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From:   Anna Schumaker <schumaker.anna@gmail.com>
Date:   Wed, 22 Mar 2023 16:55:31 -0400
Message-ID: <CAFX2JfnA=qk+=YGFjaE8nrmW=yUwthvWaeCN3X4k0065eXExuA@mail.gmail.com>
Subject: Re: [PATCH v1] SUNRPC: Fix a crash in gss_krb5_checksum()
To:     Chuck Lever <cel@kernel.org>
Cc:     anna.schumaker@netapp.com, linux-nfs@vger.kernel.org
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On Wed, Mar 22, 2023 at 3:17=E2=80=AFPM Chuck Lever <cel@kernel.org> wrote:
>
> From: Chuck Lever <chuck.lever@oracle.com>
>
> Anna says:
> > KASAN reports [...] a slab-out-of-bounds in gss_krb5_checksum(),
> > and it can cause my client to panic when running cthon basic
> > tests with krb5p.
>
> > Running faddr2line gives me:
> >
> > gss_krb5_checksum+0x4b6/0x630:
> > ahash_request_free at
> > /home/anna/Programs/linux-nfs.git/./include/crypto/hash.h:619
> > (inlined by) gss_krb5_checksum at
> > /home/anna/Programs/linux-nfs.git/net/sunrpc/auth_gss/gss_krb5_crypto.c=
:358
>
> My diagnosis is that the memcpy() at the end of gss_krb5_checksum()
> reads past the end of the buffer containing the checksum data
> because the callers have ignored gss_krb5_checksum()'s API contract:
>
>  * Caller provides the truncation length of the output token (h) in
>  * cksumout.len.
>
> Instead they provide the fixed length of the hmac buffer. This
> length happens to be larger than the value returned by
> crypto_ahash_digestsize().
>
> Change these errant callers to work like krb5_etm_{en,de}crypt().
> As a defensive measure, bound the length of the byte copy at the
> end of gss_krb5_checksum().
>
> Kunit sez:
> Testing complete. Ran 68 tests: passed: 68
> Elapsed time: 81.680s total, 5.875s configuring, 75.610s building, 0.103s=
 running
>
> Reported-by: Anna Schumaker <schumaker.anna@gmail.com>
> Fixes: 8270dbfcebea ("SUNRPC: Obscure Kerberos integrity keys")
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

This patch fixed the issue for me, thanks! Are you going to submit it
with a future 6.3-rc pull request, or should I?

Anna

> ---
>  net/sunrpc/auth_gss/gss_krb5_crypto.c |   10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/net/sunrpc/auth_gss/gss_krb5_crypto.c b/net/sunrpc/auth_gss/=
gss_krb5_crypto.c
> index 6c7c52eeed4f..212c5d57465a 100644
> --- a/net/sunrpc/auth_gss/gss_krb5_crypto.c
> +++ b/net/sunrpc/auth_gss/gss_krb5_crypto.c
> @@ -353,7 +353,9 @@ gss_krb5_checksum(struct crypto_ahash *tfm, char *hea=
der, int hdrlen,
>         err =3D crypto_ahash_final(req);
>         if (err)
>                 goto out_free_ahash;
> -       memcpy(cksumout->data, checksumdata, cksumout->len);
> +
> +       memcpy(cksumout->data, checksumdata,
> +              min_t(int, cksumout->len, crypto_ahash_digestsize(tfm)));
>
>  out_free_ahash:
>         ahash_request_free(req);
> @@ -809,8 +811,7 @@ gss_krb5_aes_encrypt(struct krb5_ctx *kctx, u32 offse=
t,
>         buf->tail[0].iov_len +=3D GSS_KRB5_TOK_HDR_LEN;
>         buf->len +=3D GSS_KRB5_TOK_HDR_LEN;
>
> -       /* Do the HMAC */
> -       hmac.len =3D GSS_KRB5_MAX_CKSUM_LEN;
> +       hmac.len =3D kctx->gk5e->cksumlength;
>         hmac.data =3D buf->tail[0].iov_base + buf->tail[0].iov_len;
>
>         /*
> @@ -873,8 +874,7 @@ gss_krb5_aes_decrypt(struct krb5_ctx *kctx, u32 offse=
t, u32 len,
>         if (ret)
>                 goto out_err;
>
> -       /* Calculate our hmac over the plaintext data */
> -       our_hmac_obj.len =3D sizeof(our_hmac);
> +       our_hmac_obj.len =3D kctx->gk5e->cksumlength;
>         our_hmac_obj.data =3D our_hmac;
>         ret =3D gss_krb5_checksum(ahash, NULL, 0, &subbuf, 0, &our_hmac_o=
bj);
>         if (ret)
>
>