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[PATCH RESEND v2 10/14] lib/mpi: mpi_read_raw_from_sgl(): don't include leading zero SGEs in nbytes

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At the very beginning of mpi_read_raw_from_sgl(), the leading zeros of
the input scatterlist are counted:

  lzeros = 0;
  for_each_sg(sgl, sg, ents, i) {
    ...
    if (/* sg contains nonzero bytes */)
      break;

    /* sg contains nothing but zeros here */
    ents--;
    lzeros = 0;
  }

Later on, the total number of trailing nonzero bytes is calculated by
subtracting the number of leading zero bytes from the total number of input
bytes:

  nbytes -= lzeros;

However, since lzeros gets reset to zero for each completely zero leading
sg in the loop above, it doesn't include those.

Besides wasting resources by allocating a too large output buffer,
this mistake propagates into the calculation of x, the number of
leading zeros within the most significant output limb:

  x = BYTES_PER_MPI_LIMB - nbytes % BYTES_PER_MPI_LIMB;

What's more, the low order bytes of the output, equal in number to the
extra bytes in nbytes, are left uninitialized.

Fix this by adjusting nbytes for each completely zero leading scatterlist
entry.

Fixes: 2d4d1eea540b ("lib/mpi: Add mpi sgl helpers")
Signed-off-by: Nicolai Stange <nicstange@gmail.com>
---
 lib/mpi/mpicoder.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/lib/mpi/mpicoder.c b/lib/mpi/mpicoder.c
index 2de2c7d..add9e81 100644
--- a/lib/mpi/mpicoder.c
+++ b/lib/mpi/mpicoder.c
@@ -447,16 +447,12 @@ MPI mpi_read_raw_from_sgl(struct scatterlist *sgl, unsigned int nbytes)
 			break;
 
 		ents--;
+		nbytes -= lzeros;
 		lzeros = 0;
 	}
 
 	sgl = sg;
-
-	if (!ents)
-		nbytes = 0;
-	else
-		nbytes -= lzeros;
-
+	nbytes -= lzeros;
 	nbits = nbytes * 8;
 	if (nbits > MAX_EXTERN_MPI_BITS) {
 		pr_info("MPI: mpi too large (%u bits)\n", nbits);
-- 
2.7.3

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[PATCH RESEND v2 00/14] lib/mpi: bug fixes and cleanup Nicolai Stange <nicstange@gmail.com> - 2016-03-21 14:30 +0100
  [PATCH RESEND v2 05/14] lib/mpi: mpi_write_sgl(): replace open coded endian conversion Nicolai Stange <nicstange@gmail.com> - 2016-03-21 14:30 +0100
  [PATCH RESEND v2 06/14] lib/mpi: mpi_read_buffer(): optimize skipping of leading zero limbs Nicolai Stange <nicstange@gmail.com> - 2016-03-21 14:30 +0100
  [PATCH RESEND v2 12/14] lib/mpi: mpi_read_raw_from_sgl(): fix nbits calculation Nicolai Stange <nicstange@gmail.com> - 2016-03-21 14:30 +0100
  [PATCH RESEND v2 10/14] lib/mpi: mpi_read_raw_from_sgl(): don't include leading zero SGEs in nbytes Nicolai Stange <nicstange@gmail.com> - 2016-03-21 14:30 +0100
  [PATCH RESEND v2 07/14] lib/mpi: mpi_read_buffer(): replace open coded endian conversion Nicolai Stange <nicstange@gmail.com> - 2016-03-21 14:30 +0100
  [PATCH RESEND v2 08/14] lib/mpi: mpi_read_buffer(): fix buffer overflow Nicolai Stange <nicstange@gmail.com> - 2016-03-21 14:30 +0100
  [PATCH RESEND v2 13/14] lib/mpi: mpi_read_raw_from_sgl(): sanitize meaning of indices Nicolai Stange <nicstange@gmail.com> - 2016-03-21 14:30 +0100
  [PATCH RESEND v2 14/14] lib/mpi: mpi_read_raw_from_sgl(): fix out-of-bounds buffer access Nicolai Stange <nicstange@gmail.com> - 2016-03-21 14:30 +0100
  [PATCH RESEND v2 11/14] lib/mpi: mpi_read_raw_from_sgl(): purge redundant clearing of nbits Nicolai Stange <nicstange@gmail.com> - 2016-03-21 14:30 +0100
  [PATCH RESEND v2 03/14] lib/mpi: mpi_write_sgl(): purge redundant pointer arithmetic Nicolai Stange <nicstange@gmail.com> - 2016-03-21 14:40 +0100
  [PATCH RESEND v2 02/14] lib/mpi: mpi_write_sgl(): fix style issue with lzero decrement Nicolai Stange <nicstange@gmail.com> - 2016-03-21 14:40 +0100
  [PATCH RESEND v2 04/14] lib/mpi: mpi_write_sgl(): fix out-of-bounds stack access Nicolai Stange <nicstange@gmail.com> - 2016-03-21 14:40 +0100
  [PATCH RESEND v2 01/14] lib/mpi: mpi_write_sgl(): fix skipping of leading zero limbs Nicolai Stange <nicstange@gmail.com> - 2016-03-21 14:40 +0100
  Re: [PATCH RESEND v2 00/14] lib/mpi: bug fixes and cleanup Tadeusz Struk <tadeusz.struk@intel.com> - 2016-03-22 06:00 +0100
    Re: [PATCH RESEND v2 00/14] lib/mpi: bug fixes and cleanup Nicolai Stange <nicstange@gmail.com> - 2016-03-22 08:10 +0100
      Re: [PATCH RESEND v2 00/14] lib/mpi: bug fixes and cleanup Tadeusz Struk <tadeusz.struk@intel.com> - 2016-03-22 15:20 +0100

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