xsk: cache csum_start/csum_offset to fix TOCTOU in xsk_skb_metadata()

Overview

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

xsk: cache csum_start/csum_offset to fix TOCTOU in xsk_skb_metadata()

The TX metadata area resides in the UMEM buffer which is memory-mapped and concurrently writable by userspace. In xsk_skb_metadata(), csum_start and csum_offset are read from shared memory for bounds validation, then read again for skb assignment. A malicious userspace application can race to overwrite these values between the two reads, bypassing the bounds check and causing out-of-bounds memory access during checksum computation in the transmit path.

Fix this by reading csum_start and csum_offset into local variables once, then using the local copies for both validation and assignment.

Note that other metadata fields (flags, launch_time) and the cached csum fields may be mutually inconsistent due to concurrent userspace writes, but this is benign: the only security-critical invariant is that each field's validated value is the same one used, which local caching guarantees.

Affected

• Linux >= 48eb03dd26304c24f03bdbb9382e89c8564e71df < eb778aed8a9b2381668fcb1247ae4fa011224768
• Linux >= 48eb03dd26304c24f03bdbb9382e89c8564e71df < 0dfe05b938435892875e07771170051346412df9
• Linux >= 48eb03dd26304c24f03bdbb9382e89c8564e71df < bfdfd2706d5fb2cd496a1506e680daf979309c8b
• Linux >= 48eb03dd26304c24f03bdbb9382e89c8564e71df < 22ba97ea9cc1f63a0d0244fae38057ed452b6ac7
• Linux 6.8

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.