| From | (Eric W. Biederman) | Date | Fri, 12 Nov 2021 09:40:01 -0600 | Subject | Re: [PATCH 09/20] signal/vm86_32: Replace open coded BUG_ON with an actual BUG_ON |
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"Eric W. Biederman" <ebiederm@xmission.com> writes:
> The function save_v86_state is only called when userspace was > operating in vm86 mode before entering the kernel. Not having vm86 > state in the task_struct should never happen. So transform the hand > rolled BUG_ON into an actual BUG_ON to make it clear what is > happening.
Now that this change has been merged into Linus' tree I have a report that it is possible to trigger this new BUG_ON. Which obviously is not good.
We could revert the change but I think that would just be shooting the messenger.
Does anyone have an idea where to start to track down what is going on?
A very quick skim through the code suggests that the only code path that calls save_v86_state that has not already accessed is current->thread.vm86 is handle_signal.
Another quick look suggests that the only place where X86_VM_MASK gets set in eflags is in do_sys_vm86. So it appears do_sys_vm86 must be called before v8086_mode returns true in handle_signal.
Which seems to suggest that the bug on can't trigger.
But that is obviously wrong.
I will keep digging but if anyone has some ideas that would be appreciated.
Eric
> Cc: Thomas Gleixner <tglx@linutronix.de> > Cc: Ingo Molnar <mingo@redhat.com> > Cc: Borislav Petkov <bp@alien8.de> > Cc: x86@kernel.org > Cc: H Peter Anvin <hpa@zytor.com> > Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com> > --- > arch/x86/kernel/vm86_32.c | 6 ++---- > 1 file changed, 2 insertions(+), 4 deletions(-) > > diff --git a/arch/x86/kernel/vm86_32.c b/arch/x86/kernel/vm86_32.c > index e5a7a10a0164..63486da77272 100644 > --- a/arch/x86/kernel/vm86_32.c > +++ b/arch/x86/kernel/vm86_32.c > @@ -106,10 +106,8 @@ void save_v86_state(struct kernel_vm86_regs *regs, int retval) > */ > local_irq_enable(); > > - if (!vm86 || !vm86->user_vm86) { > - pr_alert("no user_vm86: BAD\n"); > - do_exit(SIGSEGV); > - } > + BUG_ON(!vm86 || !vm86->user_vm86); > + > set_flags(regs->pt.flags, VEFLAGS, X86_EFLAGS_VIF | vm86->veflags_mask); > user = vm86->user_vm86;
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