Re: [PATCH v2] usb: gadget: f_mass_storage: take filesem in nofua store to fix backing-file UAF

[Alan Stern]

On Tue, Oct 20, 2026 at 12:30:50PM +1000, Yu Zhang wrote:
> fsg_store_nofua() syncs the LUN's backing file when switching from async
> to sync mode:
> 
> 	if (!nofua && curlun->nofua)
> 		fsg_lun_fsync_sub(curlun);
> 
> fsg_lun_fsync_sub() dereferences curlun->filp (vfs_fsync(filp)). Unlike
> every other store that touches the backing file -- fsg_store_ro() and
> fsg_store_cdrom() (down_read), fsg_store_file() (down_write) --
> fsg_store_nofua() holds no filesem while doing so.
> 
> A concurrent close of the backing file runs under down_write(filesem) and
> does fput(curlun->filp) in fsg_lun_close() (fsg_store_file() with an empty
> path, the SCSI START_STOP eject in do_start_stop(), or forced_eject), so
> it can free the file while fsg_store_nofua() is dereferencing it:
> 
> 	BUG: KASAN: slab-use-after-free in vfs_fsync+0x20/0xe0
> 	 vfs_fsync+0x20/0xe0
> 	 fsg_lun_fsync_sub+0xa0/0xd0
> 	 fsg_store_nofua+0x138/0x160
> 	 configfs_write_iter+0x1a7/0x250
> 	 vfs_write+0x5df/0x940
> 	Allocated by task 3459:
> 	 fsg_store_file+0x150/0x1f0
> 	 configfs_write_iter+0x1a7/0x250
> 	Freed by task 3459:
> 	 __fput+0x456/0x6c0
> 	 __fput_deferred+0x10d/0x330
> 	 fsg_store_file+0x9e/0x1f0
> 	 configfs_write_iter+0x1a7/0x250
> 
> Hold filesem for read around the fsync in fsg_store_nofua() so the backing
> file cannot be closed underneath it. The lock is only needed when the
> fsync actually runs, so it is taken only in that branch.
> 
> Reproduced on a mass_storage gadget over dummy_hcd by writing lun.N/nofua
> (1 then 0) from one task while another task rewrites and clears lun.N/file.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Yu Zhang <yuz08559@xxxxxxxxx>
> ---
> v2:
>  - Use guard(rwsem_read) instead of a bare down_read()/up_read() pair
>    (Greg KH).
>  - Take filesem only inside the branch that actually calls
>    fsg_lun_fsync_sub(), not around the whole function / the
>    curlun->nofua assignment, since the lock is only needed for the sync
>    (Alan Stern).
> 
> v1: https://lore.kernel.org/all/20261018130519.65850-1-yuz08559@xxxxxxxxx/
> 
>  drivers/usb/gadget/function/f_mass_storage.c | 8 ++++++--
>  drivers/usb/gadget/function/storage_common.c | 7 +++++--
>  drivers/usb/gadget/function/storage_common.h | 3 ++-
>  3 files changed, 13 insertions(+), 5 deletions(-)

Acked-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>

> 
> diff --git a/drivers/usb/gadget/function/f_mass_storage.c b/drivers/usb/gadget/function/f_mass_storage.c
> index b7b06cb..6712566 100644
> --- a/drivers/usb/gadget/function/f_mass_storage.c
> +++ b/drivers/usb/gadget/function/f_mass_storage.c
> @@ -2663,8 +2663,9 @@ static ssize_t nofua_store(struct device *dev, struct device_attribute *attr,
>  			   const char *buf, size_t count)
>  {
>  	struct fsg_lun		*curlun = fsg_lun_from_dev(dev);
> +	struct rw_semaphore	*filesem = dev_get_drvdata(dev);
>  
> -	return fsg_store_nofua(curlun, buf, count);
> +	return fsg_store_nofua(curlun, filesem, buf, count);
>  }
>  
>  static ssize_t file_store(struct device *dev, struct device_attribute *attr,
> @@ -3239,7 +3240,10 @@ static ssize_t fsg_lun_opts_nofua_show(struct config_item *item, char *page)
>  static ssize_t fsg_lun_opts_nofua_store(struct config_item *item,
>  				       const char *page, size_t len)
>  {
> -	return fsg_store_nofua(to_fsg_lun_opts(item)->lun, page, len);
> +	struct fsg_lun_opts *opts = to_fsg_lun_opts(item);
> +	struct fsg_opts *fsg_opts = to_fsg_opts(opts->group.cg_item.ci_parent);
> +
> +	return fsg_store_nofua(opts->lun, &fsg_opts->common->filesem, page, len);
>  }
>  
>  CONFIGFS_ATTR(fsg_lun_opts_, nofua);
> diff --git a/drivers/usb/gadget/function/storage_common.c b/drivers/usb/gadget/function/storage_common.c
> index 75831f2..51a70f2 100644
> --- a/drivers/usb/gadget/function/storage_common.c
> +++ b/drivers/usb/gadget/function/storage_common.c
> @@ -415,7 +415,8 @@ ssize_t fsg_store_ro(struct fsg_lun *curlun, struct rw_semaphore *filesem,
>  }
>  EXPORT_SYMBOL_GPL(fsg_store_ro);
>  
> -ssize_t fsg_store_nofua(struct fsg_lun *curlun, const char *buf, size_t count)
> +ssize_t fsg_store_nofua(struct fsg_lun *curlun, struct rw_semaphore *filesem,
> +			const char *buf, size_t count)
>  {
>  	bool		nofua;
>  	int		ret;
> @@ -425,8 +426,10 @@ ssize_t fsg_store_nofua(struct fsg_lun *curlun, const char *buf, size_t count)
>  		return ret;
>  
>  	/* Sync data when switching from async mode to sync */
> -	if (!nofua && curlun->nofua)
> +	if (!nofua && curlun->nofua) {
> +		guard(rwsem_read)(filesem);
>  		fsg_lun_fsync_sub(curlun);
> +	}
>  
>  	curlun->nofua = nofua;
>  
> diff --git a/drivers/usb/gadget/function/storage_common.h b/drivers/usb/gadget/function/storage_common.h
> index 11ac785..347a86b 100644
> --- a/drivers/usb/gadget/function/storage_common.h
> +++ b/drivers/usb/gadget/function/storage_common.h
> @@ -210,7 +210,8 @@ ssize_t fsg_show_cdrom(struct fsg_lun *curlun, char *buf);
>  ssize_t fsg_show_removable(struct fsg_lun *curlun, char *buf);
>  ssize_t fsg_store_ro(struct fsg_lun *curlun, struct rw_semaphore *filesem,
>  		     const char *buf, size_t count);
> -ssize_t fsg_store_nofua(struct fsg_lun *curlun, const char *buf, size_t count);
> +ssize_t fsg_store_nofua(struct fsg_lun *curlun, struct rw_semaphore *filesem,
> +			const char *buf, size_t count);
>  ssize_t fsg_store_file(struct fsg_lun *curlun, struct rw_semaphore *filesem,
>  		       const char *buf, size_t count);
>  ssize_t fsg_store_cdrom(struct fsg_lun *curlun, struct rw_semaphore *filesem,
> -- 
> 2.43.0
>