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Date: Thu, 7 Mar 2024 08:31:16 -0600
From: Seth Forshee <sforshee@kernel.org>
To: Roberto Sassu <roberto.sassu@huaweicloud.com>
Cc: zohar@linux.ibm.com, dmitry.kasatkin@gmail.com,
	eric.snowberg@oracle.com, paul@paul-moore.com, jmorris@namei.org,
	serge@hallyn.com, linux-integrity@vger.kernel.org,
	linux-security-module@vger.kernel.org, linux-kernel@vger.kernel.org,
	Roberto Sassu <roberto.sassu@huawei.com>, stable@vger.kernel.org,
	linux-fsdevel@vger.kernel.org,
	Christian Brauner <brauner@kernel.org>
Subject: Re: [PATCH] evm: Change vfs_getxattr() with __vfs_getxattr() in
 evm_calc_hmac_or_hash()
Message-ID: <ZenPtCfh6CyD2xz5@do-x1extreme>
References: <20240307122240.3560688-1-roberto.sassu@huaweicloud.com>
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In-Reply-To: <20240307122240.3560688-1-roberto.sassu@huaweicloud.com>

On Thu, Mar 07, 2024 at 01:22:39PM +0100, Roberto Sassu wrote:
> From: Roberto Sassu <roberto.sassu@huawei.com>
> 
> Use __vfs_getxattr() instead of vfs_getxattr(), in preparation for
> deprecating using the vfs_ interfaces for retrieving fscaps.
> 
> __vfs_getxattr() is only used for debugging purposes, to check if kernel
> space and user space see the same xattr value.

__vfs_getxattr() won't give you the value as seen by userspace though.
Userspace goes through vfs_getxattr() -> xattr_getsecurity() ->
cap_inode_getsecurity(), which does the conversion to the value
userspace sees. __vfs_getxattr() just gives the raw disk data.

I'm also currently working on changes to my fscaps series that will make
it so that __vfs_getxattr() also cannot be used to read fscaps xattrs.
I'll fix this and other code in EVM which will be broken by that change
as part of the next version too.

> 
> Cc: stable@vger.kernel.org # 5.14.x
> Cc: linux-fsdevel@vger.kernel.org
> Cc: Christian Brauner <brauner@kernel.org>
> Cc: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
> Fixes: 907a399de7b0 ("evm: Check xattr size discrepancy between kernel and user")
> Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
> ---
>  security/integrity/evm/evm_crypto.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/security/integrity/evm/evm_crypto.c b/security/integrity/evm/evm_crypto.c
> index b1ffd4cc0b44..168d98c63513 100644
> --- a/security/integrity/evm/evm_crypto.c
> +++ b/security/integrity/evm/evm_crypto.c
> @@ -278,8 +278,8 @@ static int evm_calc_hmac_or_hash(struct dentry *dentry,
>  		if (size < 0)
>  			continue;
>  
> -		user_space_size = vfs_getxattr(&nop_mnt_idmap, dentry,
> -					       xattr->name, NULL, 0);
> +		user_space_size = __vfs_getxattr(dentry, inode, xattr->name,
> +						 NULL, 0);
>  		if (user_space_size != size)
>  			pr_debug("file %s: xattr %s size mismatch (kernel: %d, user: %d)\n",
>  				 dentry->d_name.name, xattr->name, size,
> -- 
> 2.34.1
>