Re: [PATCH] powerpc/eeh: Avoid use after free in eeh_handle_special_event()

[Gavin Shan <gwshan@linux.vnet.ibm.com>]

On Fri, Mar 03, 2017 at 03:47:18PM +1100, Russell Currey wrote:
>eeh_handle_special_event() is called when an EEH event is detected but
>can't be narrowed down to a specific PE.  This function looks through
>every PE to find one in an erroneous state, then calls the regular event
>handler eeh_handle_normal_event() once it knows which PE has an error.
>
>However, if eeh_handle_normal_event() found that the PE cannot possibly
>be recovered, it will remove the PE and associated devices.  This leads
>to a use after free in eeh_handle_special_event() as it attempts to clear
>the "recovering" state on the PE after eeh_handle_normal_event() returns.
>
>Thus, make sure the PE is valid when attempting to clear state in
>eeh_handle_special_event().
>

>From the changelog, I don't see how the PE is free'd. Could you explain
a bit about it?

>Cc: <stable@vger.kernel.org> #3.10+
>Reported-by: Alexey Kardashevskiy <aik@ozlabs.ru>
>Signed-off-by: Russell Currey <ruscur@russell.cc>
>---
> arch/powerpc/kernel/eeh_driver.c | 13 +++++++++++++
> 1 file changed, 13 insertions(+)
>
>diff --git a/arch/powerpc/kernel/eeh_driver.c b/arch/powerpc/kernel/eeh_driver.c
>index b94887165a10..492397298a2a 100644
>--- a/arch/powerpc/kernel/eeh_driver.c
>+++ b/arch/powerpc/kernel/eeh_driver.c
>@@ -983,6 +983,19 @@ static void eeh_handle_special_event(void)
> 		if (rc == EEH_NEXT_ERR_FROZEN_PE ||
> 		    rc == EEH_NEXT_ERR_FENCED_PHB) {
> 			eeh_handle_normal_event(pe);
>+
>+			/*
>+			 * eeh_handle_normal_event() can free the PE if it
>+			 * determines that the PE cannot possibly be recovered.
>+			 * Make sure the PE still exists before changing its
>+			 * state.
>+			 */
>+			if (!pe || (pe->type & EEH_PE_INVALID)
>+			    || (pe->state & EEH_PE_REMOVED)) {
>+				pr_warn("EEH: not clearing state on bad PE\n");
>+				continue;
>+			}
>+

It seems not correct. @pe has set to the valid PE in advance, the !pe is
always false? If the PE has been free'd, how can we access @pe->type here
and how can we make sure PE_INVALID and PE_REMOVED flag wasn't overwritten
by somebody else?

> 			eeh_pe_state_clear(pe, EEH_PE_RECOVERING);
> 		} else {
> 			pci_lock_rescan_remove();

Cheers,
Gavin