Re: [PATCH 2/2] MPI: Fix mpi_powm() when exponent is 0 and the result has no limbs

[Andrey Ryabinin <ryabinin.a.a@gmail.com>]

On 11/24/2016 03:57 PM, David Howells wrote:
> David Howells <dhowells@redhat.com> wrote:
> 
>> I've integrated my patch and yours (see attached) - are you okay with the
>> result?
> 
> It helps if I commit the changes...
> 

Looks good, thanks.

> David
> ---
> commit 1dc938c3393d8ff0e04723956ed00ec21e2b2a89
> Author: Andrey Ryabinin <aryabinin@virtuozzo.com>
> Date:   Wed Nov 23 16:44:47 2016 +0000
> 
>     mpi: Fix NULL ptr dereference in mpi_powm()
>     
>     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
>     cc: stable@vger.kernel.org
> 
> diff --git a/lib/mpi/mpi-pow.c b/lib/mpi/mpi-pow.c
> index 5464c8744ea9..e24388a863a7 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 (mpi_resize(res, 1) < 0)
> +				goto enomem;
> +			rp = res->d;
> +			rp[0] = 1;
> +		}
>  		res->sign = 0;
>  		goto leave;
>  	}
>