linux-kernel - Re: [PATCH] evm: Change vfs_getxattr() with __vfs_getxattr() in evm_calc_hmac_or

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Date: Thu, 07 Mar 2024 16:02:20 +0100
From: Roberto Sassu <roberto.sassu@...weicloud.com>
To: Seth Forshee <sforshee@...nel.org>
Cc: zohar@...ux.ibm.com, dmitry.kasatkin@...il.com,
 eric.snowberg@...cle.com,  paul@...l-moore.com, jmorris@...ei.org,
 serge@...lyn.com,  linux-integrity@...r.kernel.org,
 linux-security-module@...r.kernel.org,  linux-kernel@...r.kernel.org,
 Roberto Sassu <roberto.sassu@...wei.com>,  stable@...r.kernel.org,
 linux-fsdevel@...r.kernel.org, Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH] evm: Change vfs_getxattr() with __vfs_getxattr() in
 evm_calc_hmac_or_hash()

On Thu, 2024-03-07 at 15:36 +0100, Roberto Sassu wrote:
> On Thu, 2024-03-07 at 08:31 -0600, Seth Forshee wrote:
> > On Thu, Mar 07, 2024 at 01:22:39PM +0100, Roberto Sassu wrote:
> > > From: Roberto Sassu <roberto.sassu@...wei.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.
> 
> You are right, thank you!

(Apologies, I should have been more careful).

Roberto

> Roberto
> 
> > > 
> > > Cc: stable@...r.kernel.org # 5.14.x
> > > Cc: linux-fsdevel@...r.kernel.org
> > > Cc: Christian Brauner <brauner@...nel.org>
> > > Cc: Seth Forshee (DigitalOcean) <sforshee@...nel.org>
> > > Fixes: 907a399de7b0 ("evm: Check xattr size discrepancy between kernel and user")
> > > Signed-off-by: Roberto Sassu <roberto.sassu@...wei.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
> > >