[PATCH 7/9] ptrace: forbid ptrace checks against current_cred() from VFS context

[Jann Horn <jann@thejh.net>]

This ensures that VFS implementations don't call ptrace_may_access() from
VFS read or write handlers. In order for file descriptor passing to have
its intended security properties, VFS read/write handlers must not do any
kind of privilege checking.

Signed-off-by: Jann Horn <jann@thejh.net>
---
 fs/aio.c              |  2 ++
 fs/read_write.c       | 12 ++++++++++++
 fs/splice.c           | 12 ++++++++++--
 include/linux/sched.h |  4 ++++
 kernel/ptrace.c       |  7 +++++++
 5 files changed, 35 insertions(+), 2 deletions(-)

diff --git a/fs/aio.c b/fs/aio.c
index fb8e45b..c941885 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -1454,9 +1454,11 @@ rw_common:
 
 		if (rw == WRITE)
 			file_start_write(file);
+		current->in_unprivileged_vfs++;
 
 		ret = iter_op(req, &iter);
 
+		current->in_unprivileged_vfs--;
 		if (rw == WRITE)
 			file_end_write(file);
 		kfree(iovec);
diff --git a/fs/read_write.c b/fs/read_write.c
index 66215a7..deddb93 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -472,7 +472,9 @@ ssize_t vfs_read(struct file *file, char __user *buf, size_t count, loff_t *pos)
 	if (!ret) {
 		if (count > MAX_RW_COUNT)
 			count =  MAX_RW_COUNT;
+		current->in_unprivileged_vfs++;
 		ret = __vfs_read(file, buf, count, pos);
+		current->in_unprivileged_vfs--;
 		if (ret > 0) {
 			fsnotify_access(file);
 			add_rchar(current, ret);
@@ -557,7 +559,9 @@ ssize_t vfs_write(struct file *file, const char __user *buf, size_t count, loff_
 		if (count > MAX_RW_COUNT)
 			count =  MAX_RW_COUNT;
 		file_start_write(file);
+		current->in_unprivileged_vfs++;
 		ret = __vfs_write(file, buf, count, pos);
+		current->in_unprivileged_vfs--;
 		if (ret > 0) {
 			fsnotify_modify(file);
 			add_wchar(current, ret);
@@ -838,12 +842,14 @@ static ssize_t do_readv_writev(int type, struct file *file,
 		iter_fn = file->f_op->write_iter;
 		file_start_write(file);
 	}
+	current->in_unprivileged_vfs++;
 
 	if (iter_fn)
 		ret = do_iter_readv_writev(file, &iter, pos, iter_fn, flags);
 	else
 		ret = do_loop_readv_writev(file, &iter, pos, fn, flags);
 
+	current->in_unprivileged_vfs--;
 	if (type != READ)
 		file_end_write(file);
 
@@ -1063,12 +1069,14 @@ static ssize_t compat_do_readv_writev(int type, struct file *file,
 		iter_fn = file->f_op->write_iter;
 		file_start_write(file);
 	}
+	current->in_unprivileged_vfs++;
 
 	if (iter_fn)
 		ret = do_iter_readv_writev(file, &iter, pos, iter_fn, flags);
 	else
 		ret = do_loop_readv_writev(file, &iter, pos, fn, flags);
 
+	current->in_unprivileged_vfs--;
 	if (type != READ)
 		file_end_write(file);
 
@@ -1369,7 +1377,9 @@ static ssize_t do_sendfile(int out_fd, int in_fd, loff_t *ppos,
 		fl = SPLICE_F_NONBLOCK;
 #endif
 	file_start_write(out.file);
+	current->in_unprivileged_vfs++;
 	retval = do_splice_direct(in.file, &pos, out.file, &out_pos, count, fl);
+	current->in_unprivileged_vfs--;
 	file_end_write(out.file);
 
 	if (retval > 0) {
@@ -1512,6 +1522,7 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
 	ret = mnt_want_write_file(file_out);
 	if (ret)
 		return ret;
+	current->in_unprivileged_vfs++;
 
 	ret = -EOPNOTSUPP;
 	if (file_out->f_op->copy_file_range)
@@ -1521,6 +1532,7 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
 		ret = do_splice_direct(file_in, &pos_in, file_out, &pos_out,
 				len > MAX_RW_COUNT ? MAX_RW_COUNT : len, 0);
 
+	current->in_unprivileged_vfs--;
 	if (ret > 0) {
 		fsnotify_access(file_in);
 		add_rchar(current, ret);
diff --git a/fs/splice.c b/fs/splice.c
index dd9bf7e..5fcad56 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -1134,7 +1134,7 @@ static long do_splice_to(struct file *in, loff_t *ppos,
 {
 	ssize_t (*splice_read)(struct file *, loff_t *,
 			       struct pipe_inode_info *, size_t, unsigned int);
-	int ret;
+	long ret;
 
 	if (unlikely(!(in->f_mode & FMODE_READ)))
 		return -EBADF;
@@ -1151,7 +1151,11 @@ static long do_splice_to(struct file *in, loff_t *ppos,
 	else
 		splice_read = default_file_splice_read;
 
-	return splice_read(in, ppos, pipe, len, flags);
+	current->in_unprivileged_vfs++;
+	ret = splice_read(in, ppos, pipe, len, flags);
+	current->in_unprivileged_vfs--;
+
+	return ret;
 }
 
 /**
@@ -1334,7 +1338,9 @@ long do_splice_direct(struct file *in, loff_t *ppos, struct file *out,
 	if (unlikely(ret < 0))
 		return ret;
 
+	current->in_unprivileged_vfs++;
 	ret = splice_direct_to_actor(in, &sd, direct_splice_actor);
+	current->in_unprivileged_vfs--;
 	if (ret > 0)
 		*ppos = sd.pos;
 
@@ -1401,7 +1407,9 @@ static long do_splice(struct file *in, loff_t __user *off_in,
 			return ret;
 
 		file_start_write(out);
+		current->in_unprivileged_vfs++;
 		ret = do_splice_from(ipipe, out, &offset, len, flags);
+		current->in_unprivileged_vfs--;
 		file_end_write(out);
 
 		if (!off_out)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index e4bf894..6094720 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1473,6 +1473,10 @@ struct task_struct {
 	atomic_t usage;
 	unsigned int flags;	/* per process flags, defined below */
 	unsigned int ptrace;
+	/* depth of VFS read/write; non-zero values let certain privilege checks
+	 * fail with a warning
+	 */
+	unsigned int in_unprivileged_vfs;
 
 #ifdef CONFIG_SMP
 	struct llist_node wake_entry;
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index b311ca5..d839919 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -302,6 +302,13 @@ ok:
 bool ptrace_may_access(struct task_struct *task, unsigned int mode)
 {
 	int err;
+
+	/* If you have to check for ptrace access from a VFS method, use
+	 * ptrace_may_access_noncurrent() instead.
+	 */
+	if (WARN_ON(current->in_unprivileged_vfs != 0))
+		return false;
+
 	task_lock(task);
 	err = __ptrace_may_access(task, mode, current_cred(),
 				  &current->self_privunit_id);
-- 
2.1.4