Re: [PATCH 4.19 41/48] x86/unwind/orc: Prevent unwinding before ORC initialization

Pavel Machek
2020-05-14T20:13:44Z

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Hi!

> > > From: Josh Poimboeuf <jpoimboe@redhat.com>
> > >
> > > commit 98d0c8ebf77e0ba7c54a9ae05ea588f0e9e3f46e upstream.
> > >
> > > If the unwinder is called before the ORC data has been initialized,
> > > orc_find() returns NULL, and it tries to fall back to using frame
> > > pointers. This can cause some unexpected warnings during boot.
> > >
> > > Move the 'orc_init' check from orc_find() to __unwind_init(), so that it
> > > doesn't even try to unwind from an uninitialized state.
> >
> > > @@ -563,6 +560,9 @@ EXPORT_SYMBOL_GPL(unwind_next_frame);
> > > void __unwind_start(struct unwind_state *state, struct task_struct *task,
> > > struct pt_regs *regs, unsigned long *first_frame)
> > > {
> > > + if (!orc_init)
> > > + goto done;
> > > +
> > > memset(state, 0, sizeof(*state));
> > > state->task = task;
> > >
> >
> > As this returns the *state to the caller, should the "goto done" move
> > below the memset? Otherwise we are returning partialy-initialized
> > struct, which is ... weird.
>
> Yeah, it is a little weird. In most cases it should be fine, but there
> is an edge case where if there's a corrupt ORC table and this returns
> early, 'arch_stack_walk_reliable() -> unwind_error()' could check an
> uninitialized value.
>
> Also the __unwind_start() error handling needs to set that error bit
> anyway, in its error cases. I'll fix it up.

I did this in the mean time. It moves goto around memset, and I
believe that 8 in get_reg should have been sizeof(long) [not that it
matters, x86-32 is protected by build bug on.]

Signed-off-by: Pavel Machek <pavel@ucw.cz>

Best regards,
Pavel

diff --git a/arch/x86/kernel/unwind_orc.c b/arch/x86/kernel/unwind_orc.c
index 169b96492b7c..90cb3cb2b4f1 100644
--- a/arch/x86/kernel/unwind_orc.c
+++ b/arch/x86/kernel/unwind_orc.c
@@ -375,7 +375,7 @@ static bool deref_stack_iret_regs(struct unwind_state *state, unsigned long addr
static bool get_reg(struct unwind_state *state, unsigned int reg_off,
unsigned long *val)
{
- unsigned int reg = reg_off/8;
+ unsigned int reg = reg_off/sizeof(long);

if (!state->regs)
return false;
@@ -589,12 +589,12 @@ EXPORT_SYMBOL_GPL(unwind_next_frame);
void __unwind_start(struct unwind_state *state, struct task_struct *task,
struct pt_regs *regs, unsigned long *first_frame)
{
- if (!orc_init)
- goto done;
-
memset(state, 0, sizeof(*state));
state->task = task;

+ if (!orc_init)
+ goto done;
+
/*
* Refuse to unwind the stack of a task while it's executing on another
* CPU. This check is racy, but that's ok: the unwinder has other

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