Merge patch series "fs: harden anon inodes"

Christian Brauner <brauner@kernel.org> says:

* Anonymous inodes currently don't come with a proper mode causing
  issues in the kernel when we want to add useful VFS debug assert. Fix
  that by giving them a proper mode and masking it off when we report it
  to userspace which relies on them not having any mode.

* Anonymous inodes currently allow to change inode attributes because
  the VFS falls back to simple_setattr() if i_op->setattr isn't
  implemented. This means the ownership and mode for every single user
  of anon_inode_inode can be changed. Block that as it's either useless
  or actively harmful. If specific ownership is needed the respective
  subsystem should allocate anonymous inodes from their own private
  superblock.

* Port pidfs to the new anon_inode_{g,s}etattr() helpers.

* Add proper tests for anonymous inode behavior.

The anonymous inode specific fixes should ideally be backported to all
LTS kernels.

* patches from https://lore.kernel.org/20250407-work-anon_inode-v1-0-53a44c20d44e@kernel.org:
  selftests/filesystems: add fourth test for anonymous inodes
  selftests/filesystems: add third test for anonymous inodes
  selftests/filesystems: add second test for anonymous inodes
  selftests/filesystems: add first test for anonymous inodes
  anon_inode: raise SB_I_NODEV and SB_I_NOEXEC
  pidfs: use anon_inode_setattr()
  anon_inode: explicitly block ->setattr()
  pidfs: use anon_inode_getattr()
  anon_inode: use a proper mode internally

Link: https://lore.kernel.org/20250407-work-anon_inode-v1-0-53a44c20d44e@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

7 files changed, 130 insertions, 26 deletions

diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
index 583ac81669c2..e51e7d88980a 100644
--- a/fs/anon_inodes.c
+++ b/fs/anon_inodes.c
@@ -24,10 +24,51 @@
 
 #include <linux/uaccess.h>
 
+#include "internal.h"
+
 static struct vfsmount *anon_inode_mnt __ro_after_init;
 static struct inode *anon_inode_inode __ro_after_init;
 
 /*
+ * User space expects anonymous inodes to have no file type in st_mode.
+ *
+ * In particular, 'lsof' has this legacy logic:
+ *
+ *	type = s->st_mode & S_IFMT;
+ *	switch (type) {
+ *	  ...
+ *	case 0:
+ *		if (!strcmp(p, "anon_inode"))
+ *			Lf->ntype = Ntype = N_ANON_INODE;
+ *
+ * to detect our old anon_inode logic.
+ *
+ * Rather than mess with our internal sane inode data, just fix it
+ * up here in getattr() by masking off the format bits.
+ */
+int anon_inode_getattr(struct mnt_idmap *idmap, const struct path *path,
+		       struct kstat *stat, u32 request_mask,
+		       unsigned int query_flags)
+{
+	struct inode *inode = d_inode(path->dentry);
+
+	generic_fillattr(&nop_mnt_idmap, request_mask, inode, stat);
+	stat->mode &= ~S_IFMT;
+	return 0;
+}
+
+int anon_inode_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
+		       struct iattr *attr)
+{
+	return -EOPNOTSUPP;
+}
+
+static const struct inode_operations anon_inode_operations = {
+	.getattr = anon_inode_getattr,
+	.setattr = anon_inode_setattr,
+};
+
+/*
  * anon_inodefs_dname() is called from d_path().
  */
 static char *anon_inodefs_dname(struct dentry *dentry, char *buffer, int buflen)
@@ -45,6 +86,8 @@ static int anon_inodefs_init_fs_context(struct fs_context *fc)
 	struct pseudo_fs_context *ctx = init_pseudo(fc, ANON_INODE_FS_MAGIC);
 	if (!ctx)
 		return -ENOMEM;
+	fc->s_iflags |= SB_I_NOEXEC;
+	fc->s_iflags |= SB_I_NODEV;
 	ctx->dops = &anon_inodefs_dentry_operations;
 	return 0;
 }
@@ -66,6 +109,7 @@ static struct inode *anon_inode_make_secure_inode(
 	if (IS_ERR(inode))
 		return inode;
 	inode->i_flags &= ~S_PRIVATE;
+	inode->i_op = &anon_inode_operations;
 	error =	security_inode_init_security_anon(inode, &QSTR(name),
 						  context_inode);
 	if (error) {
@@ -313,6 +357,7 @@ static int __init anon_inode_init(void)
 	anon_inode_inode = alloc_anon_inode(anon_inode_mnt->mnt_sb);
 	if (IS_ERR(anon_inode_inode))
 		panic("anon_inode_init() inode allocation failed (%ld)\n", PTR_ERR(anon_inode_inode));
+	anon_inode_inode->i_op = &anon_inode_operations;
 
 	return 0;
 }
diff --git a/fs/internal.h b/fs/internal.h
index b9b3e29a73fd..f545400ce607 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -343,3 +343,8 @@ static inline bool path_mounted(const struct path *path)
 void file_f_owner_release(struct file *file);
 bool file_seek_cur_needs_f_lock(struct file *file);
 int statmount_mnt_idmap(struct mnt_idmap *idmap, struct seq_file *seq, bool uid_map);
+int anon_inode_getattr(struct mnt_idmap *idmap, const struct path *path,
+		       struct kstat *stat, u32 request_mask,
+		       unsigned int query_flags);
+int anon_inode_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
+		       struct iattr *attr);
diff --git a/fs/libfs.c b/fs/libfs.c
index 6393d7c49ee6..e1146620346e 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -1647,7 +1647,13 @@ struct inode *alloc_anon_inode(struct super_block *s)
 	 * that it already _is_ on the dirty list.
 	 */
 	inode->i_state = I_DIRTY;
-	inode->i_mode = S_IRUSR | S_IWUSR;
+	/*
+	 * Historically anonymous inodes didn't have a type at all and
+	 * userspace has come to rely on this. Internally they're just
+	 * regular files but S_IFREG is masked off when reporting
+	 * information to userspace.
+	 */
+	inode->i_mode = S_IFREG | S_IRUSR | S_IWUSR;
 	inode->i_uid = current_fsuid();
 	inode->i_gid = current_fsgid();
 	inode->i_flags |= S_PRIVATE;
diff --git a/fs/pidfs.c b/fs/pidfs.c
index d64a4cbeb0da..10b4ee454cca 100644
--- a/fs/pidfs.c
+++ b/fs/pidfs.c
@@ -569,36 +569,14 @@ static struct vfsmount *pidfs_mnt __ro_after_init;
 static int pidfs_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
 			 struct iattr *attr)
 {
-	return -EOPNOTSUPP;
+	return anon_inode_setattr(idmap, dentry, attr);
 }
 
-
-/*
- * User space expects pidfs inodes to have no file type in st_mode.
- *
- * In particular, 'lsof' has this legacy logic:
- *
- *	type = s->st_mode & S_IFMT;
- *	switch (type) {
- *	  ...
- *	case 0:
- *		if (!strcmp(p, "anon_inode"))
- *			Lf->ntype = Ntype = N_ANON_INODE;
- *
- * to detect our old anon_inode logic.
- *
- * Rather than mess with our internal sane inode data, just fix it
- * up here in getattr() by masking off the format bits.
- */
 static int pidfs_getattr(struct mnt_idmap *idmap, const struct path *path,
 			 struct kstat *stat, u32 request_mask,
 			 unsigned int query_flags)
 {
-	struct inode *inode = d_inode(path->dentry);
-
-	generic_fillattr(&nop_mnt_idmap, request_mask, inode, stat);
-	stat->mode &= ~S_IFMT;
-	return 0;
+	return anon_inode_getattr(idmap, path, stat, request_mask, query_flags);
 }
 
 static const struct inode_operations pidfs_inode_operations = {
diff --git a/tools/testing/selftests/filesystems/.gitignore b/tools/testing/selftests/filesystems/.gitignore
index 828b66a10c63..7afa58e2bb20 100644
--- a/tools/testing/selftests/filesystems/.gitignore
+++ b/tools/testing/selftests/filesystems/.gitignore
@@ -2,3 +2,4 @@
 dnotify_test
 devpts_pts
 file_stressor
+anon_inode_test
diff --git a/tools/testing/selftests/filesystems/Makefile b/tools/testing/selftests/filesystems/Makefile
index 66305fc34c60..b02326193fee 100644
--- a/tools/testing/selftests/filesystems/Makefile
+++ b/tools/testing/selftests/filesystems/Makefile
@@ -1,7 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0
 
 CFLAGS += $(KHDR_INCLUDES)
-TEST_GEN_PROGS := devpts_pts file_stressor
+TEST_GEN_PROGS := devpts_pts file_stressor anon_inode_test
 TEST_GEN_PROGS_EXTENDED := dnotify_test
 
 include ../lib.mk
diff --git a/tools/testing/selftests/filesystems/anon_inode_test.c b/tools/testing/selftests/filesystems/anon_inode_test.c
new file mode 100644
index 000000000000..e8e0ef1460d2
--- /dev/null
+++ b/tools/testing/selftests/filesystems/anon_inode_test.c
@@ -0,0 +1,69 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#define __SANE_USERSPACE_TYPES__
+
+#include <fcntl.h>
+#include <stdio.h>
+#include <sys/stat.h>
+
+#include "../kselftest_harness.h"
+#include "overlayfs/wrappers.h"
+
+TEST(anon_inode_no_chown)
+{
+	int fd_context;
+
+	fd_context = sys_fsopen("tmpfs", 0);
+	ASSERT_GE(fd_context, 0);
+
+	ASSERT_LT(fchown(fd_context, 1234, 5678), 0);
+	ASSERT_EQ(errno, EOPNOTSUPP);
+
+	EXPECT_EQ(close(fd_context), 0);
+}
+
+TEST(anon_inode_no_chmod)
+{
+	int fd_context;
+
+	fd_context = sys_fsopen("tmpfs", 0);
+	ASSERT_GE(fd_context, 0);
+
+	ASSERT_LT(fchmod(fd_context, 0777), 0);
+	ASSERT_EQ(errno, EOPNOTSUPP);
+
+	EXPECT_EQ(close(fd_context), 0);
+}
+
+TEST(anon_inode_no_exec)
+{
+	int fd_context;
+
+	fd_context = sys_fsopen("tmpfs", 0);
+	ASSERT_GE(fd_context, 0);
+
+	ASSERT_LT(execveat(fd_context, "", NULL, NULL, AT_EMPTY_PATH), 0);
+	ASSERT_EQ(errno, EACCES);
+
+	EXPECT_EQ(close(fd_context), 0);
+}
+
+TEST(anon_inode_no_open)
+{
+	int fd_context;
+
+	fd_context = sys_fsopen("tmpfs", 0);
+	ASSERT_GE(fd_context, 0);
+
+	ASSERT_GE(dup2(fd_context, 500), 0);
+	ASSERT_EQ(close(fd_context), 0);
+	fd_context = 500;
+
+	ASSERT_LT(open("/proc/self/fd/500", 0), 0);
+	ASSERT_EQ(errno, ENXIO);
+
+	EXPECT_EQ(close(fd_context), 0);
+}
+
+TEST_HARNESS_MAIN
+