linux-kernel - [PATCH] uapi: export PROCFS_ROOT

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Message-Id: <20250708-uapi-procfs-root-ino-v1-1-6ae61e97c79b@cyphar.com>
Date: Tue, 08 Jul 2025 23:21:51 +1000
From: Aleksa Sarai <cyphar@...har.com>
To: Alexander Viro <viro@...iv.linux.org.uk>, 
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
 Aleksa Sarai <cyphar@...har.com>
Subject: [PATCH] uapi: export PROCFS_ROOT_INO

The root inode of /proc having a fixed inode number has been part of the
core kernel ABI since its inception, and recently some userspace
programs (mainly container runtimes) have started to explicitly depend
on this behaviour.

The main reason this is useful to userspace is that by checking that a
suspect /proc handle has fstype PROC_SUPER_MAGIC and is PROCFS_ROOT_INO,
they can then use openat2(RESOLVE_{NO_{XDEV,MAGICLINK},BENEATH}) to
ensure that there isn't a bind-mount that replaces some procfs file with
a different one. This kind of attack has lead to security issues in
container runtimes in the past (such as CVE-2019-19921) and libraries
like libpathrs[1] use this feature of procfs to provide safe procfs
handling functions.

There was also some trailing whitespace in the "struct proc_dir_entry"
initialiser, so fix that up as well.

[1]: https://github.com/openSUSE/libpathrs

Signed-off-by: Aleksa Sarai <cyphar@...har.com>
---
 fs/proc/root.c          | 10 +++++-----
 include/linux/proc_ns.h |  1 -
 include/uapi/linux/fs.h | 11 +++++++++++
 3 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/fs/proc/root.c b/fs/proc/root.c
index 06a297a27ba3..ed86ac710384 100644
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -363,12 +363,12 @@ static const struct inode_operations proc_root_inode_operations = {
  * This is the root "inode" in the /proc tree..
  */
 struct proc_dir_entry proc_root = {
-	.low_ino	= PROC_ROOT_INO, 
-	.namelen	= 5, 
-	.mode		= S_IFDIR | S_IRUGO | S_IXUGO, 
-	.nlink		= 2, 
+	.low_ino	= PROCFS_ROOT_INO,
+	.namelen	= 5,
+	.mode		= S_IFDIR | S_IRUGO | S_IXUGO,
+	.nlink		= 2,
 	.refcnt		= REFCOUNT_INIT(1),
-	.proc_iops	= &proc_root_inode_operations, 
+	.proc_iops	= &proc_root_inode_operations,
 	.proc_dir_ops	= &proc_root_operations,
 	.parent		= &proc_root,
 	.subdir		= RB_ROOT,
diff --git a/include/linux/proc_ns.h b/include/linux/proc_ns.h
index 6258455e49a4..4b20375f3783 100644
--- a/include/linux/proc_ns.h
+++ b/include/linux/proc_ns.h
@@ -40,7 +40,6 @@ extern const struct proc_ns_operations timens_for_children_operations;
  * We always define these enumerators
  */
 enum {
-	PROC_ROOT_INO		= 1,
 	PROC_IPC_INIT_INO	= IPC_NS_INIT_INO,
 	PROC_UTS_INIT_INO	= UTS_NS_INIT_INO,
 	PROC_USER_INIT_INO	= USER_NS_INIT_INO,
diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
index 3d7bb6580cfb..0bd678a4a10e 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -60,6 +60,17 @@
 #define RENAME_EXCHANGE		(1 << 1)	/* Exchange source and dest */
 #define RENAME_WHITEOUT		(1 << 2)	/* Whiteout source */
 
+/*
+ * The root inode of procfs is guaranteed to always have the same inode number.
+ * For programs that make heavy use of procfs, verifying that the root is a
+ * real procfs root and using openat2(RESOLVE_{NO_{XDEV,MAGICLINKS},BENEATH})
+ * will allow you to make sure you are never tricked into operating on the
+ * wrong procfs file.
+ */
+enum procfs_ino {
+	PROCFS_ROOT_INO = 1,
+};
+
 struct file_clone_range {
 	__s64 src_fd;
 	__u64 src_offset;

---
base-commit: 40e87bc3b0e06018c908c338b73268ca12e28d89
change-id: 20250708-uapi-procfs-root-ino-a180d59a9a6d

Best regards,
-- 
Aleksa Sarai <cyphar@...har.com>