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SubjectRe: [PATCH v2] cifs: fix strcat buffer overflow and reduce raciness in smb21_set_oplock_level()
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at 02:42, Pavel Shilovsky <piastryyy@gmail.com> wrote:

> ср, 8 мая 2019 г. в 01:23, Kai-Heng Feng <kai.heng.feng@canonical.com>:
>> at 02:28, Pavel Shilovsky <piastryyy@gmail.com> wrote:
>>
>>> вт, 7 мая 2019 г. в 09:13, Steve French via samba-technical
>>> <samba-technical@lists.samba.org>:
>>>> merged into cifs-2.6.git for-next
>>>>
>>>> On Tue, May 7, 2019 at 10:17 AM Christoph Probst via samba-technical
>>>> <samba-technical@lists.samba.org> wrote:
>>>>> Change strcat to strncpy in the "None" case to fix a buffer overflow
>>>>> when cinode->oplock is reset to 0 by another thread accessing the same
>>>>> cinode. It is never valid to append "None" to any other message.
>>>>>
>>>>> Consolidate multiple writes to cinode->oplock to reduce raciness.
>>>>>
>>>>> Signed-off-by: Christoph Probst <kernel@probst.it>
>>>>> ---
>>>>> fs/cifs/smb2ops.c | 14 ++++++++------
>>>>> 1 file changed, 8 insertions(+), 6 deletions(-)
>>>>>
>>>>> diff --git a/fs/cifs/smb2ops.c b/fs/cifs/smb2ops.c
>>>>> index c36ff0d..aa61dcf 100644
>>>>> --- a/fs/cifs/smb2ops.c
>>>>> +++ b/fs/cifs/smb2ops.c
>>>>> @@ -2917,26 +2917,28 @@ smb21_set_oplock_level(struct cifsInodeInfo
>>>>> *cinode, __u32 oplock,
>>>>> unsigned int epoch, bool *purge_cache)
>>>>> {
>>>>> char message[5] = {0};
>>>>> + unsigned int new_oplock = 0;
>>>>>
>>>>> oplock &= 0xFF;
>>>>> if (oplock == SMB2_OPLOCK_LEVEL_NOCHANGE)
>>>>> return;
>>>>>
>>>>> - cinode->oplock = 0;
>>>>> if (oplock & SMB2_LEASE_READ_CACHING_HE) {
>>>>> - cinode->oplock |= CIFS_CACHE_READ_FLG;
>>>>> + new_oplock |= CIFS_CACHE_READ_FLG;
>>>>> strcat(message, "R");
>>>>> }
>>>>> if (oplock & SMB2_LEASE_HANDLE_CACHING_HE) {
>>>>> - cinode->oplock |= CIFS_CACHE_HANDLE_FLG;
>>>>> + new_oplock |= CIFS_CACHE_HANDLE_FLG;
>>>>> strcat(message, "H");
>>>>> }
>>>>> if (oplock & SMB2_LEASE_WRITE_CACHING_HE) {
>>>>> - cinode->oplock |= CIFS_CACHE_WRITE_FLG;
>>>>> + new_oplock |= CIFS_CACHE_WRITE_FLG;
>>>>> strcat(message, "W");
>>>>> }
>>>>> - if (!cinode->oplock)
>>>>> - strcat(message, "None");
>>>>> + if (!new_oplock)
>>>>> + strncpy(message, "None", sizeof(message));
>>>>> +
>>>>> + cinode->oplock = new_oplock;
>>>>> cifs_dbg(FYI, "%s Lease granted on inode %p\n", message,
>>>>> &cinode->vfs_inode);
>>>>> }
>>>>> --
>>>>> 2.1.4
>>
>> Doesn’t the race still happen, but implicitly here?
>> cinode->oplock = new_oplock;
>>
>> Is it possible to just introduce a lock to force its proper ordering?
>> e.g.
>>
>> diff --git a/fs/cifs/cifsfs.c b/fs/cifs/cifsfs.c
>> index bf5b8264e119..a3c3c6156d17 100644
>> --- a/fs/cifs/cifsfs.c
>> +++ b/fs/cifs/cifsfs.c
>> @@ -267,6 +267,7 @@ cifs_alloc_inode(struct super_block *sb)
>> * server, can not assume caching of file data or metadata.
>> */
>> cifs_set_oplock_level(cifs_inode, 0);
>> + mutex_init(&cifs_inode->oplock_mutex);
>> cifs_inode->flags = 0;
>> spin_lock_init(&cifs_inode->writers_lock);
>> cifs_inode->writers = 0;
>> diff --git a/fs/cifs/cifsglob.h b/fs/cifs/cifsglob.h
>> index 37b5ddf27ff1..6dfd4ab16c4f 100644
>> --- a/fs/cifs/cifsglob.h
>> +++ b/fs/cifs/cifsglob.h
>> @@ -1214,6 +1214,7 @@ struct cifsInodeInfo {
>> struct list_head openFileList;
>> __u32 cifsAttrs; /* e.g. DOS archive bit, sparse, compressed, system */
>> unsigned int oplock; /* oplock/lease level we have */
>> + struct mutex oplock_mutex;
>> unsigned int epoch; /* used to track lease state changes */
>> #define CIFS_INODE_PENDING_OPLOCK_BREAK (0) /* oplock break in progress */
>> #define CIFS_INODE_PENDING_WRITERS (1) /* Writes in progress */
>> diff --git a/fs/cifs/smb2ops.c b/fs/cifs/smb2ops.c
>> index b20063cf774f..796b23712e71 100644
>> --- a/fs/cifs/smb2ops.c
>> +++ b/fs/cifs/smb2ops.c
>> @@ -1901,6 +1901,7 @@ smb21_set_oplock_level(struct cifsInodeInfo *cinode,
>> __u32 oplock,
>> if (oplock == SMB2_OPLOCK_LEVEL_NOCHANGE)
>> return;
>>
>> + mutex_lock(&cinode->oplock_mutex);
>> cinode->oplock = 0;
>> if (oplock & SMB2_LEASE_READ_CACHING_HE) {
>> cinode->oplock |= CIFS_CACHE_READ_FLG;
>> @@ -1916,6 +1917,8 @@ smb21_set_oplock_level(struct cifsInodeInfo *cinode,
>> __u32 oplock,
>> }
>> if (!cinode->oplock)
>> strcat(message, "None");
>> + mutex_unlock(&cinode->oplock_mutex);
>> +
>> cifs_dbg(FYI, "%s Lease granted on inode %p\n", message,
>> &cinode->vfs_inode);
>> }
>>
>> Kai-Heng
>
> Unless you calculations on the oplock value or accessing it multiple
> times with some logic involved I don't think locking will help much.
> If two threads are assigning the same variable, you can end up with
> two possible outcomes regardless of whether locking is used or not.

Yes you are right, didn’t think of this case.

>
> Locking will be needed once we start to make proper decisions based on
> previous and new values of the oplock to purge a page cache or flush
> buffered data. This still needs to be done and is out of the scope of
> this patch which aims to fix the buffer overflow error.

Thanks for your explanation.

Kai-Heng

>
> --
> Best regards,
> Pavel Shilovsky


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