Re: [PATCH] x86/mm: Remove broken vsyscall emulation code from the page fault code

[Jiri Olsa]

On Mon, Apr 29, 2024 at 10:00:51AM +0200, Ingo Molnar wrote:

SNIP

> The attached patch looks like the ObviouslyCorrect(tm) thing to do.
> 
> NOTE! This broken code goes back to this commit in 2011:
> 
>   4fc3490114bb ("x86-64: Set siginfo and context on vsyscall emulation faults")
> 
> ... and back then the reason was to get all the siginfo details right. 
> Honestly, I do not for a moment believe that it's worth getting the siginfo 
> details right here, but part of the commit says:
> 
>     This fixes issues with UML when vsyscall=emulate.
> 
> ... and so my patch to remove this garbage will probably break UML in this 
> situation.
> 
> I do not believe that anybody should be running with vsyscall=emulate in 
> 2024 in the first place, much less if you are doing things like UML. But 
> let's see if somebody screams.
> 
> Not-Yet-Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> Link: https://lore.kernel.org/r/CAHk-=wh9D6f7HUkDgZHKmDCHUQmp+Co89GP+b8+z+G56BKeyNg@xxxxxxxxxxxxxx

fwiw I can no longer trigger the invalid wait context bug
with this change

Tested-by: Jiri Olsa <jolsa@xxxxxxxxxx>

jirka

> ---
>  arch/x86/entry/vsyscall/vsyscall_64.c | 25 ++-----------------------
>  arch/x86/include/asm/processor.h      |  1 -
>  arch/x86/mm/fault.c                   | 33 +--------------------------------
>  3 files changed, 3 insertions(+), 56 deletions(-)
> 
> diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c b/arch/x86/entry/vsyscall/vsyscall_64.c
> index a3c0df11d0e6..3b0f61b2ea6d 100644
> --- a/arch/x86/entry/vsyscall/vsyscall_64.c
> +++ b/arch/x86/entry/vsyscall/vsyscall_64.c
> @@ -98,11 +98,6 @@ static int addr_to_vsyscall_nr(unsigned long addr)
>  
>  static bool write_ok_or_segv(unsigned long ptr, size_t size)
>  {
> -	/*
> -	 * XXX: if access_ok, get_user, and put_user handled
> -	 * sig_on_uaccess_err, this could go away.
> -	 */
> -
>  	if (!access_ok((void __user *)ptr, size)) {
>  		struct thread_struct *thread = &current->thread;
>  
> @@ -123,7 +118,6 @@ bool emulate_vsyscall(unsigned long error_code,
>  	struct task_struct *tsk;
>  	unsigned long caller;
>  	int vsyscall_nr, syscall_nr, tmp;
> -	int prev_sig_on_uaccess_err;
>  	long ret;
>  	unsigned long orig_dx;
>  
> @@ -234,12 +228,8 @@ bool emulate_vsyscall(unsigned long error_code,
>  		goto do_ret;  /* skip requested */
>  
>  	/*
> -	 * With a real vsyscall, page faults cause SIGSEGV.  We want to
> -	 * preserve that behavior to make writing exploits harder.
> +	 * With a real vsyscall, page faults cause SIGSEGV.
>  	 */
> -	prev_sig_on_uaccess_err = current->thread.sig_on_uaccess_err;
> -	current->thread.sig_on_uaccess_err = 1;
> -
>  	ret = -EFAULT;
>  	switch (vsyscall_nr) {
>  	case 0:
> @@ -262,23 +252,12 @@ bool emulate_vsyscall(unsigned long error_code,
>  		break;
>  	}
>  
> -	current->thread.sig_on_uaccess_err = prev_sig_on_uaccess_err;
> -
>  check_fault:
>  	if (ret == -EFAULT) {
>  		/* Bad news -- userspace fed a bad pointer to a vsyscall. */
>  		warn_bad_vsyscall(KERN_INFO, regs,
>  				  "vsyscall fault (exploit attempt?)");
> -
> -		/*
> -		 * If we failed to generate a signal for any reason,
> -		 * generate one here.  (This should be impossible.)
> -		 */
> -		if (WARN_ON_ONCE(!sigismember(&tsk->pending.signal, SIGBUS) &&
> -				 !sigismember(&tsk->pending.signal, SIGSEGV)))
> -			goto sigsegv;
> -
> -		return true;  /* Don't emulate the ret. */
> +		goto sigsegv;
>  	}
>  
>  	regs->ax = ret;
> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index 811548f131f4..78e51b0d6433 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -472,7 +472,6 @@ struct thread_struct {
>  	unsigned long		iopl_emul;
>  
>  	unsigned int		iopl_warn:1;
> -	unsigned int		sig_on_uaccess_err:1;
>  
>  	/*
>  	 * Protection Keys Register for Userspace.  Loaded immediately on
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 6b2ca8ba75b8..f26ecabc9424 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -724,39 +724,8 @@ kernelmode_fixup_or_oops(struct pt_regs *regs, unsigned long error_code,
>  	WARN_ON_ONCE(user_mode(regs));
>  
>  	/* Are we prepared to handle this kernel fault? */
> -	if (fixup_exception(regs, X86_TRAP_PF, error_code, address)) {
> -		/*
> -		 * Any interrupt that takes a fault gets the fixup. This makes
> -		 * the below recursive fault logic only apply to a faults from
> -		 * task context.
> -		 */
> -		if (in_interrupt())
> -			return;
> -
> -		/*
> -		 * Per the above we're !in_interrupt(), aka. task context.
> -		 *
> -		 * In this case we need to make sure we're not recursively
> -		 * faulting through the emulate_vsyscall() logic.
> -		 */
> -		if (current->thread.sig_on_uaccess_err && signal) {
> -			sanitize_error_code(address, &error_code);
> -
> -			set_signal_archinfo(address, error_code);
> -
> -			if (si_code == SEGV_PKUERR) {
> -				force_sig_pkuerr((void __user *)address, pkey);
> -			} else {
> -				/* XXX: hwpoison faults will set the wrong code. */
> -				force_sig_fault(signal, si_code, (void __user *)address);
> -			}
> -		}
> -
> -		/*
> -		 * Barring that, we can do the fixup and be happy.
> -		 */
> +	if (fixup_exception(regs, X86_TRAP_PF, error_code, address))
>  		return;
> -	}
>  
>  	/*
>  	 * AMD erratum #91 manifests as a spurious page fault on a PREFETCH
>