[functionfs] mainline UAF (was Re: [PATCH v3 36/50] functionfs: switch to simple_remove_by_name())

[Al Viro]

On Sat, Nov 15, 2025 at 08:21:34AM -0500, Greg Kroah-Hartman wrote:

> Ugh, messy.  But yes, this does look better, thanks for that.  Want me
> to take it through the USB tree, or will you take it through one of
> yours? (I don't remember what started this thread...)

I'll carve it up in several chunks and push to #work.functionfs; will post
tomorrow morning.  Minimal fix for ffs_epfiles_destroy() bug folded into #36
in #work.persistency - replacement for that commit below; are you OK with
that one?  It's orthogonal to the rest of the mess in there.

commit b9c24b7499916a1dbee50a4429fc04ebf7e21f03
Author: Al Viro <viro at zeniv.linux.org.uk>
Date:   Wed Sep 17 22:55:33 2025 -0400

    functionfs: switch to simple_remove_by_name()
    
    No need to return dentry from ffs_sb_create_file() or keep it around
    afterwards.
    
    To avoid subtle issues with getting to ffs from epfiles in
    ffs_epfiles_destroy(), pass the superblock as explicit argument.
    Callers have it anyway.
    
    Signed-off-by: Al Viro <viro at zeniv.linux.org.uk>

diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
index 47cfbe41fdff..6e6933a9fe45 100644
--- a/drivers/usb/gadget/function/f_fs.c
+++ b/drivers/usb/gadget/function/f_fs.c
@@ -160,8 +160,6 @@ struct ffs_epfile {
 	struct ffs_data			*ffs;
 	struct ffs_ep			*ep;	/* P: ffs->eps_lock */
 
-	struct dentry			*dentry;
-
 	/*
 	 * Buffer for holding data from partial reads which may happen since
 	 * we’re rounding user read requests to a multiple of a max packet size.
@@ -271,11 +269,11 @@ struct ffs_desc_helper {
 };
 
 static int  __must_check ffs_epfiles_create(struct ffs_data *ffs);
-static void ffs_epfiles_destroy(struct ffs_epfile *epfiles, unsigned count);
+static void ffs_epfiles_destroy(struct super_block *sb,
+				struct ffs_epfile *epfiles, unsigned count);
 
-static struct dentry *
-ffs_sb_create_file(struct super_block *sb, const char *name, void *data,
-		   const struct file_operations *fops);
+static int ffs_sb_create_file(struct super_block *sb, const char *name,
+			      void *data, const struct file_operations *fops);
 
 /* Devices management *******************************************************/
 
@@ -1866,9 +1864,8 @@ ffs_sb_make_inode(struct super_block *sb, void *data,
 }
 
 /* Create "regular" file */
-static struct dentry *ffs_sb_create_file(struct super_block *sb,
-					const char *name, void *data,
-					const struct file_operations *fops)
+static int ffs_sb_create_file(struct super_block *sb, const char *name,
+			      void *data, const struct file_operations *fops)
 {
 	struct ffs_data	*ffs = sb->s_fs_info;
 	struct dentry	*dentry;
@@ -1876,16 +1873,16 @@ static struct dentry *ffs_sb_create_file(struct super_block *sb,
 
 	dentry = d_alloc_name(sb->s_root, name);
 	if (!dentry)
-		return NULL;
+		return -ENOMEM;
 
 	inode = ffs_sb_make_inode(sb, data, fops, NULL, &ffs->file_perms);
 	if (!inode) {
 		dput(dentry);
-		return NULL;
+		return -ENOMEM;
 	}
 
 	d_add(dentry, inode);
-	return dentry;
+	return 0;
 }
 
 /* Super block */
@@ -1928,10 +1925,7 @@ static int ffs_sb_fill(struct super_block *sb, struct fs_context *fc)
 		return -ENOMEM;
 
 	/* EP0 file */
-	if (!ffs_sb_create_file(sb, "ep0", ffs, &ffs_ep0_operations))
-		return -ENOMEM;
-
-	return 0;
+	return ffs_sb_create_file(sb, "ep0", ffs, &ffs_ep0_operations);
 }
 
 enum {
@@ -2161,7 +2155,7 @@ static void ffs_data_closed(struct ffs_data *ffs)
 							flags);
 
 			if (epfiles)
-				ffs_epfiles_destroy(epfiles,
+				ffs_epfiles_destroy(ffs->sb, epfiles,
 						 ffs->eps_count);
 
 			if (ffs->setup_state == FFS_SETUP_PENDING)
@@ -2226,7 +2220,7 @@ static void ffs_data_clear(struct ffs_data *ffs)
 	 * copy of epfile will save us from use-after-free.
 	 */
 	if (epfiles) {
-		ffs_epfiles_destroy(epfiles, ffs->eps_count);
+		ffs_epfiles_destroy(ffs->sb, epfiles, ffs->eps_count);
 		ffs->epfiles = NULL;
 	}
 
@@ -2323,6 +2317,7 @@ static int ffs_epfiles_create(struct ffs_data *ffs)
 {
 	struct ffs_epfile *epfile, *epfiles;
 	unsigned i, count;
+	int err;
 
 	count = ffs->eps_count;
 	epfiles = kcalloc(count, sizeof(*epfiles), GFP_KERNEL);
@@ -2339,12 +2334,11 @@ static int ffs_epfiles_create(struct ffs_data *ffs)
 			sprintf(epfile->name, "ep%02x", ffs->eps_addrmap[i]);
 		else
 			sprintf(epfile->name, "ep%u", i);
-		epfile->dentry = ffs_sb_create_file(ffs->sb, epfile->name,
-						 epfile,
-						 &ffs_epfile_operations);
-		if (!epfile->dentry) {
-			ffs_epfiles_destroy(epfiles, i - 1);
-			return -ENOMEM;
+		err = ffs_sb_create_file(ffs->sb, epfile->name,
+					 epfile, &ffs_epfile_operations);
+		if (err) {
+			ffs_epfiles_destroy(ffs->sb, epfiles, i - 1);
+			return err;
 		}
 	}
 
@@ -2352,16 +2346,15 @@ static int ffs_epfiles_create(struct ffs_data *ffs)
 	return 0;
 }
 
-static void ffs_epfiles_destroy(struct ffs_epfile *epfiles, unsigned count)
+static void ffs_epfiles_destroy(struct super_block *sb,
+				struct ffs_epfile *epfiles, unsigned count)
 {
 	struct ffs_epfile *epfile = epfiles;
+	struct dentry *root = sb->s_root;
 
 	for (; count; --count, ++epfile) {
 		BUG_ON(mutex_is_locked(&epfile->mutex));
-		if (epfile->dentry) {
-			simple_recursive_removal(epfile->dentry, NULL);
-			epfile->dentry = NULL;
-		}
+		simple_remove_by_name(root, epfile->name, NULL);
 	}
 
 	kfree(epfiles);