CVE-2021-46972

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

ovl: fix leaked dentry

Since commit 6815f479ca90 (ovl: use only uppermetacopy state in ovl_lookup()), overlayfs doesnt put temporary dentry when there is a metacopy error, which leads to dentry leaks when shutting down the related superblock:

overlayfs: refusing to follow metacopy origin for (/file0) … BUG: Dentry (ptrval){i=3f33,n=file3} still in use (1) [unmount of overlay overlay] … WARNING: CPU: 1 PID: 432 at umount_check.cold+0x107/0x14d CPU: 1 PID: 432 Comm: unmount-overlay Not tainted 5.12.0-rc5 #1 … RIP: 0010:umount_check.cold+0x107/0x14d … Call Trace: d_walk+0x28c/0x950 ? dentry_lru_isolate+0x2b0/0x2b0 ? __kasan_slab_free+0x12/0x20 do_one_tree+0x33/0x60 shrink_dcache_for_umount+0x78/0x1d0 generic_shutdown_super+0x70/0x440 kill_anon_super+0x3e/0x70 deactivate_locked_super+0xc4/0x160 deactivate_super+0xfa/0x140 cleanup_mnt+0x22e/0x370 __cleanup_mnt+0x1a/0x30 task_work_run+0x139/0x210 do_exit+0xb0c/0x2820 ? __kasan_check_read+0x1d/0x30 ? find_held_lock+0x35/0x160 ? lock_release+0x1b6/0x660 ? mm_update_next_owner+0xa20/0xa20 ? reacquire_held_locks+0x3f0/0x3f0 ? __sanitizer_cov_trace_const_cmp4+0x22/0x30 do_group_exit+0x135/0x380 __do_sys_exit_group.isra.0+0x20/0x20 __x64_sys_exit_group+0x3c/0x50 do_syscall_64+0x45/0x70 entry_SYSCALL_64_after_hwframe+0x44/0xae … VFS: Busy inodes after unmount of overlay. Self-destruct in 5 seconds. Have a nice day…

This fix has been tested with a syzkaller reproducer.

Weakness

The product does not sufficiently track and release allocated memory after it has been used, which slowly consumes remaining memory.

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

• https://git.kernel.org/stable/c/71d58457a8afc650da5d3292a7f7029317654d95
• https://git.kernel.org/stable/c/cf3e3330bc5719fa9d658e3e2f596bde89344a94
• https://git.kernel.org/stable/c/d587cfaef72b1b6f4b2774827123bce91f497cc8
• https://git.kernel.org/stable/c/eaab1d45cdb4bb0c846bd23c3d666d5b90af7b41
• https://git.kernel.org/stable/c/71d58457a8afc650da5d3292a7f7029317654d95
• https://git.kernel.org/stable/c/cf3e3330bc5719fa9d658e3e2f596bde89344a94
• https://git.kernel.org/stable/c/d587cfaef72b1b6f4b2774827123bce91f497cc8
• https://git.kernel.org/stable/c/eaab1d45cdb4bb0c846bd23c3d666d5b90af7b41