anon_inode: use a proper mode internally

This allows the VFS to not trip over anonymous inodes and we can add
asserts based on the mode into the vfs. When we report it to userspace
we can simply hide the mode to avoid regressions. I've audited all
direct callers of alloc_anon_inode() and only secretmen overrides i_mode
and i_op inode operations but it already uses a regular file.

Link: https://lore.kernel.org/20250407-work-anon_inode-v1-1-53a44c20d44e@kernel.org
Fixes: af153bb63a336 ("vfs: catch invalid modes in may_open()")
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Cc: stable@vger.kernel.org # all LTS kernels
Reported-by: syzbot+5d8e79d323a13aa0b248@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/67ed3fb3.050a0220.14623d.0009.GAE@google.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 36 insertions, 0 deletions

diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
index 583ac81669c2..42e4b9c34f89 100644
--- a/fs/anon_inodes.c
+++ b/fs/anon_inodes.c
@@ -24,10 +24,44 @@
 
 #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;
+}
+
+static const struct inode_operations anon_inode_operations = {
+	.getattr = anon_inode_getattr,
+};
+
+/*
  * anon_inodefs_dname() is called from d_path().
  */
 static char *anon_inodefs_dname(struct dentry *dentry, char *buffer, int buflen)
@@ -66,6 +100,7 @@ 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 +348,7 @@ 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;
 }