[Lucid][CVE] UBUNTU: SAUCE: exec: do not leave bprm->interp on stack

Luis Henriques luis.henriques at canonical.com
Wed Dec 19 12:42:02 UTC 2012

From: Kees Cook <keescook at chromium.org>


BugLink: http://bugs.launchpad.net/bugs/1068888

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

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:

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

Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
 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 42b60b0..fb93997 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 0834350..356568c 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->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 df656b8..e8161ce 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1112,9 +1112,24 @@ void free_bprm(struct linux_binprm *bprm)
+	/* If a binfmt changed the interp, free it. */
+	if (bprm->interp != bprm->filename)
+		kfree(bprm->interp);
+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;
  * install the new credentials for this executable
diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
index a3d802e..d06c3a4 100644
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -122,6 +122,7 @@ 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,char ** argv,struct linux_binprm *bprm);
 extern int prepare_bprm_creds(struct linux_binprm *bprm);
 extern void install_exec_creds(struct linux_binprm *bprm);

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