Patchwork [3.5.y.z,extended,stable] Patch "exec: do not leave bprm->interp on stack" has been added to staging queue

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Submitter Herton Ronaldo Krzesinski
Date Jan. 8, 2013, 8:58 p.m.
Message ID <1357678687-20654-1-git-send-email-herton.krzesinski@canonical.com>
Download mbox | patch
Permalink /patch/210523/
State New
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Herton Ronaldo Krzesinski - Jan. 8, 2013, 8:58 p.m.
This is a note to let you know that I have just added a patch titled

    exec: do not leave bprm->interp on stack

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.5.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Herton

------

From c2c433b3a5ca98e6701e555a67a89b8558adbb53 Mon Sep 17 00:00:00 2001
From: Kees Cook <keescook@chromium.org>
Date: Thu, 20 Dec 2012 15:05:16 -0800
Subject: [PATCH 24/27] exec: do not leave bprm->interp on stack

commit b66c5984017533316fd1951770302649baf1aa33 upstream.

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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Herton Ronaldo Krzesinski <herton.krzesinski@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(-)

--
1.7.9.5

Patch

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 8726a93..5ee73d2 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);