[PATCH 6.18 0/4] dm-verity fixes

[PATCH 6.18 0/4] dm-verity fixes

[Eric Biggers]

This series backports some dm-verity fixes to 6.18 that failed to apply
earlier.

The lines added/removed are the same as the upstream commits.  The
conflicts were just in the context.

Eric Biggers (3):
  dm-verity-fec: fix the size of dm_verity_fec_io::erasures
  dm-verity-fec: fix reading parity bytes split across blocks (take 3)
  dm-verity-fec: replace {MAX,MIN}_RSN with {MIN,MAX}_ROOTS

Mikulas Patocka (1):
  dm-verity: fix buffer overflow in FEC calculation

 drivers/md/dm-verity-fec.c | 110 +++++++++++++++++--------------------
 drivers/md/dm-verity-fec.h |   6 +-
 2 files changed, 52 insertions(+), 64 deletions(-)


base-commit: 221fc2f4d0eda59d02af2e751a9282fa013a8e97
-- 
2.55.0

[PATCH 6.18 1/4] dm-verity-fec: fix the size of dm_verity_fec_io::erasures

[Eric Biggers]

commit a7fca324d7d90f7b139d4d32747c83a629fdb446 upstream.

At most 25 entries in dm_verity_fec_io::erasures are used: the maximum
number of FEC roots plus one.  Therefore, set the array size
accordingly.  This reduces the size of dm_verity_fec_io by 912 bytes.

Note: a later commit introduces a constant DM_VERITY_FEC_MAX_ROOTS,
which allows the size to be more clearly expressed as
DM_VERITY_FEC_MAX_ROOTS + 1.  This commit just fixes the size first.

Fixes: a739ff3f543a ("dm verity: add support for forward error correction")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
---
 drivers/md/dm-verity-fec.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/md/dm-verity-fec.h b/drivers/md/dm-verity-fec.h
index ec37e607cb3f0..90a0af3f35d31 100644
--- a/drivers/md/dm-verity-fec.h
+++ b/drivers/md/dm-verity-fec.h
@@ -50,7 +50,8 @@ struct dm_verity_fec {
 /* per-bio data */
 struct dm_verity_fec_io {
 	struct rs_control *rs;	/* Reed-Solomon state */
-	int erasures[DM_VERITY_FEC_MAX_RSN];	/* erasures for decode_rs8 */
+	/* erasures for decode_rs8 */
+	int erasures[DM_VERITY_FEC_RSM - DM_VERITY_FEC_MIN_RSN + 1];
 	u8 *bufs[DM_VERITY_FEC_BUF_MAX];	/* bufs for deinterleaving */
 	unsigned int nbufs;		/* number of buffers allocated */
 	u8 *output;		/* buffer for corrected output */
-- 
2.55.0

[PATCH 6.18 2/4] dm-verity-fec: fix reading parity bytes split across blocks (take 3)

[Eric Biggers]

commit 430a05cb926f6bdf53e81460a2c3a553257f3f61 upstream.

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@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
---
 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 03e59b85132aa..a8fbf97b97517 100644
--- a/drivers/md/dm-verity-fec.c
+++ b/drivers/md/dm-verity-fec.c
@@ -39,36 +39,6 @@ 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 preallocated buffer slot. */
 #define fec_for_each_prealloc_buffer(__i) \
 	for (__i = 0; __i < DM_VERITY_FEC_BUF_PREALLOC; __i++)
@@ -116,15 +86,29 @@ 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
@@ -132,8 +116,32 @@ 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);
@@ -148,26 +156,6 @@ 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;
-- 
2.55.0

[PATCH 6.18 3/4] dm-verity-fec: replace {MAX,MIN}_RSN with {MIN,MAX}_ROOTS

[Eric Biggers]

commit 82fbd6a3e29a329d439690cd7ccc4162c9cd8db6 upstream.

Every time DM_VERITY_FEC_{MAX,MIN}_RSN are used, they are subtracted
from DM_VERITY_FEC_RSM to get the bounds on the number of roots.
Therefore, replace these with {MIN,MAX}_ROOTS constants which are more
directly useful.  (Note the inversion, where MAX_RSN maps to MIN_ROOTS
and MIN_RSN maps to MAX_ROOTS.)  No functional change.

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
---
 drivers/md/dm-verity-fec.c | 6 +++---
 drivers/md/dm-verity-fec.h | 7 +++----
 2 files changed, 6 insertions(+), 7 deletions(-)

diff --git a/drivers/md/dm-verity-fec.c b/drivers/md/dm-verity-fec.c
index a8fbf97b97517..cebcc8fd25d70 100644
--- a/drivers/md/dm-verity-fec.c
+++ b/drivers/md/dm-verity-fec.c
@@ -87,7 +87,7 @@ 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, parity_pos, to_copy;
-	uint16_t par_buf[DM_VERITY_FEC_RSM - DM_VERITY_FEC_MIN_RSN];
+	uint16_t par_buf[DM_VERITY_FEC_MAX_ROOTS];
 	u8 *par, *block;
 	u64 parity_block;
 	struct bio *bio = dm_bio_from_per_bio_data(io, v->ti->per_io_data_size);
@@ -605,8 +605,8 @@ int verity_fec_parse_opt_args(struct dm_arg_set *as, struct dm_verity *v,
 
 	} else if (!strcasecmp(arg_name, DM_VERITY_OPT_FEC_ROOTS)) {
 		if (sscanf(arg_value, "%hhu%c", &num_c, &dummy) != 1 || !num_c ||
-		    num_c < (DM_VERITY_FEC_RSM - DM_VERITY_FEC_MAX_RSN) ||
-		    num_c > (DM_VERITY_FEC_RSM - DM_VERITY_FEC_MIN_RSN)) {
+		    num_c < DM_VERITY_FEC_MIN_ROOTS ||
+		    num_c > DM_VERITY_FEC_MAX_ROOTS) {
 			ti->error = "Invalid " DM_VERITY_OPT_FEC_ROOTS;
 			return -EINVAL;
 		}
diff --git a/drivers/md/dm-verity-fec.h b/drivers/md/dm-verity-fec.h
index 90a0af3f35d31..b3460103e0e10 100644
--- a/drivers/md/dm-verity-fec.h
+++ b/drivers/md/dm-verity-fec.h
@@ -13,8 +13,8 @@
 
 /* Reed-Solomon(M, N) parameters */
 #define DM_VERITY_FEC_RSM		255
-#define DM_VERITY_FEC_MAX_RSN		253
-#define DM_VERITY_FEC_MIN_RSN		231	/* ~10% space overhead */
+#define DM_VERITY_FEC_MIN_ROOTS	2	/* RS(255, 253): ~0.8% space overhead */
+#define DM_VERITY_FEC_MAX_ROOTS	24	/* RS(255, 231): ~10% space overhead */
 
 /* buffers for deinterleaving and decoding */
 #define DM_VERITY_FEC_BUF_PREALLOC	1	/* buffers to preallocate */
@@ -50,8 +50,7 @@ struct dm_verity_fec {
 /* per-bio data */
 struct dm_verity_fec_io {
 	struct rs_control *rs;	/* Reed-Solomon state */
-	/* erasures for decode_rs8 */
-	int erasures[DM_VERITY_FEC_RSM - DM_VERITY_FEC_MIN_RSN + 1];
+	int erasures[DM_VERITY_FEC_MAX_ROOTS + 1]; /* erasures for decode_rs8 */
 	u8 *bufs[DM_VERITY_FEC_BUF_MAX];	/* bufs for deinterleaving */
 	unsigned int nbufs;		/* number of buffers allocated */
 	u8 *output;		/* buffer for corrected output */
-- 
2.55.0

[PATCH 6.18 4/4] dm-verity: fix buffer overflow in FEC calculation

[Eric Biggers]

From: Mikulas Patocka <mpatocka@redhat.com>

commit 31d6e6c0ba8d5a7bd59660035a089307100c5e8e upstream.

There's a buffer overflow in dm-verity-fec:

if (neras && *neras <= v->fec->roots)
	fio->erasures[(*neras)++] = i;

This allows *neras to reach roots + 1 (the post-increment pushes it past
roots). This value is then passed as no_eras to decode_rs8(). Inside the
RS decoder (lib/reed_solomon/decode_rs.c:113-121), the erasure locator
polynomial loop writes lambda[j] where j can reach nroots + 1 — one
element past the end of lambda[] (which is sized nroots + 1, valid
indices 0..nroots). The out-of-bounds write lands on syn[0], corrupting
the syndrome buffer.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Assisted-by: Claude:claude-opus-4-6
Cc: stable@vger.kernel.org
Fixes: a739ff3f543a ("dm verity: add support for forward error correction")
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
---
 drivers/md/dm-verity-fec.c | 4 ++--
 drivers/md/dm-verity-fec.h | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/md/dm-verity-fec.c b/drivers/md/dm-verity-fec.c
index cebcc8fd25d70..d1e4fbc5a21d9 100644
--- a/drivers/md/dm-verity-fec.c
+++ b/drivers/md/dm-verity-fec.c
@@ -250,7 +250,7 @@ static int fec_read_bufs(struct dm_verity *v, struct dm_verity_io *io,
 				     (unsigned long long)block, PTR_ERR(bbuf));
 
 			/* assume the block is corrupted */
-			if (neras && *neras <= v->fec->roots)
+			if (neras && *neras < v->fec->roots)
 				fio->erasures[(*neras)++] = i;
 
 			continue;
@@ -268,7 +268,7 @@ static int fec_read_bufs(struct dm_verity *v, struct dm_verity_io *io,
 			 * skip if we have already found the theoretical
 			 * maximum number (i.e. fec->roots) of erasures
 			 */
-			if (neras && *neras <= v->fec->roots &&
+			if (neras && *neras < v->fec->roots &&
 			    fec_is_erasure(v, io, want_digest, bbuf))
 				fio->erasures[(*neras)++] = i;
 		}
diff --git a/drivers/md/dm-verity-fec.h b/drivers/md/dm-verity-fec.h
index b3460103e0e10..8552b5d3c9152 100644
--- a/drivers/md/dm-verity-fec.h
+++ b/drivers/md/dm-verity-fec.h
@@ -50,7 +50,7 @@ struct dm_verity_fec {
 /* per-bio data */
 struct dm_verity_fec_io {
 	struct rs_control *rs;	/* Reed-Solomon state */
-	int erasures[DM_VERITY_FEC_MAX_ROOTS + 1]; /* erasures for decode_rs8 */
+	int erasures[DM_VERITY_FEC_MAX_ROOTS]; /* erasures for decode_rs8 */
 	u8 *bufs[DM_VERITY_FEC_BUF_MAX];	/* bufs for deinterleaving */
 	unsigned int nbufs;		/* number of buffers allocated */
 	u8 *output;		/* buffer for corrected output */
-- 
2.55.0