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

[Kent Overstreet <kmo@daterainc.com>]

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@oracle.com>
> > Signed-off-by: Kent Overstreet <kmo@daterainc.com>
> > ---
> > 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.