Re: [PATCH 10/20] signal/vm86_32: Properly send SIGSEGV when the vm86 state cannot be saved.

[Eric W. Biederman]

"Andy Lutomirski" <luto@xxxxxxxxxx> writes:

> On Wed, Oct 20, 2021, at 10:43 AM, Eric W. Biederman wrote:
>> Instead of pretending to send SIGSEGV by calling do_exit(SIGSEGV)
>> call force_sigsegv(SIGSEGV) to force the process to take a SIGSEGV
>> and terminate.
>
> Why?  I realize it's more polite, but is this useful enough to justify
> the need for testing and potential security impacts?

The why is that do_exit as an interface needs to be refactored.

As it exists right now "do_exit" is bad enough that on a couple of older
architectures do_exit in a random location results in being able to
read/write the kernel stack using ptrace.

So to addresses the issues I need to get everything that really
shouldn't be using do_exit to use something else.

>> Update handle_signal to return immediately when save_v86_state fails
>> and kills the process.  Returning immediately without doing anything
>> except killing the process with SIGSEGV is also what signal_setup_done
>> does when setup_rt_frame fails.  Plus it is always ok to return
>> immediately without delivering a signal to a userspace handler when a
>> fatal signal has killed the current process.
>>
>
> I can mostly understand the individual sentences, but I don't
> understand what you're getting it.  If a fatal signal has killed the
> current process and we are guaranteed not to hit the exit-to-usermode
> path, then, sure, it's safe to return unless we're worried that the
> core dump code will explode.
>
> But, unless it's fixed elsewhere in your series, force_sigsegv() is
> itself quite racy, or at least looks racy -- it can race against
> another thread calling sigaction() and changing the action to
> something other than SIG_DFL.  So it does not appear to actually
> reliably kill the caller, especially if exposed to a malicious user
> program.

You are correct about the races.  I have changes in the works to make
the races go away but that is not an excuse for push a change that
is buggy without them.



>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>> Cc: Borislav Petkov <bp@xxxxxxxxx>
>> Cc: x86@xxxxxxxxxx
>> Cc: H Peter Anvin <hpa@xxxxxxxxx>
>> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
>> ---
>>  arch/x86/kernel/signal.c  | 6 +++++-
>>  arch/x86/kernel/vm86_32.c | 2 +-
>>  2 files changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
>> index f4d21e470083..25a230f705c1 100644
>> --- a/arch/x86/kernel/signal.c
>> +++ b/arch/x86/kernel/signal.c
>> @@ -785,8 +785,12 @@ handle_signal(struct ksignal *ksig, struct pt_regs *regs)
>>  	bool stepping, failed;
>>  	struct fpu *fpu = &current->thread.fpu;
>> 
>> -	if (v8086_mode(regs))
>> +	if (v8086_mode(regs)) {
>>  		save_v86_state((struct kernel_vm86_regs *) regs, VM86_SIGNAL);
>> +		/* Has save_v86_state failed and killed the process? */
>> +		if (fatal_signal_pending(current))
>> +			return;
>
> This might be an ABI break, or at least it could be if anyone cared
> about vm86.  Imagine this wasn't guarded by if (v8086_mode) and was
> just if (fatal_signal_pending(current)) return; Then all the other
> processing gets skipped if a fatal signal is pending (e.g. from a
> concurrent kill), which could cause visible oddities in a core dump, I
> think.  Maybe it's minor.

I believe it is minor, because the test happens before anything is
written to userspace.  The worst case is a signal gets dequeued and
then not written to userspace.

On a second I am not certain this test is even necessary.  Especially
if the change you suggest be made to save_v86_state is made so that
the kernel is out of v86 state and kernel things can safely happen.

>> +	}
>> 
>>  	/* Are we from a system call? */
>>  	if (syscall_get_nr(current, regs) != -1) {
>> diff --git a/arch/x86/kernel/vm86_32.c b/arch/x86/kernel/vm86_32.c
>> index 63486da77272..040fd01be8b3 100644
>> --- a/arch/x86/kernel/vm86_32.c
>> +++ b/arch/x86/kernel/vm86_32.c
>> @@ -159,7 +159,7 @@ void save_v86_state(struct kernel_vm86_regs *regs, 
>> int retval)
>>  	user_access_end();
>>  Efault:
>>  	pr_alert("could not access userspace vm86 info\n");
>> -	do_exit(SIGSEGV);
>> +	force_sigsegv(SIGSEGV);
>
> This causes us to run unwitting kernel code with the vm86 garbage
> still loaded into the relevant architectural areas (see the chunk if
> save_v86_state that's inside preempt_disable()).  So NAK, especially
> since the aforementioned race might cause the exit-to-usermode path to
> actually run with who-knows-what consequences.

Fair.  I suspect it might even make the current do_exit call run
with who-knows-what consequence.

> If you really want to make this change, please arrange for
> save_v86_state() to switch out of vm86 mode *before* anything that
> might fail so that it's guaranteed to at least put the task in a sane
> state.  And write an explicit test case that tests it.  I could help
> with the latter if you do the former.

I do really want to remove this do_exit.  If the error was causes by a
kernel malfunction we could do something like die.

As it is the code is effectively hand rolling die/oops for a userspace
caused condition.  Which is quite nasty from a maintenance point of
view.


I think your suggested changes to save_v86_state are much more robust
than my idea of simply calling force_sig... and expecting the kernel
to exit immediately.   Having to go another pass through the
exit_to_usermode_loop does not look like it is very hard to make
it robust against a kernel in a random state.

I could close the race today by replacing the force_sigsegv(SIGSEGV)
with force_sig(SIGKILL).  And that removes the coredump path from
the equation so is a bit interesting, but it really is unsatisfactory.


I will dig in and see what can be done including writing a test so that
this code path gracefully handles -EFAULT rather than tries to walk
through the rest of the kernel in a problematic state.


This change as proposed does not get this save_v86_state case to using
ordinary mechanisms to handle the problem, so as written it does not
solve the problem it set out to solve.

Eric