Re: [PATCH, Libbacktrace] Fix possible SEGV when handling stripped PIE binaries.

[Maxim Ostapenko]

On 02/03/16 16:59, Ian Lance Taylor wrote:

On Wed, Mar 2, 2016 at 12:51 AM, Maxim Ostapenko
<m.ostape...@partner.samsung.com> wrote:

When testing ASan on large system, I've noticed that sometimes it crashes
with SEGV in Libbacktrace when trying to symbolize stripped PIE (compiled
with -pie -fPIC) binaries in fully stripped environment (this means that all
dependent libraries are also stripped). Here a scenario I've observed:

1) _asan_backtrace_initialize calls elf_add passing &elf_fileline_fn as
output parameter to properly initialize it.
2) elf_add doesn't elf_fileline_fn initialize and returns -1 for stripped
PIE binary.
3) _asan_backtrace_initialize calls phdr_callback on each dependent library
via dl_iterate_phdr.
4) phdr_callback initializes elf_fileline_fn iff it found debug info in some
library (found_dwarf == 1), but this is false since all libs are stripped.
So, we still have uninitialized elf_fileline_fn value.
5) _asan_backtrace_initialize uses elf_fileline_fn to initialize proper
fileline_fn callback.
6) Libbacktrace uses fileline_fn callback later and crashes because it
contains garbage.

This patch fixes the issue by simply initializing elf_fileline_fn via
elf_nodebug in _asan_backtrace_initialize prologue.

Tested on x86_64-linux-gnu and arm-linux-gnueabi, OK for trunk?

Thanks for the analysis.  I would rather set *fileline_fn in the case
where elf_add returns -1.  Or, remove the setting of *fileline_fn =
elf_nodebug in elf_add, since that would become the default.

Ian

Thanks, does this look better (I used the second option)?

-Maxim

libbacktrace/ChangeLog:

2016-03-02  Maxim Ostapenko  <m.ostape...@partner.samsung.com>

	* elf.c (backtrace_initialize): Properly initialize elf_fileline_fn to
	avoid possible crash.
	(elf_add): Don't set *fileline_fn to elf_nodebug value in case of
	missing debug info anymore.

diff --git a/libbacktrace/elf.c b/libbacktrace/elf.c
index 05cc5c0..f85ac65 100644
--- a/libbacktrace/elf.c
+++ b/libbacktrace/elf.c
@@ -791,7 +791,6 @@ elf_add (struct backtrace_state *state, int descriptor, uintptr_t base_address,
     {
       if (!backtrace_close (descriptor, error_callback, data))
 	goto fail;
-      *fileline_fn = elf_nodebug;
       return 1;
     }
 
@@ -925,7 +924,7 @@ backtrace_initialize (struct backtrace_state *state, int descriptor,
   int ret;
   int found_sym;
   int found_dwarf;
-  fileline elf_fileline_fn;
+  fileline elf_fileline_fn = elf_nodebug;
   struct phdr_data pd;
 
   ret = elf_add (state, descriptor, 0, error_callback, data, &elf_fileline_fn,