[PATCH v2 1/3] gdb/corelow.c: fix use-after-free in build_file_mappings

[Andrew Burgess]

Lancelot SIX <lancelot.six@amd.com> writes:

> Hi,
>
> Thanks John and Andrew for the reviews and suggestions.  Here is a V2
> for patch #1 (I have added Andrew's Reviewed-By tag to patch 2 and 3).
>
> John, should I add your Reviewed-By tag in patches 2 and 3 as you
> reviewed them as well?
>
> Changes since V1:
>  - Only register the bfd in bfd_map if it got successfully opened,
>    following John's suggestion.  I have not used the exact pattern
>    suggested in the review to avoid duplicating the warning message.
>
> Best,
> Lancelot.
>
> ---
>
> In core_target::build_file_mappings, GDB tries to open files referenced
> in the core dump.
>
> The process goes like this:
>
>     struct bfd *bfd = bfd_map[filename];
>     if (bfd == nullptr)
>       {
>         bfd = bfd_map[filename]
>           = bfd_openr (expanded_fname.get (), "binary");
>         if (bfd == nullptr || !bfd_check_format (bfd, bfd_object))
>           {
>             if (bfd != nullptr)
>               bfd_close (bfd);
>             return;
>           }
>       }
>     asection *sec = bfd_make_section_anyway (bfd, "load");
>     ...
>
> The problem is that if bfd_check_format fails, we close the bfd but keep
> a reference to it in the bfd_map.
>
> If the same filename appears another time in the NT_FILE note, we enter
> this code again.  The second time, bfd_map[filename] is not nullptr and
> we try to call bfd_make_section_anyway on an already closed BFD, which
> is a use-after-free error.
>
> This patch makes sure that the bfd is only saved in the bfd_map if it
> got opened successfully.
>
> This error got exposed by a recent change in BFD (014a602b86f "Don't
> optimise bfd_seek to same position").  Since this change, opening a
> coredump which contains mapping to some special files such as a DRI
> render node (/dev/dri/renderD$NUM) exposes the issue.  This happens for
> example for processes using AMDGPU devices to offload compute tasks.

All 3 patches LGTM.

Approved-By: Andrew Burgess <aburgess@redhat.com>

Thanks,
Andrew

> ---
>  gdb/corelow.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/gdb/corelow.c b/gdb/corelow.c
> index db489b4280e..54def4198bc 100644
> --- a/gdb/corelow.c
> +++ b/gdb/corelow.c
> @@ -258,8 +258,7 @@ core_target::build_file_mappings ()
>  		return;
>  	      }
>  
> -	    bfd = bfd_map[filename] = bfd_openr (expanded_fname.get (),
> -						 "binary");
> +	    bfd = bfd_openr (expanded_fname.get (), "binary");
>  
>  	    if (bfd == nullptr || !bfd_check_format (bfd, bfd_object))
>  	      {
> @@ -284,6 +283,7 @@ core_target::build_file_mappings ()
>  	       This can be checked before/after a core file detach via
>  	       "maint info bfds".  */
>  	    gdb_bfd_record_inclusion (core_bfd, bfd);
> +	    bfd_map[filename] = bfd;
>  	  }
>  
>  	/* Make new BFD section.  All sections have the same name,
> -- 
> 2.34.1