Re: [PATCH 01/13] sysfs: Update sysfs_setxattr so it updates secdata under the sysfs_mutex

[Eric W. Biederman]

"Serge E. Hallyn" <serue@xxxxxxxxxx> writes:

> Quoting Eric W. Biederman (ebiederm@xxxxxxxxxxxx):
>> From: Eric W. Biederman <ebiederm@xxxxxxxxxxxxxxxxxx>
>> 
>> The sysfs_mutex is required to ensure updates are and will remain
>> atomic with respect to other inode iattr updates, that do not happen
>> through the filesystem.
>> 
>> Signed-off-by: Eric W. Biederman <ebiederm@xxxxxxxxxxxxxxxxxx>
>> ---
>>  fs/sysfs/inode.c |   41 +++++++++++++++++++++++++++++------------
>>  1 files changed, 29 insertions(+), 12 deletions(-)
>> 
>> diff --git a/fs/sysfs/inode.c b/fs/sysfs/inode.c
>> index e28cecf..8a08250 100644
>> --- a/fs/sysfs/inode.c
>> +++ b/fs/sysfs/inode.c
>> @@ -122,23 +122,39 @@ int sysfs_setattr(struct dentry * dentry, struct iattr * iattr)
>>  	return error;
>>  }
>> 
>> +static int sysfs_sd_setsecdata(struct sysfs_dirent *sd, void **secdata, u32 *secdata_len)
>> +{
>> +	struct sysfs_inode_attrs *iattrs;
>> +	void *old_secdata;
>> +	size_t old_secdata_len;
>> +
>> +	iattrs = sd->s_iattr;
>> +	if (!iattrs)
>> +		iattrs = sysfs_init_inode_attrs(sd);
>> +	if (!iattrs)
>> +		return -ENOMEM;
>> +
>> +	old_secdata = iattrs->ia_secdata;
>> +	old_secdata_len = iattrs->ia_secdata_len;
>> +
>> +	iattrs->ia_secdata = *secdata;
>> +	iattrs->ia_secdata_len = *secdata_len;
>> +
>> +	*secdata = old_secdata;
>> +	*secdata_len = old_secdata_len;
>> +	return 0;
>> +}
>> +
>>  int sysfs_setxattr(struct dentry *dentry, const char *name, const void *value,
>>  		size_t size, int flags)
>>  {
>>  	struct sysfs_dirent *sd = dentry->d_fsdata;
>> -	struct sysfs_inode_attrs *iattrs;
>>  	void *secdata;
>>  	int error;
>>  	u32 secdata_len = 0;
>> 
>>  	if (!sd)
>>  		return -EINVAL;
>> -	if (!sd->s_iattr)
>> -		sd->s_iattr = sysfs_init_inode_attrs(sd);
>> -	if (!sd->s_iattr)
>> -		return -ENOMEM;
>> -
>> -	iattrs = sd->s_iattr;
>> 
>>  	if (!strncmp(name, XATTR_SECURITY_PREFIX, XATTR_SECURITY_PREFIX_LEN)) {
>>  		const char *suffix = name + XATTR_SECURITY_PREFIX_LEN;
>> @@ -150,12 +166,13 @@ int sysfs_setxattr(struct dentry *dentry, const char *name, const void *value,
>>  						&secdata, &secdata_len);
>>  		if (error)
>>  			goto out;
>> -		if (iattrs->ia_secdata)
>> -			security_release_secctx(iattrs->ia_secdata,
>> -						iattrs->ia_secdata_len);
>> -		iattrs->ia_secdata = secdata;
>> -		iattrs->ia_secdata_len = secdata_len;
>> 
>> +		mutex_lock(&sysfs_mutex);
>> +		error = sysfs_sd_setsecdata(sd, &secdata, &secdata_len);
>
> You're ignoring the potential -ENOMEM return value here?
>
> Worse, if -ENOMEM, then secdata was never set, so you call
> security_release_secctx() on a random value left on the stack...

No.  It is more elegant than that.
If sysfs_sd_setsecdata fails secdata holds the freshly allocated secdata value.
If sysfs_sd_setsecdata succeeds secdata holds the old value of secdata.

Eric
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