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[PATCH 8/8] lib/mpi: mpi_read_buffer(): fix buffer overflow

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Currently, mpi_read_buffer() writes full limbs to the output buffer
and moves memory around to purge leading zero limbs afterwards.

However, with

  commit 9cbe21d8f89d ("lib/mpi: only require buffers as big as needed for
                        the integer")

the caller is only required to provide a buffer large enough to hold the
result without the leading zeros.

This might result in a buffer overflow for small MP numbers with leading
zeros.

Fix this by coping the result to its final destination within the output
buffer and not copying the leading zeros at all.

Fixes: 9cbe21d8f89d ("lib/mpi: only require buffers as big as needed for
                      the integer")
Signed-off-by: Nicolai Stange <nicstange@gmail.com>
---
 lib/mpi/mpicoder.c | 13 +++----------
 1 file changed, 3 insertions(+), 10 deletions(-)

diff --git a/lib/mpi/mpicoder.c b/lib/mpi/mpicoder.c
index a999ee1..27582e2 100644
--- a/lib/mpi/mpicoder.c
+++ b/lib/mpi/mpicoder.c
@@ -201,16 +201,9 @@ int mpi_read_buffer(MPI a, uint8_t *buf, unsigned buf_len, unsigned *nbytes,
 #else
 #error please implement for this limb size.
 #endif
-		memcpy(p, &alimb, BYTES_PER_MPI_LIMB);
-		p += BYTES_PER_MPI_LIMB;
-		if (lzeros > 0) {
-			mpi_limb_t *limb1 = (void *)p - sizeof(alimb);
-			mpi_limb_t *limb2 = (void *)p - sizeof(alimb)
-				+ lzeros;
-			*limb1 = *limb2;
-			p -= lzeros;
-			lzeros -= sizeof(alimb);
-		}
+		memcpy(p, &alimb + lzeros, BYTES_PER_MPI_LIMB - lzeros);
+		p += BYTES_PER_MPI_LIMB - lzeros;
+		lzeros = 0;
 	}
 	return 0;
 }
-- 
2.7.2

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[PATCH 0/8] lib/mpi: bug fixes and cleanup Nicolai Stange <nicstange@gmail.com> - 2016-03-17 19:10 +0100
  [PATCH 8/8] lib/mpi: mpi_read_buffer(): fix buffer overflow Nicolai Stange <nicstange@gmail.com> - 2016-03-17 19:10 +0100
  [PATCH 2/8] lib/mpi: mpi_write_sgl(): fix style issue with lzero decrement Nicolai Stange <nicstange@gmail.com> - 2016-03-17 19:10 +0100
  [PATCH 5/8] lib/mpi: mpi_write_sgl(): replace open coded endian conversion Nicolai Stange <nicstange@gmail.com> - 2016-03-17 19:10 +0100
  [PATCH 1/8] lib/mpi: mpi_write_sgl(): fix skipping of leading zero limbs Nicolai Stange <nicstange@gmail.com> - 2016-03-17 19:10 +0100
  [PATCH 4/8] lib/mpi: mpi_write_sgl(): fix out-of-bounds stack access Nicolai Stange <nicstange@gmail.com> - 2016-03-17 19:10 +0100
  [PATCH 3/8] lib/mpi: mpi_write_sgl(): purge redundant pointer arithmetic Nicolai Stange <nicstange@gmail.com> - 2016-03-17 19:10 +0100
  [PATCH 6/8] lib/mpi: mpi_read_buffer(): optimize skipping of leading zero limbs Nicolai Stange <nicstange@gmail.com> - 2016-03-17 19:10 +0100
  Re: [PATCH 0/8] lib/mpi: bug fixes and cleanup Nicolai Stange <nicstange@gmail.com> - 2016-03-20 13:50 +0100

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