CVE-2025-21887

Summary

CVE-2025-21887 is a use-after-free vulnerability affecting the Linux kernel's OverlayFS file system. The issue was discovered in the 'ovl_dentry_remote' and 'ovl_dentry_update_reval' functions, where 'dput(upper)' was called before updating 'upper->d_flags'. This sequence led to a use-after-free condition, causing memory corruption and potentially leading to arbitrary code execution or denial of service. The vulnerability was identified using the Kernel Address Sanitizer (KASAN) and was reported in the 'ovl_copy_up_one', 'ovl_copy_up_flags', and 'ovl_rename' functions. The issue has been resolved by moving 'dput(upper)' after its last use in the 'ovl_link_up' function.

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