Patch "lib/mpi: mpi_write_sgl(): fix skipping of leading zero limbs" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

lib/mpi: mpi_write_sgl(): fix skipping of leading zero limbs

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
lib-mpi-mpi_write_sgl-fix-skipping-of-leading-zero-limbs.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Nicolai Stange <[email protected]>
Date: Tue, 22 Mar 2016 13:12:35 +0100
Subject: lib/mpi: mpi_write_sgl(): fix skipping of leading zero limbs

From: Nicolai Stange <[email protected]>

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.

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

@@ -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);
 		}
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/lib-mpi-mpi_write_sgl-fix-skipping-of-leading-zero-limbs.patch