CVE-2022-48822

In the Linux kernel, the following vulnerability has been resolved:

usb: f_fs: Fix use-after-free for epfile

Consider a case where ffs_func_eps_disable is called from

ffs_func_disable as part of composition switch and at the

same time ffs_epfile_release get called from userspace.

ffs_epfile_release will free up the read buffer and call

ffs_data_closed which in turn destroys ffs->epfiles and

mark it as NULL. While this was happening the driver has

already initialized the local epfile in ffs_func_eps_disable

which is now freed and waiting to acquire the spinlock. Once

spinlock is acquired the driver proceeds with the stale value

of epfile and tries to free the already freed read buffer

causing use-after-free.

Following is the illustration of the race:

CPU1 CPU2

ffs_func_eps_disable

epfiles (local copy)

ffs_epfile_release

ffs_data_closed

if (last file closed)

ffs_data_reset

ffs_data_clear

ffs_epfiles_destroy

spin_lock

dereference epfiles

Fix this races by taking epfiles local copy & assigning it under

spinlock and if epfiles(local) is null then update it in ffs->epfiles

then finally destroy it.

Extending the scope further from the race, protecting the ep related

structures, and concurrent accesses.