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exit/syscall_user_dispatch: Send ordinary signals on failure

Use force_fatal_sig instead of calling do_exit directly. This ensures
the ordinary signal handling path gets invoked, core dumps as
appropriate get created, and for multi-threaded processes all of the
threads are terminated not just a single thread.

When asked Gabriel Krisman Bertazi <krisman@collabora.com> said [1]:
> ebiederm@xmission.com (Eric W. Biederman) asked:
>
> > Why does do_syscal_user_dispatch call do_exit(SIGSEGV) and
> > do_exit(SIGSYS) instead of force_sig(SIGSEGV) and force_sig(SIGSYS)?
> >
> > Looking at the code these cases are not expected to happen, so I would
> > be surprised if userspace depends on any particular behaviour on the
> > failure path so I think we can change this.
>
> Hi Eric,
>
> There is not really a good reason, and the use case that originated the
> feature doesn't rely on it.
>
> Unless I'm missing yet another problem and others correct me, I think
> it makes sense to change it as you described.
>
> > Is using do_exit in this way something you copied from seccomp?
>
> I'm not sure, its been a while, but I think it might be just that. The
> first prototype of SUD was implemented as a seccomp mode.

If at some point it becomes interesting we could relax
"force_fatal_sig(SIGSEGV)" to instead say
"force_sig_fault(SIGSEGV, SEGV_MAPERR, sd->selector)".

I avoid doing that in this patch to avoid making it possible
to catch currently uncatchable signals.

Cc: Gabriel Krisman Bertazi <krisman@collabora.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>
[1] https://lkml.kernel.org/r/87mtr6gdvi.fsf@collabora.com
Link: https://lkml.kernel.org/r/20211020174406.17889-14-ebiederm@xmission.com
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

Eric W. Biederman 941edc5b 26d5badb

+8 -4
+8 -4
kernel/entry/syscall_user_dispatch.c
··· 47 47 * access_ok() is performed once, at prctl time, when 48 48 * the selector is loaded by userspace. 49 49 */ 50 - if (unlikely(__get_user(state, sd->selector))) 51 - do_exit(SIGSEGV); 50 + if (unlikely(__get_user(state, sd->selector))) { 51 + force_fatal_sig(SIGSEGV); 52 + return true; 53 + } 52 54 53 55 if (likely(state == SYSCALL_DISPATCH_FILTER_ALLOW)) 54 56 return false; 55 57 56 - if (state != SYSCALL_DISPATCH_FILTER_BLOCK) 57 - do_exit(SIGSYS); 58 + if (state != SYSCALL_DISPATCH_FILTER_BLOCK) { 59 + force_fatal_sig(SIGSYS); 60 + return true; 61 + } 58 62 } 59 63 60 64 sd->on_dispatch = true;