[PATCH 3.16 86/87] lib/mpi: Fix karactx leak in mpi_powm

[Ben Hutchings]

3.16.75-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>

commit c8ea9fce2baf7b643384f36f29e4194fa40d33a6 upstream.

Sometimes mpi_powm will leak karactx because a memory allocation
failure causes a bail-out that skips the freeing of karactx.  This
patch moves the freeing of karactx to the end of the function like
everything else so that it can't be skipped.

Reported-by: syzbot+f7baccc38dcc1e094e77@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: cdec9cb5167a ("crypto: GnuPG based MPI lib - source files...")
Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Reviewed-by: Eric Biggers <ebiggers@xxxxxxxxxx>
Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 lib/mpi/mpi-pow.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

--- a/lib/mpi/mpi-pow.c
+++ b/lib/mpi/mpi-pow.c
@@ -36,6 +36,7 @@
 int mpi_powm(MPI res, MPI base, MPI exp, MPI mod)
 {
 	mpi_ptr_t mp_marker = NULL, bp_marker = NULL, ep_marker = NULL;
+	struct karatsuba_ctx karactx = {};
 	mpi_ptr_t xp_marker = NULL;
 	mpi_ptr_t tspace = NULL;
 	mpi_ptr_t rp, ep, mp, bp;
@@ -163,13 +164,11 @@ int mpi_powm(MPI res, MPI base, MPI exp,
 		int c;
 		mpi_limb_t e;
 		mpi_limb_t carry_limb;
-		struct karatsuba_ctx karactx;
 
 		xp = xp_marker = mpi_alloc_limb_space(2 * (msize + 1));
 		if (!xp)
 			goto enomem;
 
-		memset(&karactx, 0, sizeof karactx);
 		negative_result = (ep[0] & 1) && base->sign;
 
 		i = esize - 1;
@@ -293,8 +292,6 @@ int mpi_powm(MPI res, MPI base, MPI exp,
 		if (mod_shift_cnt)
 			mpihelp_rshift(rp, rp, rsize, mod_shift_cnt);
 		MPN_NORMALIZE(rp, rsize);
-
-		mpihelp_release_karatsuba_ctx(&karactx);
 	}
 
 	if (negative_result && rsize) {
@@ -311,6 +308,7 @@ int mpi_powm(MPI res, MPI base, MPI exp,
 leave:
 	rc = 0;
 enomem:
+	mpihelp_release_karatsuba_ctx(&karactx);
 	if (assign_rp)
 		mpi_assign_limb_space(res, rp, size);
 	if (mp_marker)