[PATCH][BUG 18093] Fix ldconfig segmentation fault with corrupted cache

[Aurelien Jarno <aurelien at aurel32 dot net>]

ldconfig is using an aux-cache to speed up the ld.so.cache update. It
is read by mmaping the file to a structure which contains data offsets
used as pointers. As they are not checked, it is not hard to get
ldconfig to segfault with a corrupted file. This happens for instance if
the file is truncated, which is common following a filesystem check
following a system crash.

This can be reproduced for example by truncating the file to roughly
half of it's size.

There is already in some code in elf/cache.c (load_aux_cache) to check
for a corrupted aux cache, but it happens to be broken and not enough.
The test (aux_cache->nlibs >= aux_cache_size) compares the number of
libs entry with the cache size. It's a non sense, as it basically
assumes that each library entry is a 1 byte... Instead the patch below
computes the theoretical cache size using the headers and compares it
to the real size.

Another corruption can happen if the pointers to the string table is
corrupted though in that case it means that the file has been more than
just truncated. The patch below ignores entries pointing outside of the
string table, they will be added with the current ldconfig run.

(This patch is based on an initial patch from Lennart Sorensen).


2015-03-08  Aurelien Jarno  <aurelien@aurel32.net>

	[BZ #18093]
	* elf/cache.c (load_aux_cache): Regenerate the cache if it has the
	wrong size. Ignore entries pointing outside of the mmaped memory.

diff --git a/elf/cache.c b/elf/cache.c
index 1732268..9131e08 100644
--- a/elf/cache.c
+++ b/elf/cache.c
@@ -698,7 +698,9 @@ load_aux_cache (const char *aux_cache_name)
   if (aux_cache == MAP_FAILED
       || aux_cache_size < sizeof (struct aux_cache_file)
       || memcmp (aux_cache->magic, AUX_CACHEMAGIC, sizeof AUX_CACHEMAGIC - 1)
-      || aux_cache->nlibs >= aux_cache_size)
+      || aux_cache_size != (sizeof(struct aux_cache_file) +
+			    aux_cache->nlibs * sizeof(struct aux_cache_file_entry) +
+			    aux_cache->len_strings))
     {
       close (fd);
       init_aux_cache ();
@@ -711,12 +713,13 @@ load_aux_cache (const char *aux_cache_name)
   const char *aux_cache_data
     = (const char *) &aux_cache->libs[aux_cache->nlibs];
   for (unsigned int i = 0; i < aux_cache->nlibs; ++i)
-    insert_to_aux_cache (&aux_cache->libs[i].id,
-			 aux_cache->libs[i].flags,
-			 aux_cache->libs[i].osversion,
-			 aux_cache->libs[i].soname == 0
-			 ? NULL : aux_cache_data + aux_cache->libs[i].soname,
-			 0);
+    if (aux_cache->libs[i].soname < aux_cache->len_strings)
+      insert_to_aux_cache (&aux_cache->libs[i].id,
+			   aux_cache->libs[i].flags,
+			   aux_cache->libs[i].osversion,
+			   aux_cache->libs[i].soname == 0
+			   ? NULL : aux_cache_data + aux_cache->libs[i].soname,
+			   0);
 
   munmap (aux_cache, aux_cache_size);
   close (fd);

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
aurelien@aurel32.net                 http://www.aurel32.net