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

[Takashi Iwai]

On Mon, 23 Jul 2018 02:47:18 +0200,
He, Bo wrote:
> 
> Hi, Takashi:
> 	we tested for the whole weekend, your patch works, no panic issue seen. You can safe merge you patch.

OK, thanks for testing!  Now it's merged.


Takashi
> 
> -----Original Message-----
> From: Takashi Iwai <tiwai@xxxxxxx> 
> Sent: Thursday, July 19, 2018 5:11 PM
> To: Zhang, Jun <jun.zhang@xxxxxxxxx>
> Cc: He, Bo <bo.he@xxxxxxxxx>; alsa-devel@xxxxxxxxxxxxxxxx; perex@xxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Zhang, Yanmin <yanmin.zhang@xxxxxxxxx>
> Subject: Re: [PATCH] ALSA: core: fix unsigned int pages overflow when comapred
> 
> On Thu, 19 Jul 2018 08:42:14 +0200,
> Takashi Iwai wrote:
> > 
> > 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.
> 
> And we can reduce even more lines.  A proper patch is below.
> 
> 
> thanks,
> 
> Takashi
> 
> -- 8< --
> From: Takashi Iwai <tiwai@xxxxxxx>
> Subject: [PATCH] ALSA: memalloc: Don't exceed over the requested size
> 
> snd_dma_alloc_pages_fallback() tries to allocate pages again when the allocation fails with reduced size.  But the first try actually
> *increases* the size to power-of-two, which may give back a larger chunk than the requested size.  This confuses the callers, e.g. sgbuf assumes that the size is equal or less, and it may result in a bad loop due to the underflow and eventually lead to Oops.
> 
> The code of this function seems incorrectly assuming the usage of get_order().  We need to decrease at first, then align to power-of-two.
> 
> Reported-by: he, bo <bo.he@xxxxxxxxx>
> Reported-by: zhang jun <jun.zhang@xxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
> ---
>  sound/core/memalloc.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/sound/core/memalloc.c b/sound/core/memalloc.c index 7f89d3c79a4b..753d5fc4b284 100644
> --- a/sound/core/memalloc.c
> +++ b/sound/core/memalloc.c
> @@ -242,16 +242,12 @@ int snd_dma_alloc_pages_fallback(int type, struct device *device, size_t size,
>  	int err;
>  
>  	while ((err = snd_dma_alloc_pages(type, device, size, dmab)) < 0) {
> -		size_t aligned_size;
>  		if (err != -ENOMEM)
>  			return err;
>  		if (size <= PAGE_SIZE)
>  			return -ENOMEM;
> -		aligned_size = PAGE_SIZE << get_order(size);
> -		if (size != aligned_size)
> -			size = aligned_size;
> -		else
> -			size >>= 1;
> +		size >>= 1;
> +		size = PAGE_SIZE << get_order(size);
>  	}
>  	if (! dmab->area)
>  		return -ENOMEM;
> --
> 2.18.0
>