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Sun, 18 Oct 2026 06:05:27 -0700 (PDT) Received: from EAIT-H54D9Q2FJQ.eait.uq.edu.au ([130.102.10.249]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-3acc49c263asm11415222a91.17.2026.10.18.06.05.24 (version=TLS1_3 cipher=TLS_CHACHA20_POLY1305_SHA256 bits=256/256); Sun, 18 Oct 2026 06:05:26 -0700 (PDT) From: Yu Zhang To: Greg Kroah-Hartman Cc: linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org, Yu Zhang , stable@vger.kernel.org Subject: [PATCH] usb: gadget: f_mass_storage: take filesem in nofua store to fix backing-file UAF Date: Sun, 18 Oct 2026 23:05:19 +1000 Message-ID: <20261018130519.65850-1-yuz08559@gmail.com> X-Mailer: git-send-email 2.55.0 Precedence: bulk X-Mailing-List: linux-usb@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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 Take filesem for read around the fsync in fsg_store_nofua(), as the sibling stores do, so the backing file cannot be closed underneath 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 --- drivers/usb/gadget/function/f_mass_storage.c | 6 ++++-- drivers/usb/gadget/function/storage_common.c | 5 ++++- drivers/usb/gadget/function/storage_common.h | 3 ++- 3 files changed, 12 insertions(+), 4 deletions(-) diff --git a/drivers/usb/gadget/function/f_mass_storage.c b/drivers/usb/gadget/function/f_mass_storage.c index 0000001..0000002 100644 --- a/drivers/usb/gadget/function/f_mass_storage.c +++ b/drivers/usb/gadget/function/f_mass_storage.c @@ -2663,8 +2663,9 @@ 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_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 0000003..0000004 100644 --- a/drivers/usb/gadget/function/storage_common.c +++ b/drivers/usb/gadget/function/storage_common.c @@ -415,7 +415,8 @@ } 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; @@ -424,11 +425,13 @@ if (ret) return ret; + down_read(filesem); /* Sync data when switching from async mode to sync */ if (!nofua && curlun->nofua) fsg_lun_fsync_sub(curlun); curlun->nofua = nofua; + up_read(filesem); return count; } diff --git a/drivers/usb/gadget/function/storage_common.h b/drivers/usb/gadget/function/storage_common.h index 0000005..0000006 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_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,