dm-verity: fix buffer overflow in FEC calculation

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>

diff --git a/drivers/md/dm-verity-fec.c b/drivers/md/dm-verity-fec.c
index 85ad9dc210ff0f56cc23c229987fcf9b4cef085f..c79f60df3a905cdd97bb3df740df20da69008464 100644 (file)
--- a/drivers/md/dm-verity-fec.c
+++ b/drivers/md/dm-verity-fec.c
@@ -220,7 +220,7 @@ static int fec_read_bufs(struct dm_verity *v, struct dm_verity_io *io,
                                     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;
@@ -238,7 +238,7 @@ 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 50b5e187d5cc1b0091f018e0749e96b540b9e1e9..3885b514fc23e72eccf786c23fe17c2383f18384 100644 (file)
--- a/drivers/md/dm-verity-fec.h
+++ b/drivers/md/dm-verity-fec.h
@@ -47,7 +47,7 @@ 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 *output;             /* buffer for corrected output */
        unsigned int level;             /* recursion level */
        unsigned int nbufs;             /* number of buffers allocated */