CVE-2026-46130dm-verity-fec: fix reading parity bytes split across blocks (take 3)

Official description

In the Linux kernel, the following vulnerability has been resolved: dm-verity-fec: fix reading parity bytes split across blocks (take 3) fec_decode_bufs() assumes that the parity bytes of the first RS codeword it decodes are never split across parity blocks. This assumption is false. Consider v->fec->block_size == 4096 && v->fec->roots == 17 && fio->nbufs == 1, for example. In that case, each call to fec_decode_bufs() consumes v->fec->roots * (fio->nbufs << DM_VERITY_FEC_BUF_RS_BITS) = 272 parity bytes. Considering that the parity data for each message block starts on a block boundary, the byte alignment in the parity data will iterate through 272*i mod 4096 until the 3 parity blocks have been consumed. On the 16th call (i=15), the alignment will be 4080 bytes into the first block. Only 16 bytes remain in that block, but 17 parity bytes will be needed. The code reads out-of-bounds from the parity block buffer. Fortunately this doesn't normally happen, since it can occur only for certain non-default values of fec_roots *and* when the maximum number of buffers couldn't be allocated due to low memory. For example with block_size=4096 only the following cases are affected: fec_roots=17: nbufs in [1, 3, 5, 15] fec_roots=19: nbufs in [1, 229] fec_roots=21: nbufs in [1, 3, 5, 13, 15, 39, 65, 195] fec_roots=23: nbufs in [1, 89] Regardless, fix it by refactoring how the parity blocks are read.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/03348b440f69417dc3a3c71d8271cff6b05f67fb
• https://git.kernel.org/stable/c/dac69d1b76049d7e5f565089a6a7f4502a85b9de
• https://git.kernel.org/stable/c/d47281b9a4472cfd73122393e79fbe76b651e46a
• https://git.kernel.org/stable/c/3d1b4e2d8ac0a1a1390a117f61ce0ca1c47e3bcb
• https://git.kernel.org/stable/c/430a05cb926f6bdf53e81460a2c3a553257f3f61