Linux EUVD-2026-39339

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

netfilter: nft_fib: fix stale stack leak via the OIFNAME register

For NFT_FIB_RESULT_OIFNAME the destination register is declared with len = IFNAMSIZ (four 32-bit registers), but on the lookup-fail, RTN_LOCAL and oif-mismatch paths nft_fib{4,6}_eval() only writes one register via "*dest = 0". The remaining three registers are left as whatever was on the stack in nft_do_chain()'s struct nft_regs, and a downstream expression that loads the register span can leak that uninitialised kernel stack to userspace.

The NFTA_FIB_F_PRESENT existence check has the same shape: it is only meaningful for NFT_FIB_RESULT_OIF, yet it was accepted for any result type while the eval stores a single byte via nft_reg_store8(), leaving the rest of the declared span stale.

Fix both:

• replace the bare "*dest = 0" in the eval with nft_fib_store_result(),

which strscpy_pad()s the whole IFNAMSIZ for OIFNAME (and is already used on the other early-return path), and

• restrict NFTA_FIB_F_PRESENT to NFT_FIB_RESULT_OIF and declare its

destination as a single u8, so the marked span matches the one byte the eval writes.