CVE-2021-3492

Shiftfs, an out-of-tree stacking file system included in Ubuntu Linux kernels, did not properly handle faults occurring during copy_from_user() correctly. These could lead to either a double-free situation or memory not being freed at all. An attacker could use this to cause a denial of service (kernel memory exhaustion) or gain privileges via executing arbitrary code. AKA ZDI-CAN-13562.

Weakness

The product does not sufficiently track and release allocated memory after it has been used, making the memory unavailable for reallocation and reuse.

Affected Software

Potential Mitigations

• Choose a language or tool that provides automatic memory management, or makes manual memory management less error-prone.
• For example, glibc in Linux provides protection against free of invalid pointers.
• When using Xcode to target OS X or iOS, enable automatic reference counting (ARC) [REF-391].
• To help correctly and consistently manage memory when programming in C++, consider using a smart pointer class such as std::auto_ptr (defined by ISO/IEC ISO/IEC 14882:2003), std::shared_ptr and std::unique_ptr (specified by an upcoming revision of the C++ standard, informally referred to as C++ 1x), or equivalent solutions such as Boost.

References

• http://packetstormsecurity.com/files/162614/Kernel-Live-Patch-Security-Notice-LSN-0077-1.html
• https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/focal/commit/?id=25c891a949bf918b59cbc6e4932015ba4c35c333
• https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/focal/commit/?id=8fee52ab9da87d82bc6de9ebb3480fff9b4d53e6
• https://ubuntu.com/security/notices/USN-4917-1
• https://www.openwall.com/lists/oss-security/2021/04/16/2
• https://www.zerodayinitiative.com/advisories/ZDI-21-422/
• http://packetstormsecurity.com/files/162614/Kernel-Live-Patch-Security-Notice-LSN-0077-1.html
• https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/focal/commit/?id=25c891a949bf918b59cbc6e4932015ba4c35c333
• https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/focal/commit/?id=8fee52ab9da87d82bc6de9ebb3480fff9b4d53e6
• https://ubuntu.com/security/notices/USN-4917-1
• https://www.openwall.com/lists/oss-security/2021/04/16/2
• https://www.zerodayinitiative.com/advisories/ZDI-21-422/