AZL-88935

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

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

fecdecodebufs() 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->blocksize == 4096 && v->fec->roots == 17 && fio->nbufs == 1, for example. In that case, each call to fecdecodebufs() consumes v->fec->roots * (fio->nbufs << DMVERITYFECBUFRSBITS) = 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 fecroots and when the maximum number of buffers couldn't be allocated due to low memory. For example with blocksize=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.