[PATCH 3.16 261/306] mpi: Fix NULL ptr dereference in mpi_powm() [ver #3]

Ben Hutchings
2017-02-15T23:11:39Z

From: Andrey Ryabinin <aryabinin@virtuozzo.com>

commit f5527fffff3f002b0a6b376163613b82f69de073 upstream.

This fixes CVE-2016-8650.

If mpi_powm() is given a zero exponent, it wants to immediately return
either 1 or 0, depending on the modulus. However, if the result was
initalised with zero limb space, no limbs space is allocated and a
NULL-pointer exception ensues.

Fix this by allocating a minimal amount of limb space for the result when
the 0-exponent case when the result is 1 and not touching the limb space
when the result is 0.

This affects the use of RSA keys and X.509 certificates that carry them.

BUG: unable to handle kernel NULL pointer dereference at (null)
IP: [<ffffffff8138ce5d>] mpi_powm+0x32/0x7e6
PGD 0
Oops: 0002 [#1] SMP
Modules linked in:
CPU: 3 PID: 3014 Comm: keyctl Not tainted 4.9.0-rc6-fscache+ #278
Hardware name: ASUS All Series/H97-PLUS, BIOS 2306 10/09/2014
task: ffff8804011944c0 task.stack: ffff880401294000
RIP: 0010:[<ffffffff8138ce5d>] [<ffffffff8138ce5d>] mpi_powm+0x32/0x7e6
RSP: 0018:ffff880401297ad8 EFLAGS: 00010212
RAX: 0000000000000000 RBX: ffff88040868bec0 RCX: ffff88040868bba0
RDX: ffff88040868b260 RSI: ffff88040868bec0 RDI: ffff88040868bee0
RBP: ffff880401297ba8 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000047 R11: ffffffff8183b210 R12: 0000000000000000
R13: ffff8804087c7600 R14: 000000000000001f R15: ffff880401297c50
FS: 00007f7a7918c700(0000) GS:ffff88041fb80000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 0000000401250000 CR4: 00000000001406e0
Stack:
ffff88040868bec0 0000000000000020 ffff880401297b00 ffffffff81376cd4
0000000000000100 ffff880401297b10 ffffffff81376d12 ffff880401297b30
ffffffff81376f37 0000000000000100 0000000000000000 ffff880401297ba8
Call Trace:
[<ffffffff81376cd4>] ? __sg_page_iter_next+0x43/0x66
[<ffffffff81376d12>] ? sg_miter_get_next_page+0x1b/0x5d
[<ffffffff81376f37>] ? sg_miter_next+0x17/0xbd
[<ffffffff8138ba3a>] ? mpi_read_raw_from_sgl+0xf2/0x146
[<ffffffff8132a95c>] rsa_verify+0x9d/0xee
[<ffffffff8132acca>] ? pkcs1pad_sg_set_buf+0x2e/0xbb
[<ffffffff8132af40>] pkcs1pad_verify+0xc0/0xe1
[<ffffffff8133cb5e>] public_key_verify_signature+0x1b0/0x228
[<ffffffff8133d974>] x509_check_for_self_signed+0xa1/0xc4
[<ffffffff8133cdde>] x509_cert_parse+0x167/0x1a1
[<ffffffff8133d609>] x509_key_preparse+0x21/0x1a1
[<ffffffff8133c3d7>] asymmetric_key_preparse+0x34/0x61
[<ffffffff812fc9f3>] key_create_or_update+0x145/0x399
[<ffffffff812fe227>] SyS_add_key+0x154/0x19e
[<ffffffff81001c2b>] do_syscall_64+0x80/0x191
[<ffffffff816825e4>] entry_SYSCALL64_slow_path+0x25/0x25
Code: 56 41 55 41 54 53 48 81 ec a8 00 00 00 44 8b 71 04 8b 42 04 4c 8b 67 18 45 85 f6 89 45 80 0f 84 b4 06 00 00 85 c0 75 2f 41 ff ce <49> c7 04 24 01 00 00 00 b0 01 75 0b 48 8b 41 18 48 83 38 01 0f
RIP [<ffffffff8138ce5d>] mpi_powm+0x32/0x7e6
RSP <ffff880401297ad8>
CR2: 0000000000000000
---[ end trace d82015255d4a5d8d ]---

Basically, this is a backport of a libgcrypt patch:

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>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Dmitry Kasatkin <dmitry.kasatkin@gmail.com>
cc: linux-ima-devel@lists.sourceforge.net
Signed-off-by: James Morris <james.l.morris@oracle.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
lib/mpi/mpi-pow.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)

--- 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,
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 (mpi_resize(res, 1) < 0)
+ goto enomem;
+ rp = res->d;
+ rp[0] = 1;
+ }
res->sign = 0;
goto leave;
}