Re: [PATCH] ima: Make a copy of sig and digest in asymmetric_verify()

[Roberto Sassu <roberto.sassu@huaweicloud.com>]

On Fri, 2022-11-04 at 13:20 +0100, Roberto Sassu wrote:
> From: Roberto Sassu <roberto.sassu@huawei.com>
> 
> Commit ac4e97abce9b8 ("scatterlist: sg_set_buf() argument must be in linear
> mapping") requires that both the signature and the digest resides in the
> linear mapping area.
> 
> However, more recently commit ba14a194a434c ("fork: Add generic vmalloced
> stack support"), made it possible to move the stack in the vmalloc area,
> which could make the requirement of the first commit not satisfied anymore.
> 
> If CONFIG_SG=y and CONFIG_VMAP_STACK=y, the following BUG() is triggered:

Hi Mimi

did you have the chance to have a look at this patch?

Thanks

Roberto

> [  467.077359] kernel BUG at include/linux/scatterlist.h:163!
> [  467.077939] invalid opcode: 0000 [#1] PREEMPT SMP NOPTI
> 
> [...]
> 
> [  467.095225] Call Trace:
> [  467.096088]  <TASK>
> [  467.096928]  ? rcu_read_lock_held_common+0xe/0x50
> [  467.097569]  ? rcu_read_lock_sched_held+0x13/0x70
> [  467.098123]  ? trace_hardirqs_on+0x2c/0xd0
> [  467.098647]  ? public_key_verify_signature+0x470/0x470
> [  467.099237]  asymmetric_verify+0x14c/0x300
> [  467.099869]  evm_verify_hmac+0x245/0x360
> [  467.100391]  evm_inode_setattr+0x43/0x190
> 
> The failure happens only for the digest, as the pointer comes from the
> stack, and not for the signature, which instead was allocated by
> vfs_getxattr_alloc().
> 
> Fix this by making a copy of both in asymmetric_verify(), so that the
> linear mapping requirement is always satisfied, regardless of the caller.
> 
> Cc: stable@vger.kernel.org # 4.9.x
> Fixes: ba14a194a434 ("fork: Add generic vmalloced stack support")
> Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
> ---
>  security/integrity/digsig_asymmetric.c | 19 +++++++++++++++++--
>  1 file changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/security/integrity/digsig_asymmetric.c b/security/integrity/digsig_asymmetric.c
> index 895f4b9ce8c6..635238d5c7fe 100644
> --- a/security/integrity/digsig_asymmetric.c
> +++ b/security/integrity/digsig_asymmetric.c
> @@ -122,11 +122,26 @@ int asymmetric_verify(struct key *keyring, const char *sig,
>  		goto out;
>  	}
>  
> -	pks.digest = (u8 *)data;
> +	pks.digest = kmemdup(data, datalen, GFP_KERNEL);
> +	if (!pks.digest) {
> +		ret = -ENOMEM;
> +		goto out;
> +	}
> +
>  	pks.digest_size = datalen;
> -	pks.s = hdr->sig;
> +
> +	pks.s = kmemdup(hdr->sig, siglen, GFP_KERNEL);
> +	if (!pks.s) {
> +		kfree(pks.digest);
> +		ret = -ENOMEM;
> +		goto out;
> +	}
> +
>  	pks.s_size = siglen;
> +
>  	ret = verify_signature(key, &pks);
> +	kfree(pks.digest);
> +	kfree(pks.s);
>  out:
>  	key_put(key);
>  	pr_debug("%s() = %d\n", __func__, ret);