Re: [PATCH] fs/ntfs3: validate BOOT sectors_per_clusters

[Matthew Wilcox <willy@infradead.org>]

On Fri, Apr 29, 2022 at 11:52:47AM -0700, Randy Dunlap wrote:
> Hi--
> 
> On 4/29/22 10:39, Matthew Wilcox wrote:
> > On Fri, Apr 29, 2022 at 10:27:11AM -0700, Randy Dunlap wrote:
> > > When the NTFS BOOT sectors_per_clusters field is > 0x80,
> > > it represents a shift value. First change its sign to positive
> > > and then make sure that the shift count is not too large.
> > > This prevents negative shift values and shift values that are
> > > larger than the field size.
> > > 
> > > Prevents this UBSAN error:
> > > 
> > >   UBSAN: shift-out-of-bounds in ../fs/ntfs3/super.c:673:16
> > >   shift exponent -192 is negative
> > > 
> > > Fixes: 82cae269cfa9 ("fs/ntfs3: Add initialization of super block")
> > > Signed-off-by: Randy Dunlap<rdunlap@infradead.org>
> > > Reported-by:syzbot+1631f09646bc214d2e76@syzkaller.appspotmail.com
> > > Cc: Konstantin Komarov<almaz.alexandrovich@paragon-software.com>
> > > Cc:ntfs3@lists.linux.dev
> > > Cc: Alexander Viro<viro@zeniv.linux.org.uk>
> > > Cc: Andrew Morton<akpm@linux-foundation.org>
> > > Cc: Kari Argillander<kari.argillander@stargateuniverse.net>
> > > Cc: Namjae Jeon<linkinjeon@kernel.org>
> > > ---
> > >   fs/ntfs3/super.c |    5 +++--
> > >   1 file changed, 3 insertions(+), 2 deletions(-)
> > > 
> > > --- linux-next-20220428.orig/fs/ntfs3/super.c
> > > +++ linux-next-20220428/fs/ntfs3/super.c
> > > @@ -670,7 +670,8 @@ static u32 true_sectors_per_clst(const s
> > >   {
> > >   	return boot->sectors_per_clusters <= 0x80
> > >   		       ? boot->sectors_per_clusters
> > > -		       : (1u << (0 - boot->sectors_per_clusters));
> > > +		       : -(s8)boot->sectors_per_clusters > 31 ? -1
> > > +		       : (1u << -(s8)boot->sectors_per_clusters);
> > >   }
> > This hurts my brain.  Can we do instead:
> 
> It's just C.  Lot clearer than some of our macro magic.

Well, yeah, but I don't have to understand our macro magic; I can just
assume it does what it says on the tin.

> > 
> > 	if (boot->sectors_per_clusters <= 0x80)
> > 		return boot->sectors_per_clusters;
> > 	if (boot->sectors_per_clusters < 0xA0)
> 
> The 0xA0 does not take into account the '-' negating of sectors_per_clusters
> in the shift.
> Looks like it should be
> 
> 	if (boot->sectors_per_clusters >= 0xe1)
> 		return 1U << -boot->sectors_per_clusters;

Oh!  I misunderstood how the ranges are used.  But I think a unary minus
will leave the type as unsigned (am I wrong?  C integer promotions
always confuse me), so how about:

	if (boot->sectors_per_clusters > 0xe0)
		return 1U << (0 - boot->sectors_per_clusters);

https://en.cppreference.com/w/c/language/conversion

> > 		return 1U << (boot->sectors_per_clusters - 0x80);
> > 	return 0xffffffff;
> > 
> 
> Sorry about your head.