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

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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.

Fixes: 6df90c02ba ("dm-verity FEC: Fix RS FEC repair for roots unaligned to block size (take 2)")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
This commit is contained in:
Eric Biggers 2026-02-05 20:59:24 -08:00 committed by Mikulas Patocka
parent a7fca324d7
commit 430a05cb92
1 changed files with 44 additions and 56 deletions
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100
drivers/md/dm-verity-fec.c
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@ -33,36 +33,6 @@ static inline u64 fec_interleave(struct dm_verity *v, u64 offset)
return offset + mod * (v->fec->rounds << v->data_dev_block_bits);
}
/*
* Read error-correcting codes for the requested RS block. Returns a pointer
* to the data block. Caller is responsible for releasing buf.
*/
static u8 *fec_read_parity(struct dm_verity *v, u64 rsb, int index,
unsigned int *offset, unsigned int par_buf_offset,
struct dm_buffer **buf, unsigned short ioprio)
{
u64 position, block, rem;
u8 *res;
/* We have already part of parity bytes read, skip to the next block */
if (par_buf_offset)
index++;
position = (index + rsb) * v->fec->roots;
block = div64_u64_rem(position, v->fec->io_size, &rem);
*offset = par_buf_offset ? 0 : (unsigned int)rem;
res = dm_bufio_read_with_ioprio(v->fec->bufio, block, buf, ioprio);
if (IS_ERR(res)) {
DMERR("%s: FEC %llu: parity read failed (block %llu): %ld",
v->data_dev->name, (unsigned long long)rsb,
(unsigned long long)block, PTR_ERR(res));
*buf = NULL;
}
return res;
}
/* Loop over each allocated buffer. */
#define fec_for_each_buffer(io, __i) \
for (__i = 0; __i < (io)->nbufs; __i++)
@ -102,15 +72,29 @@ static int fec_decode_bufs(struct dm_verity *v, struct dm_verity_io *io,
{
int r, corrected = 0, res;
struct dm_buffer *buf;
unsigned int n, i, j, offset, par_buf_offset = 0;
unsigned int n, i, j, parity_pos, to_copy;
uint16_t par_buf[DM_VERITY_FEC_RSM - DM_VERITY_FEC_MIN_RSN];
u8 *par, *block;
u64 parity_block;
struct bio *bio = dm_bio_from_per_bio_data(io, v->ti->per_io_data_size);
par = fec_read_parity(v, rsb, block_offset, &offset,
par_buf_offset, &buf, bio->bi_ioprio);
if (IS_ERR(par))
/*
* Compute the index of the first parity block that will be needed and
* the starting position in that block. Then read that block.
*
* io_size is always a power of 2, but roots might not be. Note that
* when it's not, a codeword's parity bytes can span a block boundary.
*/
parity_block = (rsb + block_offset) * v->fec->roots;
parity_pos = parity_block & (v->fec->io_size - 1);
parity_block >>= v->data_dev_block_bits;
par = dm_bufio_read_with_ioprio(v->fec->bufio, parity_block, &buf,
bio->bi_ioprio);
if (IS_ERR(par)) {
DMERR("%s: FEC %llu: parity read failed (block %llu): %ld",
v->data_dev->name, rsb, parity_block, PTR_ERR(par));
return PTR_ERR(par);
}
/*
* Decode the RS blocks we have in bufs. Each RS block results in
@ -118,8 +102,32 @@ static int fec_decode_bufs(struct dm_verity *v, struct dm_verity_io *io,
*/
fec_for_each_buffer_rs_block(fio, n, i) {
block = fec_buffer_rs_block(v, fio, n, i);
for (j = 0; j < v->fec->roots - par_buf_offset; j++)
par_buf[par_buf_offset + j] = par[offset + j];
/*
* Copy the next 'roots' parity bytes to 'par_buf', reading
* another parity block if needed.
*/
to_copy = min(v->fec->io_size - parity_pos, v->fec->roots);
for (j = 0; j < to_copy; j++)
par_buf[j] = par[parity_pos++];
if (to_copy < v->fec->roots) {
parity_block++;
parity_pos = 0;
dm_bufio_release(buf);
par = dm_bufio_read_with_ioprio(v->fec->bufio,
parity_block, &buf,
bio->bi_ioprio);
if (IS_ERR(par)) {
DMERR("%s: FEC %llu: parity read failed (block %llu): %ld",
v->data_dev->name, rsb, parity_block,
PTR_ERR(par));
return PTR_ERR(par);
}
for (; j < v->fec->roots; j++)
par_buf[j] = par[parity_pos++];
}
/* Decode an RS block using Reed-Solomon */
res = decode_rs8(fio->rs, block, par_buf, v->fec->rsn,
NULL, neras, fio->erasures, 0, NULL);
@ -134,26 +142,6 @@ static int fec_decode_bufs(struct dm_verity *v, struct dm_verity_io *io,
block_offset++;
if (block_offset >= 1 << v->data_dev_block_bits)
goto done;
/* Read the next block when we run out of parity bytes */
offset += (v->fec->roots - par_buf_offset);
/* Check if parity bytes are split between blocks */
if (offset < v->fec->io_size && (offset + v->fec->roots) > v->fec->io_size) {
par_buf_offset = v->fec->io_size - offset;
for (j = 0; j < par_buf_offset; j++)
par_buf[j] = par[offset + j];
offset += par_buf_offset;
} else
par_buf_offset = 0;
if (offset >= v->fec->io_size) {
dm_bufio_release(buf);
par = fec_read_parity(v, rsb, block_offset, &offset,
par_buf_offset, &buf, bio->bi_ioprio);
if (IS_ERR(par))
return PTR_ERR(par);
}
}
done:
r = corrected;