exec: do not leave bprm->interp on stack

If a series of scripts are executed, each triggering module loading via
unprintable bytes in the script header, kernel stack contents can leak
into the command line.

Normally execution of binfmt_script and binfmt_misc happens recursively.
However, when modules are enabled, and unprintable bytes exist in the
bprm->buf, execution will restart after attempting to load matching
binfmt modules.  Unfortunately, the logic in binfmt_script and
binfmt_misc does not expect to get restarted.  They leave bprm->interp
pointing to their local stack.  This means on restart bprm->interp is
left pointing into unused stack memory which can then be copied into the
userspace argv areas.

After additional study, it seems that both recursion and restart remains
the desirable way to handle exec with scripts, misc, and modules.  As
such, we need to protect the changes to interp.

This changes the logic to require allocation for any changes to the
bprm->interp.  To avoid adding a new kmalloc to every exec, the default
value is left as-is.  Only when passing through binfmt_script or
binfmt_misc does an allocation take place.

For a proof of concept, see DoTest.sh from:

   http://www.halfdog.net/Security/2012/LinuxKernelBinfmtScriptStackDataDisclosure/

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: halfdog <me@halfdog.net>
Cc: P J P <ppandit@redhat.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index 9be335fb8a7cd3f9f480d0c5a46de7cfbcdd1c1f..0c8869fdd14ebf9291eb700ccd099c910584910d 100644 (file)
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -172,7 +172,10 @@ static int load_misc_binary(struct linux_binprm *bprm)
                goto _error;
        bprm->argc ++;
 
-       bprm->interp = iname;   /* for binfmt_script */
+       /* Update interp in case binfmt_script needs it. */
+       retval = bprm_change_interp(iname, bprm);
+       if (retval < 0)
+               goto _error;
 
        interp_file = open_exec (iname);
        retval = PTR_ERR (interp_file);

diff --git a/fs/binfmt_script.c b/fs/binfmt_script.c
index 1610a91637e57d88c54b1a88d6a3ba1e4a230ac4..5027a3e149222bd5945c87d9521e6e7a6108dc77 100644 (file)
--- a/fs/binfmt_script.c
+++ b/fs/binfmt_script.c
@@ -80,7 +80,9 @@ static int load_script(struct linux_binprm *bprm)
        retval = copy_strings_kernel(1, &i_name, bprm);
        if (retval) return retval; 
        bprm->argc++;
-       bprm->interp = interp;
+       retval = bprm_change_interp(interp, bprm);
+       if (retval < 0)
+               return retval;
 
        /*
         * OK, now restart the process with the interpreter's dentry.

diff --git a/fs/exec.c b/fs/exec.c
index d8e1191cb112eb2d77154908a32699a06c4bda78..237d5342786c43499bf32cfb831607ff8c86a728 100644 (file)
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1175,9 +1175,24 @@ void free_bprm(struct linux_binprm *bprm)
                mutex_unlock(&current->signal->cred_guard_mutex);
                abort_creds(bprm->cred);
        }
+       /* If a binfmt changed the interp, free it. */
+       if (bprm->interp != bprm->filename)
+               kfree(bprm->interp);
        kfree(bprm);
 }
 
+int bprm_change_interp(char *interp, struct linux_binprm *bprm)
+{
+       /* If a binfmt changed the interp, free it first. */
+       if (bprm->interp != bprm->filename)
+               kfree(bprm->interp);
+       bprm->interp = kstrdup(interp, GFP_KERNEL);
+       if (!bprm->interp)
+               return -ENOMEM;
+       return 0;
+}
+EXPORT_SYMBOL(bprm_change_interp);
+
 /*
  * install the new credentials for this executable
  */

diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
index a4c2b565c835dfceade1065efe641d5009b85b08..bdf3965f0a29a9d6b3cf9568ce58dc457de32526 100644 (file)
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -112,6 +112,7 @@ extern int setup_arg_pages(struct linux_binprm * bprm,
                           unsigned long stack_top,
                           int executable_stack);
 extern int bprm_mm_init(struct linux_binprm *bprm);
+extern int bprm_change_interp(char *interp, struct linux_binprm *bprm);
 extern int copy_strings_kernel(int argc, const char *const *argv,
                               struct linux_binprm *bprm);
 extern int prepare_bprm_creds(struct linux_binprm *bprm);