
snd_dma_alloc_pages_fallback() returns a larger size than requested.
It would be acceptable if the internal allocator aligns a larger size,
but it shouldn't appear in the returned size outside.  I believe this
was just a misunderstanding of get_order() usage there.
(BTW, it's interesting that the allocation with a larger block worked
 while allocation with a smaller chunk failed; it must be a rare case
 and that's one of reasons this bug didn't hit frequently.)

That being said, what we should fix is rather the function
snd_dma_alloc_pages_fallback() to behave as expected, and it'll be
like the patch below.


thanks,

Takashi

--- a/sound/core/memalloc.c
+++ b/sound/core/memalloc.c
@@ -247,11 +247,10 @@ int snd_dma_alloc_pages_fallback(int type, struct device *device, size_t size,
 			return err;
 		if (size <= PAGE_SIZE)
 			return -ENOMEM;
+		size >>= 1;
 		aligned_size = PAGE_SIZE << get_order(size);
 		if (size != aligned_size)
 			size = aligned_size;
-		else
-			size >>= 1;
 	}
 	if (! dmab->area)
 		return -ENOMEM;