CVE-2021-47280

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 devices 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 devices 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 devices master mutex before retrieving the pointer from from fpriv->master. This patch passes the Syzbot reproducer test.

Weakness

The product checks the state of a resource before using that resource, but the resource’s state can change between the check and the use in a way that invalidates the results of the check. This can cause the product to perform invalid actions when the resource is in an unexpected state.

Affected Software

Potential Mitigations

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/27.html
• https://cwe.mitre.org/data/definitions/29.html

References

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