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

[Paul Moore]

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')?

-- 
paul-moore.com