Re: [PATCH 1/1] EDAC/ghes: Fix for NULL pointer dereference in ghes_edac_register()

From: Borislav Petkov
Date: Fri Sep 11 2020 - 12:48:49 EST

On Thu, Aug 27, 2020 at 02:02:27PM +0000, Shiju Jose wrote:
> I tested with your changes and it fixes the issue. I will send v2.

Btw, I don't know how it managed to work on your machine because even
with this patch, it isn't all fixed because num_dimms needs to be
cleared too, see here:

From: Borislav Petkov <bp@xxxxxxx>
Date: Fri, 11 Sep 2020 12:55:55 +0200
Subject: [PATCH] EDAC/ghes: Clear scanned data on unload


b972fdba8665 ("EDAC/ghes: Fix NULL pointer dereference in ghes_edac_register()")

didn't clear all the information from the scanned system and, more
specifically, left ghes_hw.num_dimms to its previous value. On a
second load (CONFIG_DEBUG_TEST_DRIVER_REMOVE=y), the driver would use
the leftover num_dimms value which is not 0 and thus the 0 check in
enumerate_dimms() will get bypassed and it would go directly to the
pointer deref:

d = &hw->dimms[hw->num_dimms];

which is, of course, NULL:

#PF: supervisor write access in kernel mode
#PF: error_code(0x0002) - not-present page
PGD 0 P4D 0
Oops: 0002 [#1] PREEMPT SMP
CPU: 7 PID: 1 Comm: swapper/0 Not tainted 5.9.0-rc4+ #7
Hardware name: GIGABYTE MZ01-CE1-00/MZ01-CE1-00, BIOS F02 08/29/2018
RIP: 0010:enumerate_dimms.cold+0x7b/0x375

Reset the whole ghes_hw on driver unregister so that no stale values are
used on a second system scan.

Fixes: b972fdba8665 ("EDAC/ghes: Fix NULL pointer dereference in ghes_edac_register()")
Cc: Shiju Jose <shiju.jose@xxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
drivers/edac/ghes_edac.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/drivers/edac/ghes_edac.c b/drivers/edac/ghes_edac.c
index a6b9c0b2a15c..eb6034a6fbbb 100644
--- a/drivers/edac/ghes_edac.c
+++ b/drivers/edac/ghes_edac.c
@@ -632,6 +632,7 @@ void ghes_edac_unregister(struct ghes *ghes)

system_scanned = false;
+ memset(&ghes_hw, 0, sizeof(struct ghes_hw_desc));

if (!refcount_dec_and_test(&ghes_refcount))
goto unlock;