[PATCH AUTOSEL 6.19-6.12] usb: gadget: f_fs: Fix ioctl error handling

Sasha Levin
2026-02-19T02:04:51Z

From: Sam Day <me@samcday.com>

[ Upstream commit 8e4c1d06183c25022f6b0002a5cab84979ca6337 ]

When ffs_epfile_ioctl handles FUNCTIONFS_DMABUF_* ioctls, it's currently
falling through when copy_from_user fails.

However, this fallthrough isn't being checked properly, so the handler
continues executing further than it should. It then tries the secondary
dispatch where it ultimately gives up and returns -ENOTTY.

The end result is invalid ioctl invocations will yield a -ENOTTY rather
than an -EFAULT.

It's a common pattern elsewhere in the kernel code to directly return
-EFAULT when copy_from_user fails. So we update ffs_epfile_ioctl to do
the same and fix this issue.

Signed-off-by: Sam Day <me@samcday.com>
Link: https://patch.msgid.link/20260108-ffs-dmabuf-ioctl-fix-v1-1-e51633891a81@samcday.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

LLM Generated explanations, may be completely bogus:

Good. So 6.12.y exists and would contain this DMABUF code (introduced in
6.9).

### Stable Criteria Assessment

1. **Obviously correct and tested**: The fix is trivially correct —
returning `-EFAULT` immediately on `copy_from_user` failure is the
standard kernel pattern. The commit was accepted by Greg KH (USB
maintainer and stable tree maintainer).

2. **Fixes a real bug**: Yes — wrong error code returned to userspace
and unnecessary blocking call on error path. This affects userspace
programs that depend on correct error codes for their error handling.

3. **Small and contained**: Yes — only 3 error paths changed in a single
function, net -6 lines. No structural changes.

4. **No new features**: Correct — purely fixes error handling.

5. **Risk assessment**: Extremely low risk. The change only affects
error paths (when `copy_from_user` fails). The non-error paths
(successful `copy_from_user`) are completely unchanged.

### Additional Concerns

- **Unnecessary blocking**: The current buggy code causes the function
to call `ffs_epfile_wait_ep()` when `copy_from_user` fails. This is a
wait function that could block, causing unnecessary delays for the
failing ioctl. The fix eliminates this.
- **Dependencies**: No dependencies on other patches needed. The fix is
self-contained.
- **Affected versions**: Only kernels v6.9+ that contain the DMABUF
ioctl support.

### Verification

- **git log** confirmed that DMABUF ioctls were introduced in commit
`7b07a2a7ca02a` (v6.9)
- **Read of f_fs.c lines 1730-1835** confirmed the bug mechanism: after
`break` from first switch, code falls through to
`ffs_epfile_wait_ep()` then second switch where default case returns
`-ENOTTY`, overwriting `-EFAULT`
- **git tag** confirmed the code exists in stable tree 6.12.y (which
includes all code from v6.9+)
- **Commit message** clearly explains the bug and fix mechanism
- **Signed-off-by Greg KH** confirms the USB maintainer accepted this
fix
- Could NOT independently verify whether userspace programs have been
affected (unverified, but the wrong error code is a definite API
contract violation)

### Summary

This is a clear, small, correct bug fix for error handling in the USB
gadget FunctionFS DMABUF ioctl handler. The bug causes:
1. Wrong error code (`-ENOTTY` instead of `-EFAULT`) returned to
userspace
2. Unnecessary blocking in `ffs_epfile_wait_ep()` on error paths

The fix is minimal (3 error paths changed to return directly), obviously
correct (follows standard kernel patterns), has zero risk to non-error
paths, and was accepted by Greg KH. It meets all stable kernel criteria.

**YES**

drivers/usb/gadget/function/f_fs.c | 18 ++++++------------
1 file changed, 6 insertions(+), 12 deletions(-)

diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
index 928f51fddc64e..e75d5d8b5ac91 100644
--- a/drivers/usb/gadget/function/f_fs.c
+++ b/drivers/usb/gadget/function/f_fs.c
@@ -1744,10 +1744,8 @@ static long ffs_epfile_ioctl(struct file *file, unsigned code,
{
int fd;

- if (copy_from_user(&fd, (void __user *)value, sizeof(fd))) {
- ret = -EFAULT;
- break;
- }
+ if (copy_from_user(&fd, (void __user *)value, sizeof(fd)))
+ return -EFAULT;

return ffs_dmabuf_attach(file, fd);
}
@@ -1755,10 +1753,8 @@ static long ffs_epfile_ioctl(struct file *file, unsigned code,
{
int fd;

- if (copy_from_user(&fd, (void __user *)value, sizeof(fd))) {
- ret = -EFAULT;
- break;
- }
+ if (copy_from_user(&fd, (void __user *)value, sizeof(fd)))
+ return -EFAULT;

return ffs_dmabuf_detach(file, fd);
}
@@ -1766,10 +1762,8 @@ static long ffs_epfile_ioctl(struct file *file, unsigned code,
{
struct usb_ffs_dmabuf_transfer_req req;

- if (copy_from_user(&req, (void __user *)value, sizeof(req))) {
- ret = -EFAULT;
- break;
- }
+ if (copy_from_user(&req, (void __user *)value, sizeof(req)))
+ return -EFAULT;

return ffs_dmabuf_transfer(file, &req);
}
--
2.51.0