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

[Yu Zhang <yuz08559@gmail.com>]

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 <yuz08559@gmail.com>
---
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