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Message-ID: <20260206045942.52965-6-ebiggers@kernel.org>
Date: Thu, 5 Feb 2026 20:59:24 -0800
From: Eric Biggers <ebiggers@...nel.org>
To: dm-devel@...ts.linux.dev,
Alasdair Kergon <agk@...hat.com>,
Mike Snitzer <snitzer@...nel.org>,
Mikulas Patocka <mpatocka@...hat.com>,
Benjamin Marzinski <bmarzins@...hat.com>
Cc: Sami Tolvanen <samitolvanen@...gle.com>,
linux-kernel@...r.kernel.org,
Eric Biggers <ebiggers@...nel.org>,
stable@...r.kernel.org
Subject: [PATCH 05/22] 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.
Fixes: 6df90c02bae4 ("dm-verity FEC: Fix RS FEC repair for roots unaligned to block size (take 2)")
Cc: stable@...r.kernel.org
Signed-off-by: Eric Biggers <ebiggers@...nel.org>
---
drivers/md/dm-verity-fec.c | 100 ++++++++++++++++---------------------
1 file changed, 44 insertions(+), 56 deletions(-)
diff --git a/drivers/md/dm-verity-fec.c b/drivers/md/dm-verity-fec.c
index d4a9367a2fee6..fcfacaec2989a 100644
--- a/drivers/md/dm-verity-fec.c
+++ b/drivers/md/dm-verity-fec.c
@@ -31,40 +31,10 @@ static inline u64 fec_interleave(struct dm_verity *v, u64 offset)
mod = do_div(offset, v->fec->rsn);
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++)
/* Loop over each RS block in each allocated buffer. */
@@ -100,28 +70,66 @@ static int fec_decode_bufs(struct dm_verity *v, struct dm_verity_io *io,
struct dm_verity_fec_io *fio, u64 rsb, int byte_index,
unsigned int block_offset, int neras)
{
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
* one corrected target byte and consumes fec->roots parity bytes.
*/
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);
if (res < 0) {
r = res;
@@ -132,30 +140,10 @@ static int fec_decode_bufs(struct dm_verity *v, struct dm_verity_io *io,
fio->output[block_offset] = block[byte_index];
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;
error:
dm_bufio_release(buf);
--
2.52.0
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