drm: Fix use-after-free read in drm_getunique()

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: drm: Fix use-after-free read in drm_getunique() There is a time-of-check-to-time-of-use error in drm_getunique() due to retrieving file_priv->master prior to locking the device's master mutex. An example can be seen in the crash report of the use-after-free error found by Syzbot: https://syzkaller.appspot.com/bug?id=148d2f1dfac64af52ffd27b661981a540724f803 In the report, the master pointer was used after being freed. This is because another process had acquired the device's master mutex in drm_setmaster_ioctl(), then overwrote fpriv->master in drm_new_set_master(). The old value of fpriv->master was subsequently freed before the mutex was unlocked. To fix this, we lock the device's master mutex before retrieving the pointer from from fpriv->master. This patch passes the Syzbot reproducer test.

References

• https://git.kernel.org/stable/c/17dab9326ff263c62dab1dbac4492e2938a049e4
• https://git.kernel.org/stable/c/7d233ba700ceb593905ea82b42dadb4ec8ef85e9
• https://git.kernel.org/stable/c/b246b4c70c1250e7814f409b243000f9c0bf79a3
• https://git.kernel.org/stable/c/491d52e0078860b33b6c14f0a7ac74ca1b603bd6
• https://git.kernel.org/stable/c/f773f8cccac13c7e7bbd9182e7996c727742488e
• https://git.kernel.org/stable/c/b436acd1cf7fac0ba987abd22955d98025c80c2b