ACK/cmnt: [Trusty SRU][PATCH 1/1] mpi: Fix NULL ptr dereference in mpi_powm() [ver #3]

[Stefan Bader]

On 05.09.2017 10:31, Kleber Sacilotto de Souza wrote:
> From: Andrey Ryabinin <aryabinin at virtuozzo.com>
> 
> 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 at virtuozzo.com>
> Signed-off-by: David Howells <dhowells at redhat.com>
> cc: Dmitry Kasatkin <dmitry.kasatkin at gmail.com>
> cc: linux-ima-devel at lists.sourceforge.net
> cc: stable at vger.kernel.org
> Signed-off-by: James Morris <james.l.morris at oracle.com>

CVE-2016-8650

> (cherry picked from commit f5527fffff3f002b0a6b376163613b82f69de073)
> Signed-off-by: Kleber Sacilotto de Souza <kleber.souza at canonical.com>
Acked-by: Stefan Bader <stefan.bader at canonical.com>

> ---

I think I missed this on some of the previous patches. I believe the matching of
insertchanges does not find things like "This fixes CVE...". So I would suggest
to add the number explicitly in the SOB area. Even if it seems to appear in the
commit text.

-Stefan
>  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 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;
>  	}
> 


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