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: Paul Gortmaker <paul.gortmaker@windriver.com>

diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index 42b60b04ea061fc01419b2330899f307e4bed895..fb939976d58cb32101b896d25e20870ecc52cf6b 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 aca9d55afb220fddd5998a5d37bfd7a402bb5995..73d51f39c89a9c9c66279bf03019dc587f313c22 100644 (file)
--- a/fs/binfmt_script.c
+++ b/fs/binfmt_script.c
@@ -81,7 +81,9 @@ 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 4afb996086d52b47810c6316bfc2b2069d765728..0ee94fe2fe37a9d649cbd3325df965e268d0d350 100644 (file)
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1119,9 +1119,24 @@ void free_bprm(struct linux_binprm *bprm)
                mutex_unlock(&current->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 074b620d5d8b9a6599bacfe2049b3375c13d939f..8e0957df83bba0517b52f43faa38a0fca3f8eee9 100644 (file)
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -131,6 +131,7 @@ 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);