Re: [PATCH] Avoid infinite loop in malformed packfiles

[Ori Bernstein <ori@eigenstate.org>]

On Sun, 23 Aug 2020 08:26:14 +0200, René Scharfe <l.s.r@web.de> wrote:

> Am 23.08.20 um 05:11 schrieb Ori Bernstein:
> > In packfile.c:1680, there's an infinite loop that tries to get
> > to the base of a packfile. With offset deltas, the offset needs
> > to be greater than 0, so it's always walking backwards, and the
> > search is guaranteed to terminate.
> >
> > With reference deltas, there's no check for a cycle in the
> > references, so a cyclic reference will cause git to loop
> > infinitely, growing the delta_stack infinitely, which will
> > cause it to consume all available memory as as a full CPU
> > core.
> 
> "as as"?  Perhaps "and"?

I think I meant 'As well as' -- will fix.
 
> 
> b5c0cbd8083 (pack-objects: use bitfield for object_entry::depth,
> 2018-04-14) limited the delta depth for new packs to 4095, so 10000
> seems reasonable.  Users with unreasonable packs would need to repack
> them with an older version of Git, though.  Not sure if that would
> affect anyone in practice.
> 
> >  #define UNPACK_ENTRY_STACK_PREALLOC 64
> 
> Hmm, setting a hard limit may allow to allocate the whole stack on the,
> ehm, stack.  That would get rid of the hybrid stack/heap allocation and
> thus simplify the code a bit.  10000 entries with 24 bytes each would be
> quite big, though, but that might be OK without recursion.  (And not in
> this patch anyway, of course.)
> 
> >  struct unpack_entry_stack_ent {
> >  	off_t obj_offset;
> > @@ -1715,6 +1716,12 @@ void *unpack_entry(struct repository *r, struct packed_git *p, off_t obj_offset,
> >  			break;
> >  		}
> >
> > +		if (delta_stack_nr > UNPACK_ENTRY_STACK_LIMIT) {
> > +			error("overlong delta chain at offset %jd from %s",
> > +			      (uintmax_t)curpos, p->pack_name);
> > +			goto out;
> > +		}
> 
> Other error handlers in this loop set data to NULL.  That's actually
> unnecessary because it's NULL to begin with and the loop is exited after
> setting it to some other value.  So not doing it here is fine.  (And a
> separate cleanup patch could remove the dead stores in the other
> handlers.)

Is there anything you'd like me to do in this patch, other than fixing
the typo?

-- 
    Ori Bernstein