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

[Shiju Jose]

Hi Boris,

Sorry for the delay.

>-----Original Message-----
>From: Borislav Petkov [mailto:bp@xxxxxxxxx]
>Sent: 11 September 2020 17:48
>To: Shiju Jose <shiju.jose@xxxxxxxxxx>
>Cc: linux-edac@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
>mchehab@xxxxxxxxxx; tony.luck@xxxxxxxxx; james.morse@xxxxxxx;
>Linuxarm <linuxarm@xxxxxxxxxx>; Robert Richter <rric@xxxxxxxxxx>
>Subject: Re: [PATCH 1/1] EDAC/ghes: Fix for NULL pointer dereference in
>ghes_edac_register()
>
>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:

I debug with adding more logs. 
I found that in our platform hw->num_dimms was 32 when called ghes_edac_register() second time
when probe a new ghes instance,  the check !(hw->num_dimms % 16) in the enumerate_dimms() passed and 
it allocated memory for  hw->dimms. Thus it did not fail with NULL pointer dereference in ghes_edac_register().
With the your new fix hw->num_dimms reset to 0.

>
>---
>From: Borislav Petkov <bp@xxxxxxx>
>Date: Fri, 11 Sep 2020 12:55:55 +0200
>Subject: [PATCH] EDAC/ghes: Clear scanned data on unload
>
>Commit
>
>  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)
> 	mutex_lock(&ghes_reg_mutex);
>
> 	system_scanned = false;
>+	memset(&ghes_hw, 0, sizeof(struct ghes_hw_desc));
>
> 	if (!refcount_dec_and_test(&ghes_refcount))
> 		goto unlock;
>--
>2.21.0
>
>--
>Regards/Gruss,
>    Boris.
>
>https://people.kernel.org/tglx/notes-about-netiquette

Thanks,
Shiju