ubifs: fix out-of-bounds read in signature length check

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

ubifs: fix out-of-bounds read in signature length check

ubifs_sb_verify_signature() bounds the on-disk ubifs_sig_node->len field
before handing the signature payload to verify_pkcs7_signature(), but the
check has the wrong sign:

if (le32_to_cpu(signode->len) > snod->len + sizeof(struct ubifs_sig_node))

The signature bytes start sizeof(struct ubifs_sig_node) (UBIFS_SIG_NODE_SZ,
64 bytes) into the node, so the payload is at most

snod->len - sizeof(struct ubifs_sig_node)

bytes long. Adding the header size instead of subtracting it accepts a
declared length up to 2 * UBIFS_SIG_NODE_SZ larger than the node actually
holds -- past the end of c->sbuf, which is vmalloc(c->leb_size).
verify_pkcs7_signature() -> pkcs7_parse_message() -> asn1_ber_decoder()
is then handed that inflated length and reads beyond the allocation while
walking the DER headers. The node length comes straight from the mounted
image, so a crafted signed UBIFS image reaches this via
ubifs_read_superblock() before the signature is cryptographically checked.

snod->len is guaranteed to be >= UBIFS_SIG_NODE_SZ by the node scanner
(c->ranges[UBIFS_SIG_NODE].min_len == UBIFS_SIG_NODE_SZ), so the corrected
subtraction cannot underflow. Legitimately signed images are unaffected: a
correct superblock never declares a signature longer than the node it is
embedded in.