[PATCH] procfs: Prevent unpriveleged processes accessing fdinfo

[Kalesh Singh <kaleshsingh@google.com>]

The file permissions on the fdinfo dir from were changed from
S_IRUSR|S_IXUSR to S_IRUGO|S_IXUGO, and a PTRACE_MODE_READ check was
added for opening the fdinfo files [1]. However, the ptrace permission
check was not added to the directory, allowing anyone to get the open FD
numbers by reading the fdinfo directory.

Add the missing ptrace permission check for opening the fdinfo directory.
The check is also added for readdir, lseek in the case that
an unprivileged process inherits an open FD to the fdinfo dir after an
exec.

For the same reason, similar checks are added for fdinfo files which
previously only checked the ptrace permission in open.

[1] https://lkml.kernel.org/r/20210308170651.919148-1-kaleshsingh@google.com

Fixes: 7bc3fa0172a4 ("procfs: allow reading fdinfo with PTRACE_MODE_READ")
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
---
 fs/proc/fd.c | 65 ++++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 61 insertions(+), 4 deletions(-)

diff --git a/fs/proc/fd.c b/fs/proc/fd.c
index 172c86270b31..aea59e243bae 100644
--- a/fs/proc/fd.c
+++ b/fs/proc/fd.c
@@ -72,7 +72,7 @@ static int seq_show(struct seq_file *m, void *v)
 	return 0;
 }
 
-static int seq_fdinfo_open(struct inode *inode, struct file *file)
+static int proc_fdinfo_access_allowed(struct inode *inode)
 {
 	bool allowed = false;
 	struct task_struct *task = get_proc_task(inode);
@@ -86,13 +86,44 @@ static int seq_fdinfo_open(struct inode *inode, struct file *file)
 	if (!allowed)
 		return -EACCES;
 
+	return 0;
+}
+
+static int seq_fdinfo_open(struct inode *inode, struct file *file)
+{
+	int ret = proc_fdinfo_access_allowed(inode);
+
+	if (ret)
+		return ret;
+
 	return single_open(file, seq_show, inode);
 }
 
+static ssize_t seq_fdinfo_read(struct file *file, char __user *buf, size_t size,
+		loff_t *ppos)
+{
+	int ret = proc_fdinfo_access_allowed(file_inode(file));
+
+	if (ret)
+		return ret;
+
+	return seq_read(file, buf, size, ppos);
+}
+
+static loff_t seq_fdinfo_lseek(struct file *file, loff_t offset, int whence)
+{
+	int ret = proc_fdinfo_access_allowed(file_inode(file));
+
+	if (ret)
+		return ret;
+
+	return seq_lseek(file, offset, whence);
+}
+
 static const struct file_operations proc_fdinfo_file_operations = {
 	.open		= seq_fdinfo_open,
-	.read		= seq_read,
-	.llseek		= seq_lseek,
+	.read		= seq_fdinfo_read,
+	.llseek		= seq_fdinfo_lseek,
 	.release	= single_release,
 };
 
@@ -344,17 +375,43 @@ proc_lookupfdinfo(struct inode *dir, struct dentry *dentry, unsigned int flags)
 
 static int proc_readfdinfo(struct file *file, struct dir_context *ctx)
 {
+	int ret = proc_fdinfo_access_allowed(file_inode(file));
+
+	if (ret)
+		return ret;
+
 	return proc_readfd_common(file, ctx,
 				  proc_fdinfo_instantiate);
 }
 
+static loff_t proc_llseek_fdinfo(struct file *file, loff_t offset, int whence)
+{
+	int ret = proc_fdinfo_access_allowed(file_inode(file));
+
+	if (ret)
+		return ret;
+
+	return generic_file_llseek(file, offset, whence);
+}
+
+static int proc_open_fdinfo(struct inode *inode, struct file *file)
+{
+	int ret = proc_fdinfo_access_allowed(inode);
+
+	if (ret)
+		return ret;
+
+	return 0;
+}
+
 const struct inode_operations proc_fdinfo_inode_operations = {
 	.lookup		= proc_lookupfdinfo,
 	.setattr	= proc_setattr,
 };
 
 const struct file_operations proc_fdinfo_operations = {
+	.open		= proc_open_fdinfo,
 	.read		= generic_read_dir,
 	.iterate_shared	= proc_readfdinfo,
-	.llseek		= generic_file_llseek,
+	.llseek		= proc_llseek_fdinfo,
 };

base-commit: e9f1cbc0c4114880090c7a578117d3b9cf184ad4
-- 
2.32.0.93.g670b81a890-goog