[PATCH v2] elf: Earlier missing dynamic segment check in _dl_map_object_from_fd

[PATCH v2] elf: Earlier missing dynamic segment check in _dl_map_object_from_fd

[Florian Weimer]

Separated debuginfo files have PT_DYNAMIC with p_filesz == 0.  We
need to check for that before the _dl_map_segments call because
that could attempt to write to mappings that extend beyond the end
of the file, resulting in SIGBUS.

---
v2: Mark the result of the || expression as unlikely, and not just the
    first branch.
 elf/dl-load.c | 22 ++++++++++++----------
 1 file changed, 12 insertions(+), 10 deletions(-)

diff --git a/elf/dl-load.c b/elf/dl-load.c
index a1f1682188..9f4fa9617d 100644
--- a/elf/dl-load.c
+++ b/elf/dl-load.c
@@ -1135,6 +1135,7 @@ _dl_map_object_from_fd (const char *name, const char *origname, int fd,
     struct loadcmd loadcmds[l->l_phnum];
     size_t nloadcmds = 0;
     bool has_holes = false;
+    bool empty_dynamic = false;
 
     /* The struct is initialized to zero so this is not necessary:
     l->l_ld = 0;
@@ -1147,7 +1148,9 @@ _dl_map_object_from_fd (const char *name, const char *origname, int fd,
 	     segments are mapped in.  We record the addresses it says
 	     verbatim, and later correct for the run-time load address.  */
 	case PT_DYNAMIC:
-	  if (ph->p_filesz)
+	  if (ph->p_filesz == 0)
+	    empty_dynamic = true; /* Usually separate debuginfo.  */
+	  else
 	    {
 	      /* Debuginfo only files from "objcopy --only-keep-debug"
 		 contain a PT_DYNAMIC segment with p_filesz == 0.  Skip
@@ -1270,6 +1273,13 @@ _dl_map_object_from_fd (const char *name, const char *origname, int fd,
 	goto lose;
       }
 
+    /* This check recognizes most separate debuginfo files.  */
+    if (__glibc_unlikely ((l->l_ld == 0 && type == ET_DYN) || empty_dynamic))
+      {
+	errstring = N_("object file has no dynamic section");
+	goto lose;
+      }
+
     /* Length of the sections to be loaded.  */
     maplength = loadcmds[nloadcmds - 1].allocend - loadcmds[0].mapstart;
 
@@ -1287,15 +1297,7 @@ _dl_map_object_from_fd (const char *name, const char *origname, int fd,
       }
   }
 
-  if (l->l_ld == 0)
-    {
-      if (__glibc_unlikely (type == ET_DYN))
-	{
-	  errstring = N_("object file has no dynamic section");
-	  goto lose;
-	}
-    }
-  else
+  if (l->l_ld != 0)
     l->l_ld = (ElfW(Dyn) *) ((ElfW(Addr)) l->l_ld + l->l_addr);
 
   elf_get_dynamic_info (l, false, false);

Re: [PATCH v2] elf: Earlier missing dynamic segment check in _dl_map_object_from_fd

[H.J. Lu]

On Fri, Nov 5, 2021 at 11:27 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> Separated debuginfo files have PT_DYNAMIC with p_filesz == 0.  We
> need to check for that before the _dl_map_segments call because
> that could attempt to write to mappings that extend beyond the end
> of the file, resulting in SIGBUS.
>
> ---
> v2: Mark the result of the || expression as unlikely, and not just the
>     first branch.
>  elf/dl-load.c | 22 ++++++++++++----------
>  1 file changed, 12 insertions(+), 10 deletions(-)
>
> diff --git a/elf/dl-load.c b/elf/dl-load.c
> index a1f1682188..9f4fa9617d 100644
> --- a/elf/dl-load.c
> +++ b/elf/dl-load.c
> @@ -1135,6 +1135,7 @@ _dl_map_object_from_fd (const char *name, const char *origname, int fd,
>      struct loadcmd loadcmds[l->l_phnum];
>      size_t nloadcmds = 0;
>      bool has_holes = false;
> +    bool empty_dynamic = false;
>
>      /* The struct is initialized to zero so this is not necessary:
>      l->l_ld = 0;
> @@ -1147,7 +1148,9 @@ _dl_map_object_from_fd (const char *name, const char *origname, int fd,
>              segments are mapped in.  We record the addresses it says
>              verbatim, and later correct for the run-time load address.  */
>         case PT_DYNAMIC:
> -         if (ph->p_filesz)
> +         if (ph->p_filesz == 0)
> +           empty_dynamic = true; /* Usually separate debuginfo.  */
> +         else
>             {
>               /* Debuginfo only files from "objcopy --only-keep-debug"
>                  contain a PT_DYNAMIC segment with p_filesz == 0.  Skip
> @@ -1270,6 +1273,13 @@ _dl_map_object_from_fd (const char *name, const char *origname, int fd,
>         goto lose;
>        }
>
> +    /* This check recognizes most separate debuginfo files.  */
> +    if (__glibc_unlikely ((l->l_ld == 0 && type == ET_DYN) || empty_dynamic))
> +      {
> +       errstring = N_("object file has no dynamic section");
> +       goto lose;
> +      }
> +
>      /* Length of the sections to be loaded.  */
>      maplength = loadcmds[nloadcmds - 1].allocend - loadcmds[0].mapstart;
>
> @@ -1287,15 +1297,7 @@ _dl_map_object_from_fd (const char *name, const char *origname, int fd,
>        }
>    }
>
> -  if (l->l_ld == 0)
> -    {
> -      if (__glibc_unlikely (type == ET_DYN))
> -       {
> -         errstring = N_("object file has no dynamic section");
> -         goto lose;
> -       }
> -    }
> -  else
> +  if (l->l_ld != 0)
>      l->l_ld = (ElfW(Dyn) *) ((ElfW(Addr)) l->l_ld + l->l_addr);
>
>    elf_get_dynamic_info (l, false, false);
>

LGTM.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

Thanks.

-- 
H.J.