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Subject: Re: [PATCH 1/1] crypto: ccp: use file mode for sev ioctl permissions
To:     Connor Kuehl <ckuehl@redhat.com>, herbert@gondor.apana.org.au,
        davem@davemloft.net
Cc:     gary.hook@amd.com, erdemaktas@google.com, rientjes@google.com,
        brijesh.singh@amd.com, npmccallum@redhat.com, bsd@redhat.com,
        linux-crypto@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20200306172010.1213899-1-ckuehl@redhat.com>
 <20200306172010.1213899-2-ckuehl@redhat.com>
From:   Tom Lendacky <thomas.lendacky@amd.com>
Message-ID: <df67f101-383c-ff87-6425-2959e32996fb@amd.com>
Date:   Tue, 10 Mar 2020 14:53:13 -0500
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On 3/6/20 11:20 AM, Connor Kuehl wrote:
> Instead of using CAP_SYS_ADMIN which is restricted to the root user,
> check the file mode for write permissions before executing commands that
> can affect the platform. This allows for more fine-grained access
> control to the SEV ioctl interface. This would allow a SEV-only user
> or group the ability to administer the platform without requiring them
> to be root or granting them overly powerful permissions.
> 
> For example:
> 
> chown root:root /dev/sev
> chmod 600 /dev/sev
> setfacl -m g:sev:r /dev/sev
> setfacl -m g:sev-admin:rw /dev/sev
> 
> In this instance, members of the "sev-admin" group have the ability to
> perform all ioctl calls (including the ones that modify platform state).
> Members of the "sev" group only have access to the ioctls that do not
> modify the platform state.
> 
> This also makes opening "/dev/sev" more consistent with how file
> descriptors are usually handled. By only checking for CAP_SYS_ADMIN,
> the file descriptor could be opened read-only but could still execute
> ioctls that modify the platform state. This patch enforces that the file
> descriptor is opened with write privileges if it is going to be used to
> modify the platform state.
> 
> This flexibility is completely opt-in, and if it is not desirable by
> the administrator then they do not need to give anyone else access to
> /dev/sev.
> 
> Signed-off-by: Connor Kuehl <ckuehl@redhat.com>

Acked-by: Tom Lendacky <thomas.lendacky@amd.com>

> ---
>  drivers/crypto/ccp/sev-dev.c | 33 +++++++++++++++++----------------
>  1 file changed, 17 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/crypto/ccp/sev-dev.c b/drivers/crypto/ccp/sev-dev.c
> index e467860f797d..416b80938a3e 100644
> --- a/drivers/crypto/ccp/sev-dev.c
> +++ b/drivers/crypto/ccp/sev-dev.c
> @@ -283,11 +283,11 @@ static int sev_get_platform_state(int *state, int *error)
>  	return rc;
>  }
>  
> -static int sev_ioctl_do_reset(struct sev_issue_cmd *argp)
> +static int sev_ioctl_do_reset(struct sev_issue_cmd *argp, bool writable)
>  {
>  	int state, rc;
>  
> -	if (!capable(CAP_SYS_ADMIN))
> +	if (!writable)
>  		return -EPERM;
>  
>  	/*
> @@ -331,12 +331,12 @@ static int sev_ioctl_do_platform_status(struct sev_issue_cmd *argp)
>  	return ret;
>  }
>  
> -static int sev_ioctl_do_pek_pdh_gen(int cmd, struct sev_issue_cmd *argp)
> +static int sev_ioctl_do_pek_pdh_gen(int cmd, struct sev_issue_cmd *argp, bool writable)
>  {
>  	struct sev_device *sev = psp_master->sev_data;
>  	int rc;
>  
> -	if (!capable(CAP_SYS_ADMIN))
> +	if (!writable)
>  		return -EPERM;
>  
>  	if (sev->state == SEV_STATE_UNINIT) {
> @@ -348,7 +348,7 @@ static int sev_ioctl_do_pek_pdh_gen(int cmd, struct sev_issue_cmd *argp)
>  	return __sev_do_cmd_locked(cmd, NULL, &argp->error);
>  }
>  
> -static int sev_ioctl_do_pek_csr(struct sev_issue_cmd *argp)
> +static int sev_ioctl_do_pek_csr(struct sev_issue_cmd *argp, bool writable)
>  {
>  	struct sev_device *sev = psp_master->sev_data;
>  	struct sev_user_data_pek_csr input;
> @@ -356,7 +356,7 @@ static int sev_ioctl_do_pek_csr(struct sev_issue_cmd *argp)
>  	void *blob = NULL;
>  	int ret;
>  
> -	if (!capable(CAP_SYS_ADMIN))
> +	if (!writable)
>  		return -EPERM;
>  
>  	if (copy_from_user(&input, (void __user *)argp->data, sizeof(input)))
> @@ -539,7 +539,7 @@ static int sev_update_firmware(struct device *dev)
>  	return ret;
>  }
>  
> -static int sev_ioctl_do_pek_import(struct sev_issue_cmd *argp)
> +static int sev_ioctl_do_pek_import(struct sev_issue_cmd *argp, bool writable)
>  {
>  	struct sev_device *sev = psp_master->sev_data;
>  	struct sev_user_data_pek_cert_import input;
> @@ -547,7 +547,7 @@ static int sev_ioctl_do_pek_import(struct sev_issue_cmd *argp)
>  	void *pek_blob, *oca_blob;
>  	int ret;
>  
> -	if (!capable(CAP_SYS_ADMIN))
> +	if (!writable)
>  		return -EPERM;
>  
>  	if (copy_from_user(&input, (void __user *)argp->data, sizeof(input)))
> @@ -698,7 +698,7 @@ static int sev_ioctl_do_get_id(struct sev_issue_cmd *argp)
>  	return ret;
>  }
>  
> -static int sev_ioctl_do_pdh_export(struct sev_issue_cmd *argp)
> +static int sev_ioctl_do_pdh_export(struct sev_issue_cmd *argp, bool writable)
>  {
>  	struct sev_device *sev = psp_master->sev_data;
>  	struct sev_user_data_pdh_cert_export input;
> @@ -708,7 +708,7 @@ static int sev_ioctl_do_pdh_export(struct sev_issue_cmd *argp)
>  
>  	/* If platform is not in INIT state then transition it to INIT. */
>  	if (sev->state != SEV_STATE_INIT) {
> -		if (!capable(CAP_SYS_ADMIN))
> +		if (!writable)
>  			return -EPERM;
>  
>  		ret = __sev_platform_init_locked(&argp->error);
> @@ -801,6 +801,7 @@ static long sev_ioctl(struct file *file, unsigned int ioctl, unsigned long arg)
>  	void __user *argp = (void __user *)arg;
>  	struct sev_issue_cmd input;
>  	int ret = -EFAULT;
> +	bool writable = file->f_mode & FMODE_WRITE;
>  
>  	if (!psp_master || !psp_master->sev_data)
>  		return -ENODEV;
> @@ -819,25 +820,25 @@ static long sev_ioctl(struct file *file, unsigned int ioctl, unsigned long arg)
>  	switch (input.cmd) {
>  
>  	case SEV_FACTORY_RESET:
> -		ret = sev_ioctl_do_reset(&input);
> +		ret = sev_ioctl_do_reset(&input, writable);
>  		break;
>  	case SEV_PLATFORM_STATUS:
>  		ret = sev_ioctl_do_platform_status(&input);
>  		break;
>  	case SEV_PEK_GEN:
> -		ret = sev_ioctl_do_pek_pdh_gen(SEV_CMD_PEK_GEN, &input);
> +		ret = sev_ioctl_do_pek_pdh_gen(SEV_CMD_PEK_GEN, &input, writable);
>  		break;
>  	case SEV_PDH_GEN:
> -		ret = sev_ioctl_do_pek_pdh_gen(SEV_CMD_PDH_GEN, &input);
> +		ret = sev_ioctl_do_pek_pdh_gen(SEV_CMD_PDH_GEN, &input, writable);
>  		break;
>  	case SEV_PEK_CSR:
> -		ret = sev_ioctl_do_pek_csr(&input);
> +		ret = sev_ioctl_do_pek_csr(&input, writable);
>  		break;
>  	case SEV_PEK_CERT_IMPORT:
> -		ret = sev_ioctl_do_pek_import(&input);
> +		ret = sev_ioctl_do_pek_import(&input, writable);
>  		break;
>  	case SEV_PDH_CERT_EXPORT:
> -		ret = sev_ioctl_do_pdh_export(&input);
> +		ret = sev_ioctl_do_pdh_export(&input, writable);
>  		break;
>  	case SEV_GET_ID:
>  		pr_warn_once("SEV_GET_ID command is deprecated, use SEV_GET_ID2\n");
>