linux-kernel - Re: [PATCH v2] efifb: prevent null dereferences by removing unused array indices from dmi

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Message-ID: <52283CD0.9010108@ti.com>
Date:	Thu, 5 Sep 2013 11:12:00 +0300
From:	Tomi Valkeinen <tomi.valkeinen@...com>
To:	James Bates <james.h.bates@...il.com>
CC:	David Herrmann <dh.herrmann@...il.com>,
	Peter Jones <pjones@...hat.com>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	<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 James,

Can you resend this? I don't know how I could apply the patch from this
mail without re-creating it manually.

 Tomi

On 16/08/13 23:19, James Bates 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 <mailto: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):
> 
> 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
> <mailto: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 &&
> +     !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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