CVE-2025-21800: 
Linux Debian vulnerability analysis and mitigation

CVE-2025-21800 affects the Linux kernel's net/mlx5 Hardware Steering (HWS) component, specifically related to the HWSSET32 macro handling. The vulnerability was discovered on February 27, 2025, and involves a shift-out-of-bounds issue when dealing with negative bit offsets in the definer's HWSSET32 macro (NVD).

The vulnerability occurs in the drivers/net/ethernet/mellanox/mlx5/core/steering/hws/definer.c file when the bit offset for HWS_SET32 macro is negative. This triggers an UBSAN (Undefined Behavior Sanitizer) complaint about shift-out-of-bounds at line 177:2, specifically when the shift exponent is -8. The issue was introduced in commit 74a778b4a63f which added definers handling functionality (Kernel Commit).

The vulnerability could potentially lead to undefined behavior in the Linux kernel's network stack, specifically affecting the Mellanox MLX5 driver's hardware steering functionality. However, the exact impact severity has not been fully assessed as the CVSS score was not yet assigned at the time of disclosure (NVD).

A fix has been implemented by modifying the bit offset calculation in the HWSSET32 macro. The correction changes the expression from '(bitoff) % BITSINDW' to '(bitoff + BITSINDW) % BITSIN_DW' to properly handle negative offsets. This fix has been merged into the Linux kernel (Kernel Commit).