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

[Alexey Kardashevskiy]

On 03/03/17 16:55, Gavin Shan wrote:
> 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?


This is a backtrace when kfree(pe) is done:

dump_stack+0xb0/0xf0 (unreliable)
eeh_rmv_from_parent_pe+0x2f8/0x330
eeh_remove_device+0x128/0x170
pcibios_release_device+0x2c/0x70
pci_release_dev+0x5c/0xb0
device_release+0x58/0xf0
kobject_put+0x144/0x2e0
put_device+0x24/0x40
pci_remove_bus_device+0x14c/0x190
pci_hp_remove_devices+0xac/0x170
eeh_handle_normal_event+0x120/0x560
eeh_handle_special_event+0x328/0x3b0
eeh_handle_event+0x74/0xa0
eeh_event_handler+0x260/0x280
kthread+0x14c/0x190
ret_from_kernel_thread+0x5c/0x74


> 
>> Cc: <stable at vger.kernel.org> #3.10+
>> Reported-by: Alexey Kardashevskiy <aik at ozlabs.ru>
>> Signed-off-by: Russell Currey <ruscur at 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
> 


-- 
Alexey