Date:	Fri, 16 Aug 2013 22:36:13 +0200
From:	David Herrmann <dh.herrmann@...il.com>
To:	James Bates <james.h.bates@...il.com>
Cc:	Peter Jones <pjones@...hat.com>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Tomi Valkeinen <tomi.valkeinen@...com>,
	"linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2] efifb: prevent null dereferences by removing unused
 array indices from dmi_list

Hi

On Fri, Aug 16, 2013 at 10:19 PM, James Bates <james.h.bates@...il.com> wrote:
> On Fri, 2013-08-16 at 15:37 +0200, David Herrmann wrote:
>
> Hi
> (CC'ing hpa)
>
> Yepp, this patch looks good:
> Reviewed-by: David Herrmann <dh.herrmann@...il.com>
>
> However, I'd like to see some code to prevent this from happening
> again. It isn't really obvious that removing an entry will result in a
> NULL-deref. I am not the maintainer of this code, but I'd really like
> to see a "(dmi_list[i].optname && !strcmp())" check in efifb_setup().
> Otherwise we _will_ run into this again.
>
> Side note: this code got moved to arch/x86/kernel/sysfb_efi.c in the
> x86 tree. I am adding hpa here so he will remember this once Linus
> gets a merge conflict (iff the sysfb changes get merged through the
> x86 tree).
>
> Thanks
> David
>
> Sure thing, here is the patch again, with the extra check in efifb_setup()
> (it turns out one can simply
> switch around the order of the checks: implicitly initialized dmi_list items
> will have base == 0):

You can put such comments below the "---" in your patch so they will
not show up in the commit-message (between commit message and the
diffstat).

> Full patch v2:
>
> the dmi_list array is initialized using gnu designated initializers, and
> therefore
> contains fewer explicitly defined entries as there are elements in it. This
> is because the enum above with M_blabla constants contains more items than
> the designated
> initializer. Those elements not explicitly initialized are implicitly set to
> 0.
>
> Now efifb_setup(), L.322 & L.323, loops through all these array elements,
> and performs
> a strcmp o a field (optname) in each item. For non explicitly initialized
> elements this
> will be a null pointer:
>
>                         for (i = 0; i < M_UNKNOWN; i++) {
>                                 if (!strcmp(this_opt, dmi_list[i].optname)
> &&
>
> On my macbook6,1 the predefined values are for some reason incorrect, and
> most parameters
> are preset correctly by my efi bootloader (elilo). but stride/line_length is
> not detected
> correctly and so I wish to set it explicitly using a
> "video=efifb:stride:2048" command-line
> argument. Because of the above null dereference, an exception (presumably)
> occurs before
> the parsing code (L.333) is ever reached. I say presumably since the mac
> hangs on boot
> without a console, and I can therefore not see any output.
>
> By removing the unused values from the enum, and thus preventing implicitly
> initialized items
> in the dmi_list array, the null dereference does not occur, my customer
> command-line arg is
> parsed correctly, and my console displays correctly.
>
> This patch removes the unused enum values, and also guards against any
> future implicit
> initializing by inverting the check order in the if statement, and checking
> first whether
> dmi_list[i].base is null.
>
> Signed-off-by: James Bates <james.h.bates@...oo.com>
> ---
> drivers/video/efifb.c | 7 ++-----
> 1 file changed, 2 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/video/efifb.c b/drivers/video/efifb.c
> index 50fe668..161757b 100644
>
> --- a/drivers/video/efifb.c
> +++ b/drivers/video/efifb.c
> @@ -50,12 +50,9 @@ enum {
> M_MINI_3_1, /* Mac Mini, 3,1th gen */
> M_MINI_4_1, /* Mac Mini, 4,1th gen */
> M_MB, /* MacBook */
> - M_MB_2, /* MacBook, 2nd rev. */
> - M_MB_3, /* MacBook, 3rd rev. */
> M_MB_5_1, /* MacBook, 5th rev. */
> M_MB_6_1, /* MacBook, 6th rev. */
> M_MB_7_1, /* MacBook, 7th rev. */
> - M_MB_SR, /* MacBook, 2nd gen, (Santa Rosa) */
> M_MBA, /* MacBook Air */
> M_MBA_3, /* Macbook Air, 3rd rev */
> M_MBP, /* MacBook Pro */
> @@ -323,8 +320,8 @@ static int __init efifb_setup(char *options)
> if (!*this_opt) continue;
>
>
> for (i = 0; i < M_UNKNOWN; i++) {
> - if (!strcmp(this_opt, dmi_list[i].optname) &&
> -     dmi_list[i].base != 0) {
> + if (dmi_list[i].base != 0 &&

>From a quick glance, M_MBA_3 has "base == 0" but is a valid entry. Hm,
I think we can be sure that there will never be a framebuffer at phys
0, so yeah, I am fine with this.

Reviewed-by: David Herrmann <dh.herrmann@...il.com>

Regards
David

> +     !strcmp(this_opt, dmi_list[i].optname)) {
> screen_info.lfb_base = dmi_list[i].base;
> screen_info.lfb_linelength = dmi_list[i].stride;
> screen_info.lfb_width = dmi_list[i].width;
> --
>
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