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

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)) {