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

On Fri, Mar 03, 2017 at 04:59:11PM +1100, Alexey Kardashevskiy wrote:

On 03/03/17 15:47, Russell Currey wrote:

quoted

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: <redacted> #3.10+
Reported-by: Alexey Kardashevskiy <redacted>
Signed-off-by: Russell Currey <redacted>
---
 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.

Correct, it won't cause a kernel crash as @pe is deferencing linear mapped
area whose address is always valid. I think the proper fix would be to use
eeh_handle_normal_event() to indicate the @pe has been released and don't
access it any more.

quoted

+				pr_warn("EEH: not clearing state on bad PE\n");

The message like this isn't meaningful, no need to have it. The messages that
have prefix "EEH:" is informative messages. We definitely needn't this here.
However, the message might be not needed in next revision.

quoted

+				continue;
+			}
+
 			eeh_pe_state_clear(pe, EEH_PE_RECOVERING);
 		} else {
 			pci_lock_rescan_remove();

Thanks,
Gavin