Re: [PATCH v1] fs: Fix inconsistent f_mode

["Mickaël Salaün" <mic@digikod.net>]

On 01/03/2022 10:22, Christian Brauner wrote:
> On Mon, Feb 28, 2022 at 10:59:35PM +0100, Mickaël Salaün wrote:
>> From: Mickaël Salaün <mic@linux.microsoft.com>
>>
>> While transitionning to ACC_MODE() with commit 5300990c0370 ("Sanitize
>> f_flags helpers") and then fixing it with commit 6d125529c6cb ("Fix
>> ACC_MODE() for real"), we lost an open flags consistency check.  Opening
>> a file with O_WRONLY | O_RDWR leads to an f_flags containing MAY_READ |
>> MAY_WRITE (thanks to the ACC_MODE() helper) and an empty f_mode.
>> Indeed, the OPEN_FMODE() helper transforms 3 (an incorrect value) to 0.
>>
>> Fortunately, vfs_read() and vfs_write() both check for FMODE_READ, or
>> respectively FMODE_WRITE, and return an EBADF error if it is absent.
>> Before commit 5300990c0370 ("Sanitize f_flags helpers"), opening a file
>> with O_WRONLY | O_RDWR returned an EINVAL error.  Let's restore this safe
>> behavior.
> 
> That specific part seems a bit risky at first glance. Given that the
> patch referenced is from 2009 this means we've been allowing O_WRONLY |
> O_RDWR to succeed for almost 13 years now.

Yeah, it's an old bug, but we should keep in mind that a file descriptor 
created with such flags cannot be used to read nor write. However, 
unfortunately, it can be used for things like ioctl, fstat, chdir… I 
don't know if there is any user of this trick.

Either way, there is an inconsistency between those using ACC_MODE() and 
those using OPEN_FMODE(). If we decide to take a side for the behavior 
of one or the other, without denying to create such FD, it could also 
break security policies. We have to choose what to potentially break…


> 
>>
>> To make it consistent with ACC_MODE(), this patch also changes
>> OPEN_FMODE() to return FMODE_READ | FMODE_WRITE for O_WRONLY | O_RDWR.
>> This may help protect from potential spurious issues.
>>
>> This issue could result in inconsistencies with AppArmor, Landlock and
>> SELinux, but the VFS checks would still forbid read and write accesses.
>> Tomoyo uses the ACC_MODE() transformation which is correct, and Smack
>> doesn't check the file mode.  Filesystems using OPEN_FMODE() should also
>> be protected by the VFS checks.
>>
>> Fixes: 5300990c0370 ("Sanitize f_flags helpers")
>> Cc: Al Viro <viro@zeniv.linux.org.uk>
>> Cc: Casey Schaufler <casey@schaufler-ca.com>
>> Cc: Darrick J. Wong <djwong@kernel.org>
>> Cc: Eric Paris <eparis@parisplace.org>
>> Cc: John Johansen <john.johansen@canonical.com>
>> Cc: Kentaro Takeda <takedakn@nttdata.co.jp>
>> Cc: Miklos Szeredi <miklos@szeredi.hu>
>> Cc: Paul Moore <paul@paul-moore.com>
>> Cc: Stephen Smalley <stephen.smalley.work@gmail.com>
>> Cc: Steve French <sfrench@samba.org>
>> Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
>> Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
>> Link: https://lore.kernel.org/r/20220228215935.748017-1-mic@digikod.net
>> ---
>>   fs/file_table.c    | 3 +++
>>   include/linux/fs.h | 5 +++--
>>   2 files changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/file_table.c b/fs/file_table.c
>> index 7d2e692b66a9..b936f69525d0 100644
>> --- a/fs/file_table.c
>> +++ b/fs/file_table.c
>> @@ -135,6 +135,9 @@ static struct file *__alloc_file(int flags, const struct cred *cred)
>>   	struct file *f;
>>   	int error;
>>   
>> +	if ((flags & O_ACCMODE) == O_ACCMODE)
>> +		return ERR_PTR(-EINVAL);
>> +
>>   	f = kmem_cache_zalloc(filp_cachep, GFP_KERNEL);
>>   	if (unlikely(!f))
>>   		return ERR_PTR(-ENOMEM);
>> diff --git a/include/linux/fs.h b/include/linux/fs.h
>> index e2d892b201b0..83bc5aaf1c41 100644
>> --- a/include/linux/fs.h
>> +++ b/include/linux/fs.h
>> @@ -3527,8 +3527,9 @@ int __init list_bdev_fs_names(char *buf, size_t size);
>>   #define __FMODE_NONOTIFY	((__force int) FMODE_NONOTIFY)
>>   
>>   #define ACC_MODE(x) ("\004\002\006\006"[(x)&O_ACCMODE])
>> -#define OPEN_FMODE(flag) ((__force fmode_t)(((flag + 1) & O_ACCMODE) | \
>> -					    (flag & __FMODE_NONOTIFY)))
>> +#define OPEN_FMODE(flag) ((__force fmode_t)( \
>> +			(((flag + 1) & O_ACCMODE) ?: O_ACCMODE) | \
>> +			(flag & __FMODE_NONOTIFY)))
>>   
>>   static inline bool is_sxid(umode_t mode)
>>   {
>>
>> base-commit: 7e57714cd0ad2d5bb90e50b5096a0e671dec1ef3
>> -- 
>> 2.35.1
>>