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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index 613aa06182358806cd65f077df4e687593806455..e1724392d0503db567f81cd476bc6c7b62553eb9 100644 (file)
--- 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 d3b8c1f6315554df64d3491b52d83d2a73f4a359..df49d486b0c0a6aaa442f81e675bca10c0aa9752 100644 (file)
--- 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 6c4791d4634a90a6b435018f3d822fcbae00a4b2..51d8629736393063b5a27ee67b170a0e72944991 100644 (file)
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1206,9 +1206,24 @@ 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 366422bc1633cecda39235276569f283faf0bf46..eb53e15ede68f4fef2b4c6ed9909e051be667a70 100644 (file)
--- 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);