[RFC][PATCH] exec: Use init rlimits for setuid exec

[Kees Cook <keescook@chromium.org>]

In an attempt to provide sensible rlimit defaults for setuid execs, this
inherits the namespace's init rlimits:

$ ulimit -s
8192
$ ulimit -s unlimited
$ /bin/sh -c 'ulimit -s'
unlimited
$ sudo /bin/sh -c 'ulimit -s'
8192

This is modified from Brad Spengler/PaX Team's hard-coded setuid exec
stack rlimit (8MB) in the last public patch of grsecurity/PaX based on
my understanding of the code. Changes or omissions from the original
code are mine and don't reflect the original grsecurity/PaX code.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
Instead of copying all rlimits, we could also pick specific ones to copy
(e.g. RLIMIT_STACK, or ones from Andy's list) or exclude from copying
(probably better to blacklist than whitelist).

I think this is the right way to find the ns init task, but maybe it
needs locking?
---
 fs/exec.c | 34 ++++++++++++++++++++++++++++++----
 1 file changed, 30 insertions(+), 4 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 904199086490..80e8b2bd4284 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1675,6 +1675,12 @@ static int exec_binprm(struct linux_binprm *bprm)
 	return ret;
 }
 
+static inline bool is_setuid_exec(struct linux_binprm *bprm)
+{
+	return (!uid_eq(bprm->cred->euid, current_euid()) ||
+		!gid_eq(bprm->cred->egid, current_egid()));
+}
+
 /*
  * sys_execve() executes a new program.
  */
@@ -1687,6 +1693,7 @@ static int do_execveat_common(int fd, struct filename *filename,
 	struct linux_binprm *bprm;
 	struct file *file;
 	struct files_struct *displaced;
+	struct rlimit saved_rlim[RLIM_NLIMITS];
 	int retval;
 
 	if (IS_ERR(filename))
@@ -1771,24 +1778,38 @@ static int do_execveat_common(int fd, struct filename *filename,
 	if (retval < 0)
 		goto out;
 
+	/*
+	 * From here forward, we've got credentials set up and we're
+	 * using resources, so do rlimit replacement before we start
+	 * copying strings. (Note that the RLIMIT_NPROC check has
+	 * already happened.)
+	 */
+	BUILD_BUG_ON(sizeof(saved_rlim) != sizeof(current->signal->rlim));
+	if (is_setuid_exec(bprm)) {
+		memcpy(saved_rlim, current->signal->rlim, sizeof(saved_rlim));
+		memcpy(current->signal->rlim,
+		       task_active_pid_ns(current)->child_reaper->signal->rlim,
+		       sizeof(current->signal->rlim));
+	}
+
 	retval = copy_strings_kernel(1, &bprm->filename, bprm);
 	if (retval < 0)
-		goto out;
+		goto out_restore;
 
 	bprm->exec = bprm->p;
 	retval = copy_strings(bprm->envc, envp, bprm);
 	if (retval < 0)
-		goto out;
+		goto out_restore;
 
 	retval = copy_strings(bprm->argc, argv, bprm);
 	if (retval < 0)
-		goto out;
+		goto out_restore;
 
 	would_dump(bprm, bprm->file);
 
 	retval = exec_binprm(bprm);
 	if (retval < 0)
-		goto out;
+		goto out_restore;
 
 	/* execve succeeded */
 	current->fs->in_exec = 0;
@@ -1802,6 +1823,11 @@ static int do_execveat_common(int fd, struct filename *filename,
 		put_files_struct(displaced);
 	return retval;
 
+out_restore:
+	if (is_setuid_exec(bprm)) {
+		memcpy(current->signal->rlim, saved_rlim, sizeof(saved_rlim));
+	}
+
 out:
 	if (bprm->mm) {
 		acct_arg_size(bprm, 0);
-- 
2.7.4


-- 
Kees Cook
Pixel Security