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SubjectRe: [RFC][PATCH 0/3] add FALLOC_FL_NO_HIDE_STALE flag in fallocate
On Wed, 18 Apr 2012, Zheng Liu wrote:

> On Tue, Apr 17, 2012 at 12:40:14PM -0500, Eric Sandeen wrote:
> > On 4/17/12 11:53 AM, Zheng Liu wrote:
> > > Hi list,
> > >
> > > fallocate is a useful system call because it can preallocate some disk blocks
> > > for a file and keep blocks contiguous. However, it has a defect that file
> > > system will convert an uninitialized extent to be an initialized when the user
> > > wants to write some data to this file, because file system create an
> > > unititalized extent while it preallocates some blocks in fallocate (e.g. ext4).
> >
> > That's a security-driven design feature, not a defect. :)

Exactly! It really surprise me that you call it a defect, moreover the
name you've chosen for the flag is IMO wrong. With security hole like
this you really want give user very strong feeling that he is NOT supposed
to use it if he does not know what he is doing. Maybe something like
FALLOC_FL_EXPOSE_USER_DATA because that is what it does.

> >
> > > Especially, it causes a severe degradation when the user tries to do some
> > > random write operations, which frequently modifies the metadata of this file.
> > > We meet this problem in our product system at Taobao. Last month, in ext4
> > > workshop, we discussed this problem and the Google faces the same problem. So
> > > a new flag, FALLOC_FL_NO_HIDE_STALE, is added in order to solve this problem.
> >
> > Which then opens up severe security problems.
> >
> > > When this flag is set, file system will create an inititalized extent for this
> > > file. So it avoids the conversion from uninitialized to initialized. If users
> > > want to use this flag, they must guarantee that file has been initialized by
> > > themselves before it is read at the same offset. This flag is added in vfs so
> > > that other file systems can also support this flag to improve the performance.
> > >
> > > I try to make ext4 support this new flag, and run a simple test in my own
> > > desktop to verify it. The machine has a Intel(R) Core(TM)2 Duo CPU E8400, 4G
> > > memory and a WDC WD1600AAJS-75M0A0 160G SATA disk. I use the following script
> > > to tset the performance.
> > >
> > > #/bin/sh
> > > mkfs.ext4 ${DEVICE}
> > > mount -t ext4 ${DEVICE} ${TARGET}
> > > fallocate -l 27262976 ${TARGET}/test # the size of the file is 256M (*)
> >
> > That's only 26MB, but the below loop writes to a max offset of around
> > 256M.
>
> Yes, you are right. I preallocate a file that is 256MB.
>
> >
> > > time for((i=0;i<2000;i++)); do dd if=/dev/zero of=/mnt/sda1/test_256M \
> > > conv=notrunc bs=4k count=1 seek=`expr $i \* 16` oflag=sync,direct \
> > > 2>/dev/null; done
> >
> > You fallocate ${TARGET}/test but dd to /mnt/sda1/test_256M ? I presume
> > those should be the same file.
>
> Yes, it is the same file.

But it is not in your test case. Does it mean that your test case is
wrong ?

>
> >
> > So the testcase as shown above seems fairly wrong, no? Is that what you
> > used for the numbers below?
> >
> > > * I write a wrapper program to call fallocate(2) with FALLOC_FL_NO_HIDE_STALE
> > > flag because the userspace tool doesn't support the new flag.
> > >
> > > The result:
> > > w/o w/
> > > real 1m16.043s 0m17.946s -76.4%
> > > user 0m0.195s 0m0.192s -1.54%
> > > sys 0m0.468s 0m0.462s -1.28%
> >
> > I think that the missing information here is some profiling to show where
> > the time was spent in the "w/o" case. What, exactly, in ext4 extent
> > management is so darned slow that we have to poke security holes in the
> > filesytem to get decent performance?
> >
> > However,, the above also seems like an alarmingly large difference, and
> > one that I can't attribute to unwritten extent conversion overhead.
> >
> > If I test the seeky dd to a prewritten file (to eliminate extent
> > conversion):
> >
> > # dd if=/dev/zero of=/mnt/scratch/test bs=1M count=256
> > # sync
> >
> > vs. to a fallocated file (which requires extent conversion):
> >
> > # fallocate -l 256m /mnt/scratch/test
> >
> > and then do your seeky dd test after each of the above:
> >
> > # time for((i=0;i<2000;i++)); do dd if=/dev/zero of=/mnt/scratch/test \
> > conv=notrunc bs=4k count=1 seek=`expr $i \* 16` oflag=sync,direct \
> > 2>/dev/null; done
> >
> > On ext4, I get about 59.9 seconds in the pre-written case, 65.2 seconds in the fallocated case.
> >
> > On xfs, I get about 52.5 seconds in the pre-written case, 52.7 seconds in the fallocated case.
> >
> > I don't see anywhere near the slowdown you show above, certainly
> > nothing bad enough to warrant opening the big security hole.
> > Am I missing something?
>
> I will run more detailed benchmarks to trace this issue. If I have a
> lastest result, I will send a new mail to let you know. :)

Yes, please do. I do not think that in this case simply doing 'time
dd' is enough. We need to know what is happening inside the kernel to see
what is causing the slowdown. It may very well be that we might be able
to fix it, rather than introduce crazy security hole to bypass the
problem.

>
> I fully understand that this flag will bring a security hole, and I
> totally agree that we should dig *root cause* in ext4. But, IMHO, a
> proper interface, which is limited by a proper capabilities, might be
> useful for the user.

No, you're not getting the point. In kernel (or fs) we are supposed to
prevent from security problems as much as possible and it is
indisputably the job of file systems to prevent reading data which are
NOT in the file system and might have belonged to the other user. If you
want to do this, just use raw device.

Limiting this by proper capabilities is so not enough from tons of
reasons. Application have bugs, which may very well cause the stale data
to be exported directly to the user of the application which might not
even have root capabilities. Consider buggy database returning stale
data (possibly deleted secret informations) from the on user to
another, it _is_ possible and things like that _will_ happen with
this "feature", what is even worse that in this case the application
writers do actually use the fallocate flag right.

Moreover, a lot of application are getting rid of the root capabilities
once they are executed and initialized from the obvious reason. Consider
the case where the application is doing fallocate-expose-stale-data
before betting rid of the root cap, then after it gets rid of the root
cap, the "regular user" have access to the other users old data.

And it might not even be the case of buggy application, consider very
simple case where root fallocate-expose-stale-data a file and did not
set read permission right, then *any* user can read other users data.
This will definitely happen as this kind of errors happens all the time.

And also you're assuming that the application developer or root
administrator know exactly what the problem is and can see all the
corner cases as are the few I've described above, that's not the case,
they will get it wrong in some cases especially when it will be promoted
as performance boost.

I do not like this "feature" at all, what about rather fixing the
problem ? Xfs does not seem to have this issue, at least not as
noticeable, so it is definitely possible.

Thanks!
-Lukas

>
> Regards,
> Zheng
>
> >
> > The ext4 delta is a bit larger, though, so it seems worth investigating
> > the *root cause* of the extra overhead if it's problematic in your
> > workloads...
> >
> > -Eric
> >
> >
> > > Obviously, this flag will bring an secure issue because the malicious user
> > > could use this flag to get other user's data if (s)he doesn't do a
> > > initialization before reading this file. Thus, a sysctl parameter
> > > 'fs.falloc_no_hide_stale' is defined in order to let administrator to determine
> > > whether or not this flag is enabled. Currently, this flag is disabled by
> > > default. I am not sure whether this is enough or not. Another option is that
> > > a new Kconfig entry is created to remove this flag during the kernel is
> > > complied. So any suggestions or comments are appreciated.
> > >
> > > Regards,
> > > Zheng
> > >
> > > Zheng Liu (3):
> > > vfs: add FALLOC_FL_NO_HIDE_STALE flag in fallocate
> > > vfs: add security check for _NO_HIDE_STALE flag
> > > ext4: add FALLOC_FL_NO_HIDE_STALE support
> > >
> > > fs/ext4/extents.c | 7 +++++--
> > > fs/open.c | 12 +++++++++++-
> > > include/linux/falloc.h | 5 +++++
> > > include/linux/sysctl.h | 1 +
> > > kernel/sysctl.c | 10 ++++++++++
> > > 5 files changed, 32 insertions(+), 3 deletions(-)
> > > --
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> >
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