[PATCH 4.4 192/192] lib/mpi: mpi_write_sgl(): fix skipping of leading zero limbs

[Greg Kroah-Hartman]

4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Nicolai Stange <nicsta...@gmail.com>

commit f2d1362ff7d266b3d2b1c764d6c2ef4a3b457f23 upstream.

Currently, if the number of leading zeros is greater than fits into a
complete limb, mpi_write_sgl() skips them by iterating over them limb-wise.

However, it fails to adjust its internal leading zeros tracking variable,
lzeros, accordingly: it does a

  p -= sizeof(alimb);
  continue;

which should really have been a

  lzeros -= sizeof(alimb);
  continue;

Since lzeros never decreases if its initial value >= sizeof(alimb), nothing
gets copied by mpi_write_sgl() in that case.

Instead of skipping the high order zero limbs within the loop as shown
above, fix the issue by adjusting the copying loop's bounds.

Fixes: 2d4d1eea540b ("lib/mpi: Add mpi sgl helpers")
Signed-off-by: Nicolai Stange <nicsta...@gmail.com>
Signed-off-by: Herbert Xu <herb...@gondor.apana.org.au>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>

---
 lib/mpi/mpicoder.c |   21 +++++++++------------
 1 file changed, 9 insertions(+), 12 deletions(-)

--- a/lib/mpi/mpicoder.c
+++ b/lib/mpi/mpicoder.c
@@ -367,7 +367,9 @@ int mpi_write_to_sgl(MPI a, struct scatt
        buf_len = sgl->length;
        p2 = sg_virt(sgl);
 
-       for (i = a->nlimbs - 1; i >= 0; i--) {
+       for (i = a->nlimbs - 1 - lzeros / BYTES_PER_MPI_LIMB,
+                       lzeros %= BYTES_PER_MPI_LIMB;
+               i >= 0; i--) {
                alimb = a->d[i];
                p = (u8 *)&alimb2;
 #if BYTES_PER_MPI_LIMB == 4
@@ -388,17 +390,12 @@ int mpi_write_to_sgl(MPI a, struct scatt
 #error please implement for this limb size.
 #endif
                if (lzeros > 0) {
-                       if (lzeros >= sizeof(alimb)) {
-                               p -= sizeof(alimb);
-                               continue;
-                       } else {
-                               mpi_limb_t *limb1 = (void *)p - sizeof(alimb);
-                               mpi_limb_t *limb2 = (void *)p - sizeof(alimb)
-                                                       + lzeros;
-                               *limb1 = *limb2;
-                               p -= lzeros;
-                               y = lzeros;
-                       }
+                       mpi_limb_t *limb1 = (void *)p - sizeof(alimb);
+                       mpi_limb_t *limb2 = (void *)p - sizeof(alimb)
+                               + lzeros;
+                       *limb1 = *limb2;
+                       p -= lzeros;
+                       y = lzeros;
                        lzeros -= sizeof(alimb);
                }