linux-kernel - Re: [PATCH 1/1] crypto: ccp: use file mode for sev ioctl permissions

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Message-ID: <20200312124001.GG28885@gondor.apana.org.au>
Date:   Thu, 12 Mar 2020 23:40:01 +1100
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Connor Kuehl <ckuehl@...hat.com>
Cc:     thomas.lendacky@....com, davem@...emloft.net, gary.hook@....com,
        erdemaktas@...gle.com, rientjes@...gle.com, brijesh.singh@....com,
        npmccallum@...hat.com, bsd@...hat.com,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] crypto: ccp: use file mode for sev ioctl permissions

On Fri, Mar 06, 2020 at 09:20:10AM -0800, 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@...hat.com>
> ---
>  drivers/crypto/ccp/sev-dev.c | 33 +++++++++++++++++----------------
>  1 file changed, 17 insertions(+), 16 deletions(-)

Patch applied.  Thanks.
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt