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?
>
> 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.
> 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 = ¤t->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.
> + }
>
> /* 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.
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.
--Andy
