Re: [PATCH] evm: check for remount ro in progress before writing

[Mimi Zohar <zohar@linux.vnet.ibm.com>]

On Thu, 2018-03-01 at 13:38 +0100, Sascha Hauer wrote:
> EVM might update the evm xattr while the VFS performs a remount to
> readonly mode. This is not properly checked for, additionally check
> the s_readonly_remount superblock flag before writing.
> The bug can for example be observed with UBIFS. UBIFS checks the free
> space on the device before and after a remount. With EVM enabled the
> free space sometimes differs between both checks.
> 
> Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
> ---
>  security/integrity/evm/evm_main.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
> index a8d502827270..4a147a998aa7 100644
> --- a/security/integrity/evm/evm_main.c
> +++ b/security/integrity/evm/evm_main.c
> @@ -185,6 +185,7 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
>  					iint->flags |= EVM_IMMUTABLE_DIGSIG;
>  				evm_status = INTEGRITY_PASS_IMMUTABLE;
>  			} else if (!IS_RDONLY(d_backing_inode(dentry)) &&
> +				   !(dentry->d_inode->i_sb->s_readonly_remount) &&

The other 2 conditions are using d_back_inode().  Shouldn't this one
be too?

Mimi

>  				   !IS_IMMUTABLE(d_backing_inode(dentry))) {
>  				evm_update_evmxattr(dentry, xattr_name,
>  						    xattr_value,