From 20ae2081584450e552735a3df968ce5b5946a607 Mon Sep 17 00:00:00 2001

From: Kees Cook <keescook@chromium.org>

Date: Mon, 26 Nov 2012 08:56:37 -0500

Subject: [PATCH 1/2] 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>

---

 fs/binfmt_misc.c        |  5 ++++-

 fs/binfmt_script.c      |  4 +++-

 fs/exec.c               | 15 +++++++++++++++

 include/linux/binfmts.h |  1 +

 4 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c

index 790b3cd..772428d 100644

--- a/fs/binfmt_misc.c

+++ b/fs/binfmt_misc.c

@@ -176,7 +176,10 @@ static int load_misc_binary(struct linux_binprm *bprm, struct pt_regs *regs)

 		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 d3b8c1f..df49d48 100644

--- a/fs/binfmt_script.c

+++ b/fs/binfmt_script.c

@@ -82,7 +82,9 @@ static int load_script(struct linux_binprm *bprm,struct pt_regs *regs)

 	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 fab2c6d..59896ae 100644

--- a/fs/exec.c

+++ b/fs/exec.c

@@ -1202,9 +1202,24 @@ 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 366422b..eb53e15 100644

--- a/include/linux/binfmts.h

+++ b/include/linux/binfmts.h

@@ -128,6 +128,7 @@ 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);

-- 

1.8.0