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Mon, 19 Oct 2026 19:31:02 -0700 (PDT) Received: from EAIT-H54D9Q2FJQ.eait.uq.edu.au ([130.102.10.249]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-89febdc2209sm3958456b3a.33.2026.10.19.19.30.57 (version=TLS1_3 cipher=TLS_CHACHA20_POLY1305_SHA256 bits=256/256); Mon, 19 Oct 2026 19:31:00 -0700 (PDT) From: Yu Zhang To: Greg Kroah-Hartman Cc: Alan Stern , linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org, Yu Zhang , stable@vger.kernel.org Subject: [PATCH v2] usb: gadget: f_mass_storage: take filesem in nofua store to fix backing-file UAF Date: Tue, 20 Oct 2026 12:30:50 +1000 Message-ID: <20261020023050.87701-1-yuz08559@gmail.com> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20261018130519.65850-1-yuz08559@gmail.com> References: <20261018130519.65850-1-yuz08559@gmail.com> X-Mailing-List: linux-usb@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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@vger.kernel.org Signed-off-by: Yu Zhang --- 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@gmail.com/ 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(-) 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