[RESEND] drivers: video: metronomefb: avoid out-of-bounds array access

Commit Message

Michal Nazarewicz Nov. 29, 2013, 4:51 p.m. UTC

From: Michal Nazarewicz <[e-mail]>

load_waveform function checks whether padding bytes in stuff2a
and stuff2b are all zero, but does so by treating those arrays
as a single longer array.  Since the structure is packed, and
the size sum matches, it all works, but creates some confusion
in the code.

This commit changes the stuff2a and stuff2b arrays into pad1 and
pad2 fields such that they cover the same bytes as the arrays
covered, and changes the check in the load_waveform function so
that the fields are read instead of iterating over an arary.

It also renames the other “stuff” fields to “ignore*” fields to
give them more semantic meaning.

Signed-off-by: Michal Nazarewicz <[e-mail]>
---
 drivers/video/metronomefb.c | 17 ++++++++---------
 1 file changed, 8 insertions(+), 9 deletions(-)

Comments

Tomi Valkeinen Jan. 9, 2014, 12:21 p.m. UTC | #1

On 2013-11-29 18:51, Michal Nazarewicz wrote:
> From: Michal Nazarewicz <[e-mail]>
> 
> load_waveform function checks whether padding bytes in stuff2a
> and stuff2b are all zero, but does so by treating those arrays
> as a single longer array.  Since the structure is packed, and
> the size sum matches, it all works, but creates some confusion
> in the code.
> 
> This commit changes the stuff2a and stuff2b arrays into pad1 and
> pad2 fields such that they cover the same bytes as the arrays
> covered, and changes the check in the load_waveform function so
> that the fields are read instead of iterating over an arary.
> 
> It also renames the other “stuff” fields to “ignore*” fields to
> give them more semantic meaning.
> 
> Signed-off-by: Michal Nazarewicz <[e-mail]>
> ---
>  drivers/video/metronomefb.c | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/video/metronomefb.c b/drivers/video/metronomefb.c
> index 195cc2d..4f36a2b 100644
> --- a/drivers/video/metronomefb.c
> +++ b/drivers/video/metronomefb.c
> @@ -126,7 +126,7 @@ static struct fb_var_screeninfo metronomefb_var = {
>  
>  /* the waveform structure that is coming from userspace firmware */
>  struct waveform_hdr {
> -	u8 stuff[32];
> +	u8 ignore1[32];
>  
>  	u8 wmta[3];
>  	u8 fvsn;
> @@ -134,13 +134,14 @@ struct waveform_hdr {
>  	u8 luts;
>  	u8 mc;
>  	u8 trc;
> -	u8 stuff3;
> +	u8 ignore2;
>  
>  	u8 endb;
>  	u8 swtb;
> -	u8 stuff2a[2];
> +	u32 pad1; /* u16 halfof(pad1) */
>  
> -	u8 stuff2b[3];
> +	/* u16 halfof(pad1) */
> +	u8 pad2;

I don't quite follow what those comments mean...

I know nothing of the hw in question, but I guess there's some reason
the stuff2a and stuff2b has been separate, and there's even a blank line
between. If they pad a particular block in the data structure, doesn't
your change make it confusing?

It wouldn't be too pad to have those 5 fields as separate ones. One u16
for the stuff2a, and u16 + u8 for the stuff2b. It'd be simple to check
that those are zero.

 Tomi

Patch

diff --git a/drivers/video/metronomefb.c b/drivers/video/metronomefb.c
index 195cc2d..4f36a2b 100644
--- a/drivers/video/metronomefb.c
+++ b/drivers/video/metronomefb.c
@@ -126,7 +126,7 @@  static struct fb_var_screeninfo metronomefb_var = {
 
 /* the waveform structure that is coming from userspace firmware */
 struct waveform_hdr {
-	u8 stuff[32];
+	u8 ignore1[32];
 
 	u8 wmta[3];
 	u8 fvsn;
@@ -134,13 +134,14 @@  struct waveform_hdr {
 	u8 luts;
 	u8 mc;
 	u8 trc;
-	u8 stuff3;
+	u8 ignore2;
 
 	u8 endb;
 	u8 swtb;
-	u8 stuff2a[2];
+	u32 pad1; /* u16 halfof(pad1) */
 
-	u8 stuff2b[3];
+	/* u16 halfof(pad1) */
+	u8 pad2;
 	u8 wfm_cs;
 } __attribute__ ((packed));
 
@@ -210,11 +211,9 @@  static int load_waveform(u8 *mem, size_t size, int m, int t,
 	}
 	wfm_hdr->mc += 1;
 	wfm_hdr->trc += 1;
-	for (i = 0; i < 5; i++) {
-		if (*(wfm_hdr->stuff2a + i) != 0) {
-			dev_err(dev, "Error: unexpected value in padding\n");
-			return -EINVAL;
-		}
+	if (wfm_hdr->pad1 || wfm_hdr->pad2) {
+		dev_err(dev, "Error: unexpected value in padding\n");
+		return -EINVAL;
 	}
 
 	/* calculating trn. trn is something used to index into