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

[sashiko-bot@kernel.org]

> 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:
> [ ... ]
> 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@vger.kernel.org
> Signed-off-by: Yu Zhang <yuz08559@gmail.com>

Sashiko has reviewed this patch and found no issues.

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Sashiko AI review · https://sashiko.dev/#/patchset/20261018130519.65850-1-yuz08559@gmail.com?part=1