Re: [PATCH] bcache: fix BUG_ON due to integer overflow with GC_SECTORS_USED

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On Tue, Feb 11, 2014 at 10:18:05AM -0800, Kent Overstreet wrote:
> On Tue, Feb 11, 2014 at 10:14:31AM -0800, Greg KH wrote:
> > On Tue, Feb 11, 2014 at 10:02:57AM -0800, Darrick J. Wong wrote:
> > > The BUG_ON at the end of __bch_btree_mark_key can be triggered due to
> > > an integer overflow error:
> > > 
> > > BITMASK(GC_SECTORS_USED, struct bucket, gc_mark, 2, 13);
> > > ...
> > > SET_GC_SECTORS_USED(g, min_t(unsigned,
> > > 	     GC_SECTORS_USED(g) + KEY_SIZE(k),
> > > 	     (1 << 14) - 1));
> > > BUG_ON(!GC_SECTORS_USED(g));
> > > 
> > > In bcache.h, the SECTORS_USED bitfield is defined to be 13 bits wide.
> > > While the SET_ code tries to ensure that the field doesn't overflow by
> > > clamping it to (1<<14)-1 == 16383, this is incorrect because 16383
> > > requires 14 bits.  Therefore, if GC_SECTORS_USED() + KEY_SIZE() =
> > > 8192, the SET_ statement tries to store 8192 into a 13-bit field.  In
> > > a 13-bit field, 8192 becomes zero, thus triggering the BUG_ON.
> > > 
> > > Therefore, create a field width constant and a max value constant, and
> > > use those to create the bitfield and check the inputs to
> > > SET_GC_SECTORS_USED.  Arguably the BITMASK() template ought to have
> > > BUG_ON checks for too-large values, but that's a separate patch.
> > > 
> > > Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> > > Signed-off-by: Kent Overstreet <kmo@xxxxxxxxxxxxx>
> > > ---
> > > Greg, stable team et. all - can you apply this asap to 3.13? It hasn't hit
> > > Linus's tree yet (it's on its way via Jens), but multiple users are hitting this
> > > BUG_ON() and the fix has been tested.
> > 
> > Think about what you just wrote.
> > 
> > Then go read Documentation/stable_kernel_rules.txt.
> > 
> > You should know better...
> 
> I figured if any patch qualified for skipping ahead this one would - and it
> always takes awhile for stuff to make it through Jens, but ah well.
> 
> It's 947174476701fbc84ea8c7ec9664270f9d80b076, currently in Jens' for-linus
> tree, I'll ping you again when I see it go in since I forgot to add the stable
> tag to the patch.

You might want to fix the From: header as well... ;)

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