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Subject[PATCH] mpi: Fix NULL ptr dereference in mpi_powm()
Date
Parsing certain certificates (see [1]) triggers NULL-ptr
dereference in mpi_powm():

BUG: unable to handle kernel NULL pointer dereference at (null)
IP: [<ffffffff818eb118>] mpi_powm+0xf8/0x10b0
...
Call Trace:
[<ffffffff817e06a6>] _rsa_dec.isra.2+0x66/0x80
[<ffffffff817e07c3>] rsa_verify+0x103/0x1c0
[<ffffffff817e1683>] pkcs1pad_verify+0x1c3/0x220
[<ffffffff8184549a>] public_key_verify_signature+0x3fa/0x4d0
[<ffffffff818473c7>] x509_check_for_self_signed+0x167/0x1e0
[<ffffffff8184607e>] x509_cert_parse+0x27e/0x300
[<ffffffff81846cae>] x509_key_preparse+0x3e/0x330
[<ffffffff818444cf>] asymmetric_key_preparse+0x6f/0x100
[<ffffffff8178bec0>] key_create_or_update+0x260/0x5f0
[<ffffffff8178e6d9>] SyS_add_key+0x199/0x2a0
[<ffffffff821d823b>] entry_SYSCALL_64_fastpath+0x1e/0xad

This happens because mpi_alloc(0) doesn't allocate the limb space.
Fix this by allocating the result if needed.

Basically, this is a backport of libgcrypt patch [2].

[1] http://seclists.org/fulldisclosure/2016/Nov/76
[2] http://git.gnupg.org/cgi-bin/gitweb.cgi?p=libgcrypt.git;a=patch;h=6e1adb05d290aeeb1c230c763970695f4a538526

Fixes: cdec9cb5167a ("crypto: GnuPG based MPI lib - source files (part 1)")
Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: <stable@vger.kernel.org>
---
lib/mpi/mpi-pow.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/lib/mpi/mpi-pow.c b/lib/mpi/mpi-pow.c
index 5464c87..9e28d12 100644
--- a/lib/mpi/mpi-pow.c
+++ b/lib/mpi/mpi-pow.c
@@ -64,8 +64,13 @@ int mpi_powm(MPI res, MPI base, MPI exp, MPI mod)
if (!esize) {
/* Exponent is zero, result is 1 mod MOD, i.e., 1 or 0
* depending on if MOD equals 1. */
- rp[0] = 1;
res->nlimbs = (msize == 1 && mod->d[0] == 1) ? 0 : 1;
+ if (res->nlimbs) {
+ if (RESIZE_IF_NEEDED(res, 1) < 0)
+ goto enomem;
+ rp = res->d;
+ rp[0] = 1;
+ }
res->sign = 0;
goto leave;
}
--
2.7.3
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