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

Eric W. Biederman
2009-11-03T21:09:15Z

"Serge E. Hallyn" <serue@us.ibm.com> writes:

> Quoting Eric W. Biederman (ebiederm@xmission.com):
>> From: Eric W. Biederman <ebiederm@aristanetworks.com>
>>
>> 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@aristanetworks.com>
>> ---
>> 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