Re: Semantics of blktrace with lockdown (integrity) enabled kernel.

[Paul Moore]

On Thu, Apr 6, 2023 at 3:30 PM Junxiao Bi <junxiao.bi@xxxxxxxxxx> wrote:
> On 4/6/23 11:39 AM, Paul Moore wrote:
> > On Thu, Apr 6, 2023 at 1:38 PM Konrad Rzeszutek Wilk
> > <konrad.wilk@xxxxxxxxxx> wrote:
> >> Hey Jens, Paul, James, Nathan,
> >>
> >> We are trying to use blktrace with a kernel that has lockdown enabled and find that it cannot run.
> >>
> >> Specifically the issue is that we are trying to do is pretty simple:
> >>
> >> strace -f blktrace -d /dev/sda -w 60
> >>
> >> [pid 148882] <... mprotect resumed>)    = 0
> >> [pid 148881] openat(AT_FDCWD, "/sys/kernel/debug/block/sda/trace0", O_RDONLY|O_NONBLOCK <unfinished ...>
> >> [pid 148882] sched_setaffinity(0, 8, [1]) = 0
> >> [pid 148881] <... openat resumed>)      = -1 EPERM (Operation not permitted)
> >>
> >> which fails. The analysis from Eric (CCed) is that
> >>
> >> All debugfs entries do not exist until blktrace is run.  It is opening
> >> /sys/kernel/debug/block/sda/trace0 which isn’t there normally. While running the utility,
> >> to place something in it, it must have the write permission set.  When exiting out of
> >> blktrace, the entry is gone, both on a machine running with secure boot enabled
> >> and one with it disabled.  Which also indicates the write permission was set,
> >> otherwise the entry would still be there.
> >>
> >> The fix is simple enough (see attachment) but we are not sure about the semantics of what
> >> lockdown has in mind.
> >>
> >> Looking at the include/linux/security.h the LOCKDOWN_TRACEFS exists which would
> >> imply that it is expected that operations with tracefs *should* work with lockdown (integrity mode).
> >>
> >> But at the same point, debugfs writable attributes are a nono with lockdown.
> >>
> >> So what is the right way forward?
> > What did you use as a basis for your changes?  I'm looking at the
> > patch you sent and it appears to be making a change to a
> > debugfs_lockdown_whitelisted() function defined in
> > fs/debugfs/internal.h which does not exist in Linus' tree.  If I
> > search through all of the archives on lore.kernel.org the only hit I
> > get is your email, so it seems doubtful it is in a subsystem tree
> > which hasn't made its way to Linus yet.
> >
> > Before we go any further, can you please verify that your issue is
> > reproducible on a supported, upstream tree (preferably Linus')?
>
> The patch attached is applied to oracle kernel which is just used to
> demo the idea of a possible fix.
>
> Upstream will have the same issue because blktrace uses relay files from
> debugfs to transfer trace information from kernel to userspace ...

For future reference, the problem with sending patches that aren't
based on an upstream tree is that it both wastes reviewer time trying
to figure out where the basis of the patch, and it makes one question
if the issue is present in an upstream kernel or if there is some
out-of-tree patch in the unknown kernel which is the root cause.

Maybe you've tested everything and know it is a problem with the
upstream code, but when we see a patch that doesn't match up with
anything, how are we supposed to know?

-- 
paul-moore.com