[v2,3/7] x86: stop calling fixup_exception() from kprobe_fault_handler()

[v2,3/7] x86: stop calling fixup_exception() from kprobe_fault_handler()

[Masami Hiramatsu]

On Mon, 27 Aug 2018 21:22:19 +0200
Jann Horn <jannh@google.com> wrote:

> On Mon, Aug 27, 2018 at 9:02 PM Andy Lutomirski <luto@kernel.org> wrote:
> >
> > On Mon, Aug 27, 2018 at 11:56 AM, Jann Horn <jannh@google.com> wrote:
> > > This removes the call into exception fixup that was added in
> > > commit c28f896634f2 ("[PATCH] kprobes: fix broken fault handling for
> > > x86_64").
> > >
> > > On X86, kprobe_fault_handler() is called from two places:
> > > do_general_protection() (for #GP) and kprobes_fault() (for #PF).
> > > In both paths, the fixup_exception() call in the kprobe fault handler is
> > > redundant.
> > >
> > > For #GP, fixup_exception() is called immediately before
> > > kprobe_fault_handler() is invoked - if someone wanted to fix up our #GP,
> > > they've already done so, no need to try again. (This assumes that the
> > > kprobe's fault handler isn't going to do something crazy like changing RIP
> > > so that it suddenly points to an instruction that does userspace access.)
> >
> > This needs review by someone who understands kprobes better than I do.
> > What happens if someone puts a kprobe on a uaccess instruction and the
> > uaccess subsequently faults?
> 
> Ugh, good point. I'll admit to not having thought about that properly.
> 
> I think that's the "if (unlikely(regs->ip == (unsigned
> long)cur->ainsn.insn))" branch in kprobe_fault_handler(), which I'm
> not touching.

Correct, probing on uaccess is handled by that block.
So this fixup_exception() is just for safeness.
As Jann said, no_context() handles it correctly, we don't need it.

Acked-by: Masami Hiramatsu <mhiramat@kernel.org>

Thank you!

> 
> For #PF, both without and with my patch, stuff should get fixed up by
> the normal pagefault handler, since the fixup happens after the kprobe
> handler has fiddled with the exception state.
> 
> For #GP, we're already past the fixup call, and I think both without
> and with my patch, nothing will catch it - so I think that's a bug,
> but I don't think it's one I'm introducing.
> 
> > > For #PF on a kernel address from kernel space, after the kprobe fault
> > > handler has run, we'll go into no_context(), which calls fixup_exception().
> > >
> > > Signed-off-by: Jann Horn <jannh@google.com>
> > > ---
> > >  arch/x86/kernel/kprobes/core.c | 7 -------
> > >  1 file changed, 7 deletions(-)
> > >
> > > diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> > > index 467ac22691b0..7315ac202aad 100644
> > > --- a/arch/x86/kernel/kprobes/core.c
> > > +++ b/arch/x86/kernel/kprobes/core.c
> > > @@ -1021,13 +1021,6 @@ int kprobe_fault_handler(struct pt_regs *regs, int trapnr)
> > >                 if (cur->fault_handler && cur->fault_handler(cur, regs, trapnr))
> > >                         return 1;
> > >
> > > -               /*
> > > -                * In case the user-specified fault handler returned
> > > -                * zero, try to fix up.
> > > -                */
> > > -               if (fixup_exception(regs, trapnr))
> > > -                       return 1;
> > > -
> > >                 /* fixup routine could not handle it. */
> > >         }
> > >
> > > --
> > > 2.19.0.rc0.228.g281dcd1b4d0-goog
> > >

[v2,3/7] x86: stop calling fixup_exception() from kprobe_fault_handler()

[Andy Lutomirski]

On Mon, Aug 27, 2018 at 12:22 PM, Jann Horn <jannh@google.com> wrote:
> On Mon, Aug 27, 2018 at 9:02 PM Andy Lutomirski <luto@kernel.org> wrote:
>>
>> On Mon, Aug 27, 2018 at 11:56 AM, Jann Horn <jannh@google.com> wrote:
>> > This removes the call into exception fixup that was added in
>> > commit c28f896634f2 ("[PATCH] kprobes: fix broken fault handling for
>> > x86_64").
>> >
>> > On X86, kprobe_fault_handler() is called from two places:
>> > do_general_protection() (for #GP) and kprobes_fault() (for #PF).
>> > In both paths, the fixup_exception() call in the kprobe fault handler is
>> > redundant.
>> >
>> > For #GP, fixup_exception() is called immediately before
>> > kprobe_fault_handler() is invoked - if someone wanted to fix up our #GP,
>> > they've already done so, no need to try again. (This assumes that the
>> > kprobe's fault handler isn't going to do something crazy like changing RIP
>> > so that it suddenly points to an instruction that does userspace access.)
>>
>> This needs review by someone who understands kprobes better than I do.
>> What happens if someone puts a kprobe on a uaccess instruction and the
>> uaccess subsequently faults?
>
> Ugh, good point. I'll admit to not having thought about that properly.
>
> I think that's the "if (unlikely(regs->ip == (unsigned
> long)cur->ainsn.insn))" branch in kprobe_fault_handler(), which I'm
> not touching.
>
> For #PF, both without and with my patch, stuff should get fixed up by
> the normal pagefault handler, since the fixup happens after the kprobe
> handler has fiddled with the exception state.
>
> For #GP, we're already past the fixup call, and I think both without
> and with my patch, nothing will catch it - so I think that's a bug,
> but I don't think it's one I'm introducing.

Fair enough.  If there is indeed a problem, it'll be easier to fix
sanely with your patch applied :)

[v2,3/7] x86: stop calling fixup_exception() from kprobe_fault_handler()

[Jann Horn]

On Mon, Aug 27, 2018 at 9:02 PM Andy Lutomirski <luto@kernel.org> wrote:
>
> On Mon, Aug 27, 2018 at 11:56 AM, Jann Horn <jannh@google.com> wrote:
> > This removes the call into exception fixup that was added in
> > commit c28f896634f2 ("[PATCH] kprobes: fix broken fault handling for
> > x86_64").
> >
> > On X86, kprobe_fault_handler() is called from two places:
> > do_general_protection() (for #GP) and kprobes_fault() (for #PF).
> > In both paths, the fixup_exception() call in the kprobe fault handler is
> > redundant.
> >
> > For #GP, fixup_exception() is called immediately before
> > kprobe_fault_handler() is invoked - if someone wanted to fix up our #GP,
> > they've already done so, no need to try again. (This assumes that the
> > kprobe's fault handler isn't going to do something crazy like changing RIP
> > so that it suddenly points to an instruction that does userspace access.)
>
> This needs review by someone who understands kprobes better than I do.
> What happens if someone puts a kprobe on a uaccess instruction and the
> uaccess subsequently faults?

Ugh, good point. I'll admit to not having thought about that properly.

I think that's the "if (unlikely(regs->ip == (unsigned
long)cur->ainsn.insn))" branch in kprobe_fault_handler(), which I'm
not touching.

For #PF, both without and with my patch, stuff should get fixed up by
the normal pagefault handler, since the fixup happens after the kprobe
handler has fiddled with the exception state.

For #GP, we're already past the fixup call, and I think both without
and with my patch, nothing will catch it - so I think that's a bug,
but I don't think it's one I'm introducing.

> > For #PF on a kernel address from kernel space, after the kprobe fault
> > handler has run, we'll go into no_context(), which calls fixup_exception().
> >
> > Signed-off-by: Jann Horn <jannh@google.com>
> > ---
> >  arch/x86/kernel/kprobes/core.c | 7 -------
> >  1 file changed, 7 deletions(-)
> >
> > diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> > index 467ac22691b0..7315ac202aad 100644
> > --- a/arch/x86/kernel/kprobes/core.c
> > +++ b/arch/x86/kernel/kprobes/core.c
> > @@ -1021,13 +1021,6 @@ int kprobe_fault_handler(struct pt_regs *regs, int trapnr)
> >                 if (cur->fault_handler && cur->fault_handler(cur, regs, trapnr))
> >                         return 1;
> >
> > -               /*
> > -                * In case the user-specified fault handler returned
> > -                * zero, try to fix up.
> > -                */
> > -               if (fixup_exception(regs, trapnr))
> > -                       return 1;
> > -
> >                 /* fixup routine could not handle it. */
> >         }
> >
> > --
> > 2.19.0.rc0.228.g281dcd1b4d0-goog
> >

[v2,3/7] x86: stop calling fixup_exception() from kprobe_fault_handler()

[Andy Lutomirski]

On Mon, Aug 27, 2018 at 11:56 AM, Jann Horn <jannh@google.com> wrote:
> This removes the call into exception fixup that was added in
> commit c28f896634f2 ("[PATCH] kprobes: fix broken fault handling for
> x86_64").
>
> On X86, kprobe_fault_handler() is called from two places:
> do_general_protection() (for #GP) and kprobes_fault() (for #PF).
> In both paths, the fixup_exception() call in the kprobe fault handler is
> redundant.
>
> For #GP, fixup_exception() is called immediately before
> kprobe_fault_handler() is invoked - if someone wanted to fix up our #GP,
> they've already done so, no need to try again. (This assumes that the
> kprobe's fault handler isn't going to do something crazy like changing RIP
> so that it suddenly points to an instruction that does userspace access.)

This needs review by someone who understands kprobes better than I do.
What happens if someone puts a kprobe on a uaccess instruction and the
uaccess subsequently faults?

>
> For #PF on a kernel address from kernel space, after the kprobe fault
> handler has run, we'll go into no_context(), which calls fixup_exception().
>
> Signed-off-by: Jann Horn <jannh@google.com>
> ---
>  arch/x86/kernel/kprobes/core.c | 7 -------
>  1 file changed, 7 deletions(-)
>
> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> index 467ac22691b0..7315ac202aad 100644
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -1021,13 +1021,6 @@ int kprobe_fault_handler(struct pt_regs *regs, int trapnr)
>                 if (cur->fault_handler && cur->fault_handler(cur, regs, trapnr))
>                         return 1;
>
> -               /*
> -                * In case the user-specified fault handler returned
> -                * zero, try to fix up.
> -                */
> -               if (fixup_exception(regs, trapnr))
> -                       return 1;
> -
>                 /* fixup routine could not handle it. */
>         }
>
> --
> 2.19.0.rc0.228.g281dcd1b4d0-goog
>

[v2,3/7] x86: stop calling fixup_exception() from kprobe_fault_handler()

[Jann Horn]

This removes the call into exception fixup that was added in
commit c28f896634f2 ("[PATCH] kprobes: fix broken fault handling for
x86_64").

On X86, kprobe_fault_handler() is called from two places:
do_general_protection() (for #GP) and kprobes_fault() (for #PF).
In both paths, the fixup_exception() call in the kprobe fault handler is
redundant.

For #GP, fixup_exception() is called immediately before
kprobe_fault_handler() is invoked - if someone wanted to fix up our #GP,
they've already done so, no need to try again. (This assumes that the
kprobe's fault handler isn't going to do something crazy like changing RIP
so that it suddenly points to an instruction that does userspace access.)

For #PF on a kernel address from kernel space, after the kprobe fault
handler has run, we'll go into no_context(), which calls fixup_exception().

Signed-off-by: Jann Horn <jannh@google.com>
---
 arch/x86/kernel/kprobes/core.c | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index 467ac22691b0..7315ac202aad 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -1021,13 +1021,6 @@ int kprobe_fault_handler(struct pt_regs *regs, int trapnr)
 		if (cur->fault_handler && cur->fault_handler(cur, regs, trapnr))
 			return 1;
 
-		/*
-		 * In case the user-specified fault handler returned
-		 * zero, try to fix up.
-		 */
-		if (fixup_exception(regs, trapnr))
-			return 1;
-
 		/* fixup routine could not handle it. */
 	}