On 7/23/23 7:22 PM, Steve Grubb wrote:
On Saturday, July 22, 2023 2:01:34 AM EDT Matthew Garrett wrote:
A discussion within Debian again brought up the problem that:
1) Automounting of removable media exposes the kernel to a lot of
untrusted input
2) Kernel upstream are not terribly concerned with ensuring that kernel
filesystems are resilient against deliberately malformed filesystems so
are mostly not proactively looking for bugs there
3) Uncommonly used filesystems are less likely to be tested against
adverse input in the real world and so are more likely to contain
exploitable bugs
There are various cases where people do need to make use of uncommon
filesystems, but the majority of them aren't related to removable media.
udisks2 supports setting the UDISKS_AUTO variable to 0 on devices that
shouldn't be automounted, which means something like:
SUBSYSTEM!="block", GOTO="udisks_insecure_fs_end"
ENV{ID_FS_TYPE}=="hfs", ENV{UDISKS_AUTO}="0"
# repeat as necessary for anything else that shouldn't be automounted
LABEL="udisks_insecure_fs_end"
ought to be enough. So:
a) Does this seem like a good idea?
b) If so, is dealing with it via udev rules the right approach? This way
seems desktop-agnostic
c) Where should it ship, and what should the process be for disabling it
for people who need this functionality?
Long-term I'd love to see more work put into having FUSE support for
these and leaving the in-kernel fs to stuff we know is trustworthy, but
that's rather more work.
If "a malicious input can't cause problems" is the threshold for
trustworthy, I'm not sure we have any trustworthy filesystems as this point.
https://syzkaller.appspot.com/upstream/s/ext4
https://syzkaller.appspot.com/upstream/s/xfs
https://syzkaller.appspot.com/upstream/s/btrfs
https://syzkaller.appspot.com/upstream/s/fat
A while back, I wrote the fsfuzzer specifically to find, in a repeatable way,
filesystem bugs so they can be fixed:
https://github.com/stevegrubb/fsfuzzer
It does not support all filesystems, but it is easy to add support through
adding the correct mounter to the scrips. It has found *so* *many* filesystem
bugs over time.
That was awesome, back in the day! syzbot/syzcaller is the new shiny
here though, finding filesystem flaws day after day that (with all due
respect) fsfuzzer could never have reached (think: fuzzing metadata and
then fixing up the checksum so it passes initial validation on read.)
And frankly that is some of my motivation to find an improvement here. A
small cadre of filesystem developers are near the breaking point trying
to keep up with an army of machines running syzkaller.
-Eric
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