dm-crypt: fix a tiny race condition in crypt_dec_pending

Overview

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

dm-crypt: fix a tiny race condition in crypt_dec_pending

crypt_dec_pending reads io->error before calling atomic_dec_and_test. Another context, for example crypt_endio called from an interrupt, may set io->error and drop its reference between the read and the decrement. crypt_dec_pending then drops the last reference and completes the bio with the stale status - so a read that failed and was never decrypted, or a write that failed, is reported as successful.

The read was placed before the decrement by commit b35f8caa0890 ("dm crypt: wait for endio to complete before destruction"), because that commit freed dm_crypt_io before calling bio_endio. This is no longer the case, dm_crypt_io lives in the per-bio data now.

Read io->error after atomic_dec_and_test instead. atomic_dec_and_test is fully ordered, so no additional barrier is needed.

Affected

• Linux >= b35f8caa0890169000fec22902290d9a15274cbd < ac5d887ab93592fc5e3605fe30c1d8d232d609b3
• Linux >= b35f8caa0890169000fec22902290d9a15274cbd < 4f16bb982b27b9f5a15301f622c318001fe521f8
• Linux >= b35f8caa0890169000fec22902290d9a15274cbd < 148845aa1921d95ef5dc851c76e6a284f6657df6
• Linux 9c8f3fcd3ffd3283cc0971c8919058356f20a963
• Linux >= 2.6.28.9 < 2.6.29
• Linux 2.6.29

Remediation

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