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.

Weakness

Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code.

Affected Software

Extended Description

The use of previously-freed memory can have any number of adverse consequences, ranging from the corruption of valid data to the execution of arbitrary code, depending on the instantiation and timing of the flaw. The simplest way data corruption may occur involves the system’s reuse of the freed memory. Use-after-free errors have two common and sometimes overlapping causes:

In this scenario, the memory in question is allocated to another pointer validly at some point after it has been freed. The original pointer to the freed memory is used again and points to somewhere within the new allocation. As the data is changed, it corrupts the validly used memory; this induces undefined behavior in the process. If the newly allocated data happens to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.

Potential Mitigations

References

• https://git.kernel.org/stable/c/0042178a69eb77a979e36a50dcce9794a3140ef8
• https://git.kernel.org/stable/c/32048f4be071f9a6966744243f1786f45bb22dc2
• https://git.kernel.org/stable/c/3e078b18753669615301d946297bafd69294ad2c
• https://git.kernel.org/stable/c/72a8aee863af099d4434314c4536d6c9a61dcf3c
• https://git.kernel.org/stable/c/c9fc422c9a43e3d58d246334a71f3390401781dc
• https://git.kernel.org/stable/c/cfe5f6fd335d882bcc829a1c8a7d462a455c626e
• https://git.kernel.org/stable/c/ebe2b1add1055b903e2acd86b290a85297edc0b3
• https://git.kernel.org/stable/c/0042178a69eb77a979e36a50dcce9794a3140ef8
• https://git.kernel.org/stable/c/32048f4be071f9a6966744243f1786f45bb22dc2
• https://git.kernel.org/stable/c/3e078b18753669615301d946297bafd69294ad2c
• https://git.kernel.org/stable/c/72a8aee863af099d4434314c4536d6c9a61dcf3c
• https://git.kernel.org/stable/c/c9fc422c9a43e3d58d246334a71f3390401781dc
• https://git.kernel.org/stable/c/cfe5f6fd335d882bcc829a1c8a7d462a455c626e
• https://git.kernel.org/stable/c/ebe2b1add1055b903e2acd86b290a85297edc0b3