lib/mpi: Fix buffer overrun when SG is too long

The helper mpi_read_raw_from_sgl sets the number of entries in
the SG list according to nbytes.  However, if the last entry
in the SG list contains more data than nbytes, then it may overrun
the buffer because it only allocates enough memory for nbytes.

Fixes: 2d4d1eea ("lib/mpi: Add mpi sgl helpers")
Reported-by: NRoberto Sassu <roberto.sassu@huaweicloud.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>
Reviewed-by: NEric Biggers <ebiggers@google.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>

lib/mpi/mpicoder.c

@@ -504,7 +504,8 @@ MPI mpi_read_raw_from_sgl(struct scatterlist *sgl, unsigned int nbytes)
 
 	while (sg_miter_next(&miter)) {
 		buff = miter.addr;
-		len = miter.length;
+		len = min_t(unsigned, miter.length, nbytes);
+		nbytes -= len;
 
 		for (x = 0; x < len; x++) {
 			a <<= 8;