RE: [PATCH 1/1] gdb: Fix segfault in "start" with fuzzed binary

["Willgerodt, Felix" <felix.willgerodt@intel.com>]

> -----Original Message-----
> From: Andrew Burgess <aburgess@redhat.com>
> Sent: Dienstag, 20. Februar 2024 12:58
> To: Willgerodt, Felix <felix.willgerodt@intel.com>; gdb-patches@sourceware.org
> Subject: Re: [PATCH 1/1] gdb: Fix segfault in "start" with fuzzed binary
> 
> Felix Willgerodt <felix.willgerodt@intel.com> writes:
> 
> > I found this while playing around with fuzzing.  Though I did fuzz with
> > "-ex start", so this isn't a security issue.  But any comments welcome.
> >
> > What I observed is this segfault:
> >
> > ~~~
> > Thread 1 "gdb-up" received signal SIGSEGV, Segmentation fault.
> > 0x0000555555d8883a in objfile::arch (this=0x0) at
> /user/sources/gdb/gdb/objfiles.h:509
> > 509         return per_bfd->gdbarch;
> > (gdb) bt 5
> >     filter_=..., cond_string_=..., extra_string_=..., disposition_=disp_del,
> thread_=-1, simd_lane_num_=-1, task_=-1, inferior_=1, ignore_count_=0,
> >     from_tty=0, enabled_=1, flags=0, display_canonical_=0) at
> /user/sources/gdb/gdb/breakpoint.c:8960
> > (More stack frames follow...)
> > (gdb) frame 1
> > 7645        return sal.section->objfile->arch ();
> > (gdb) p sal.section
> > $1 = (obj_section *) 0x555558828bf8
> > (gdb) p *sal.section
> > $2 = {the_bfd_section = 0x0, objfile = 0x0, ovly_mapped = 0}
> > ~~~
> >
> > The parsed binary has a weird .text section header:
> >
> >   [14] .text             LOUSER+0x6c0000  0000000000001040  00001040
> >        00000000000000f9  0000000000000000  WX       0     0     16
> >
> > It is marked as writeable (I think) and the type is also different.  For
> > reference here is the one from the normal binary that I started fuzzing with:
> >
> >   [14] .text             PROGBITS         0000000000001040  00001040
> >        00000000000000f9  0000000000000000  AX       0     0     16
> >
> > I couldn't find where GDB actually parses this.  Nor could I figure out why
> > the section has a nullptr as objfile.
> 
> So I think add_to_objfile_sections is where the nullptr is appearing.
> This is where the obj_section::objfile field is set, but only if the
> section is allocatable, which after your fuzzing it's not, so the
> ::objfile field ends up being left as its default value.
> 
> Even after this patch, it's not obvious that the ::objfile field might
> be nullptr (looking at struct obj_section in objfiles.h), so maybe it's
> worth extending the comment there to reflect that.
> 
> I did wonder if we're wrong to even create a symtab_and_line in this
> case, we're claiming to have found some debug information for the
> program image from a particular section which actually wasn't mapped
> in.  But I think fixing that would be a much bigger task, we'd need to
> chase back all the places where we load debug information which claims
> to be within a section which is then not going to be allocated.
> 

Hi Andrew,

thanks for even looking at this. Since it isn't a security issue I was wondering
if we even care much about this. I don't really know if we would ever see
such an ELF file and care about not crashing with it after a start.
But the patch I wrote did seem harmless enough to post as a proposal.

I did check add_to_objfile_sections() and I don't see the nullptr being added there.
So it must be somewhere else. (Wouldn't it even segfault there if objfile
would be nullptr?)

Regards,
Felix
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