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

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

On Fri, Mar 03, 2017 at 03:47:18PM +1100, 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().

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

quoted

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)) {
+				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?

quoted

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

Cheers,
Gavin

-- 
Alexey