FAILED: patch "[PATCH] usb: gadget: f_fs: copy only received bytes on short ep0 read" failed to apply to 5.10-stable tree

The patch below does not apply to the 5.10-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.10.y
git checkout FETCH_HEAD
git cherry-pick -x 4e036c10e7f4df5d951c69cc3697bc8e209c6d02
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026060428-starter-stimulate-a40b@gregkh' --subject-prefix 'PATCH 5.10.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 4e036c10e7f4df5d951c69cc3697bc8e209c6d02 Mon Sep 17 00:00:00 2001
From: Michael Bommarito <[email protected]>
Date: Sun, 19 Apr 2026 12:03:59 -0400
Subject: [PATCH] usb: gadget: f_fs: copy only received bytes on short ep0 read

ffs_ep0_read() allocates its control-OUT data buffer with kmalloc() (not kzalloc) at the Length value from the Setup packet, then copies that full len to userspace regardless of how many bytes were actually received:

    data = kmalloc(len, GFP_KERNEL);
    ...
    ret = __ffs_ep0_queue_wait(ffs, data, len);
    if ((ret > 0) && (copy_to_user(buf, data, len)))
            ret = -EFAULT;

__ffs_ep0_queue_wait() returns req->actual, which on a short control OUT transfer is strictly less than len. The copy_to_user() call still copies len bytes, so on a short OUT the last (len - ret) bytes of the kmalloc() buffer -- uninitialised slab residue -- are delivered to the FunctionFS daemon.

Short ep0 OUT completions are specified USB control-transfer behavior and are produced by in-tree UDCs:

* dwc2 continues on req->actual < req->length for ep0 DATA OUT (short-not-ok is the only ep0-OUT stall path).
* aspeed_udc ends ep0 OUT on rx_len < ep->ep.maxpacket.
* renesas_usbf logs "ep0 short packet" and completes the request.
* dwc3 stalls on short IN but not on short OUT.

A short ep0 OUT is therefore not evidence of a broken UDC; it is a normal condition f_fs has to cope with. The sibling gadgetfs implementation in drivers/usb/gadget/legacy/inode.c already does this correctly via min(len, dev->req->actual) before copy_to_user(). This patch brings f_fs.c to the same safe pattern rather than trimming at a defensive layer.

The bug is reached from the FunctionFS device node, which in real deployments is owned by the privileged gadget daemon (adbd, UMS, composite gadget services, etc.); it is not reachable from unprivileged userspace. Linux host stacks normally reject short-wLength control OUTs before they reach the gadget, so reproducing this required a build that bypasses that host-side check. With the bypass in place, a 1-byte payload on a 64-byte Setup produces 63 bytes of non-canary slab residue in the daemon's read buffer.

Fix by copying only ret (actually received) bytes to userspace.

index 002c3441bea3..815639506520 100644
@@ -619,7 +619,7 @@ static ssize_t ffs_ep0_read(struct file *file, char __user *buf,
 
 		/* unlocks spinlock */
 		ret = __ffs_ep0_queue_wait(ffs, data, len);
-		if ((ret > 0) && (copy_to_user(buf, data, len)))
+		if ((ret > 0) && (copy_to_user(buf, data, ret)))
 			ret = -EFAULT;
 		goto done_mutex;