cache_seg_id_valid(9)

💅 cache_seg_id_valid(9)

okay so this is a page from the Kernel API Manual (section 9, kernel API, iykyk) and honestly? it's a little baddie of a function whose whole job is a vibe check on a cache segment id. 🎀

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✨ NAME ✨

cache_seg_id_valid — bestie, this validates a cache segment id that was read from the cache device. that's it. that's the whole job. 👑

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🦋 SYNOPSIS 🦋

bool cache_seg_id_valid (struct pcache_cache *cache , u32 cache_seg_id
);

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🌸 ARGUMENTS 🌸

• cache — Pointer to the pcache_cache structure. literally the whole cache, handing itself over so we can peek at its segment table. 🫶
• cache_seg_id — the segment id, decoded from on-media metadata. yeah. it came from the disk. we do not trust it. 🙈

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🔥 DESCRIPTION 🔥

okay so here's the tea: on-media segment ids are only protected by a CRC. that's it. one lil CRC. and the way that an attacker who can format the cache device can just… compute that CRC over whatever value they feel like picking. 😭 so it's giving "not a real security boundary," fr fr.

so the move is: reject any id that would index cache->segments[] out of bounds before it gets dereferenced. do the bounds check first, dereference later. no cap. that's the entire assignment and it ate. 💅

like, do NOT skip the check. out-of-bounds index becoming a dereference is a whole situation and nobody wants that on their conscience. 💀

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👑 SEE ALSO 👑

Kernel file ./drivers/md/dm-pcache/cache.h — plus the whole friend group:

• cache_seg_is_ctrl_seg(9)
• cache_key_cutfront(9)
• cache_key_cutback(9)
• cache_key_data_crc(9)
• kset_onmedia_valid(9)
• cache_seg_remain(9)
• cache_key_invalid(9)
• cache_key_lstart(9)
• cache_key_lend(9)
• cache_pos_onmedia_crc(9)

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⭐ that's the whole page, bestie — brought to you by ./drivers/md/dm-pcache, dated October 2026, and yes the section 9 is exactly as iconic as it sounds. go validate some ids responsibly 💜🌈