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

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 03/03/17 15:47, 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().
> 
> 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)) {


The bug is that pe becomes stale after eeh_handle_normal_event() returned
and dereferencing it afterwards is broken.



> +				pr_warn("EEH: not clearing state on bad PE\n");
> +				continue;
> +			}
> +
>  			eeh_pe_state_clear(pe, EEH_PE_RECOVERING);
>  		} else {
>  			pci_lock_rescan_remove();
> 


-- 
Alexey