[PATCH] mtd: physmap: physmap-bt1-rom: Fix unintentional stack access

[PATCH] mtd: physmap: physmap-bt1-rom: Fix unintentional stack access

[Gustavo A. R. Silva]

Cast &data to (char *) in order to avoid unintentionally accessing
the stack.

Notice that data is of type u32, so any increment to &data
will be in the order of 4-byte chunks, and this piece of code
is actually intended to be a byte offset.

Fixes: b3e79e7682e0 ("mtd: physmap: Add Baikal-T1 physically mapped ROM support")
Addresses-Coverity-ID: 1497765 ("Out-of-bounds access")
Cc: stable@vger.kernel.org
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
 drivers/mtd/maps/physmap-bt1-rom.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/mtd/maps/physmap-bt1-rom.c b/drivers/mtd/maps/physmap-bt1-rom.c
index a35450002284..58782cfaf71c 100644
--- a/drivers/mtd/maps/physmap-bt1-rom.c
+++ b/drivers/mtd/maps/physmap-bt1-rom.c
@@ -79,7 +79,7 @@ static void __xipram bt1_rom_map_copy_from(struct map_info *map,
 	if (shift) {
 		chunk = min_t(ssize_t, 4 - shift, len);
 		data = readl_relaxed(src - shift);
-		memcpy(to, &data + shift, chunk);
+		memcpy(to, (char *)&data + shift, chunk);
 		src += chunk;
 		to += chunk;
 		len -= chunk;
-- 
2.27.0


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Re: [PATCH] mtd: physmap: physmap-bt1-rom: Fix unintentional stack access

[Miquel Raynal]

Hi Gustavo,

"Gustavo A. R. Silva" <gustavoars@kernel.org> wrote on Fri, 12 Feb 2021
04:40:22 -0600:

> Cast &data to (char *) in order to avoid unintentionally accessing
> the stack.
> 
> Notice that data is of type u32, so any increment to &data
> will be in the order of 4-byte chunks, and this piece of code
> is actually intended to be a byte offset.

I don't have the same reading. I don't say that Coverity report is
wrong, but let's discuss this a bit further.

Given that &data is of type u32 *, you say that "&data + shift"
produces increments of 4-bytes, ie. we would access "&data + 4 *
shift"? Because I don't think this is the case (again, I may be wrong).

I'm sure this would be the case if we dereferenced data like
data[shift] though, but in this case I don't see what this cast is
fixing. Can you enlighten me?

Could the out-of-bounds warning come from the fact that shift
might be bigger than the data array spread?

> Fixes: b3e79e7682e0 ("mtd: physmap: Add Baikal-T1 physically mapped ROM support")
> Addresses-Coverity-ID: 1497765 ("Out-of-bounds access")
> Cc: stable@vger.kernel.org
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> ---
>  drivers/mtd/maps/physmap-bt1-rom.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/mtd/maps/physmap-bt1-rom.c b/drivers/mtd/maps/physmap-bt1-rom.c
> index a35450002284..58782cfaf71c 100644
> --- a/drivers/mtd/maps/physmap-bt1-rom.c
> +++ b/drivers/mtd/maps/physmap-bt1-rom.c
> @@ -79,7 +79,7 @@ static void __xipram bt1_rom_map_copy_from(struct map_info *map,
>  	if (shift) {
>  		chunk = min_t(ssize_t, 4 - shift, len);
>  		data = readl_relaxed(src - shift);
> -		memcpy(to, &data + shift, chunk);
> +		memcpy(to, (char *)&data + shift, chunk);
>  		src += chunk;
>  		to += chunk;
>  		len -= chunk;

Thanks,
Miquèl

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Re: [PATCH] mtd: physmap: physmap-bt1-rom: Fix unintentional stack access

[Gustavo A. R. Silva]

On 2/12/21 08:12, Miquel Raynal wrote:
> Hi Gustavo,
> 
> "Gustavo A. R. Silva" <gustavoars@kernel.org> wrote on Fri, 12 Feb 2021
> 04:40:22 -0600:
> 
>> Cast &data to (char *) in order to avoid unintentionally accessing
>> the stack.
>>
>> Notice that data is of type u32, so any increment to &data
>> will be in the order of 4-byte chunks, and this piece of code
>> is actually intended to be a byte offset.
> 
> I don't have the same reading. I don't say that Coverity report is
> wrong, but let's discuss this a bit further.
> 
> Given that &data is of type u32 *, you say that "&data + shift"
> produces increments of 4-bytes, ie. we would access "&data + 4 *
> shift"? Because I don't think this is the case (again, I may be wrong).

Yep; this is pointer arithmetic. If you have an object ptr of type u32 *:

u32 *ptr;

and let's say it points to address 100. If you increment it by one:

ptr++

ptr will now point to address 104, not to 101.

Now, if instead, you first cast ptr to 'char *' and increment it by 1,
then it will point to address 101.

> 
> I'm sure this would be the case if we dereferenced data like
> data[shift] though, but in this case I don't see what this cast is
> fixing. Can you enlighten me?
> 
> Could the out-of-bounds warning come from the fact that shift
> might be bigger than the data array spread?

Whats Coverity wants to say here is basically that using &data as an
array might corrupt adjacent memory.

Thanks
--
Gustavo

> 
>> Fixes: b3e79e7682e0 ("mtd: physmap: Add Baikal-T1 physically mapped ROM support")
>> Addresses-Coverity-ID: 1497765 ("Out-of-bounds access")
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
>> ---
>>  drivers/mtd/maps/physmap-bt1-rom.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/mtd/maps/physmap-bt1-rom.c b/drivers/mtd/maps/physmap-bt1-rom.c
>> index a35450002284..58782cfaf71c 100644
>> --- a/drivers/mtd/maps/physmap-bt1-rom.c
>> +++ b/drivers/mtd/maps/physmap-bt1-rom.c
>> @@ -79,7 +79,7 @@ static void __xipram bt1_rom_map_copy_from(struct map_info *map,
>>  	if (shift) {
>>  		chunk = min_t(ssize_t, 4 - shift, len);
>>  		data = readl_relaxed(src - shift);
>> -		memcpy(to, &data + shift, chunk);
>> +		memcpy(to, (char *)&data + shift, chunk);
>>  		src += chunk;
>>  		to += chunk;
>>  		len -= chunk;
> 
> Thanks,
> Miquèl
> 

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Re: [PATCH] mtd: physmap: physmap-bt1-rom: Fix unintentional stack access

[Miquel Raynal]

Hi Gustavo,

"Gustavo A. R. Silva" <gustavo@embeddedor.com> wrote on Fri, 12 Feb
2021 08:45:33 -0600:

> On 2/12/21 08:12, Miquel Raynal wrote:
> > Hi Gustavo,
> > 
> > "Gustavo A. R. Silva" <gustavoars@kernel.org> wrote on Fri, 12 Feb 2021
> > 04:40:22 -0600:
> >   
> >> Cast &data to (char *) in order to avoid unintentionally accessing
> >> the stack.
> >>
> >> Notice that data is of type u32, so any increment to &data
> >> will be in the order of 4-byte chunks, and this piece of code
> >> is actually intended to be a byte offset.  
> > 
> > I don't have the same reading. I don't say that Coverity report is
> > wrong, but let's discuss this a bit further.
> > 
> > Given that &data is of type u32 *, you say that "&data + shift"
> > produces increments of 4-bytes, ie. we would access "&data + 4 *
> > shift"? Because I don't think this is the case (again, I may be wrong).  
> 
> Yep; this is pointer arithmetic. If you have an object ptr of type u32 *:
> 
> u32 *ptr;
> 
> and let's say it points to address 100. If you increment it by one:
> 
> ptr++
> 
> ptr will now point to address 104, not to 101.
> 
> Now, if instead, you first cast ptr to 'char *' and increment it by 1,
> then it will point to address 101.

Yep, I got confused with the proper addition compared to dereferencing.

Patch looks legitimate.

Thanks,
Miquèl

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Re: [PATCH] mtd: physmap: physmap-bt1-rom: Fix unintentional stack access

[Gustavo A. R. Silva]

On 2/12/21 08:50, Miquel Raynal wrote:

>> Now, if instead, you first cast ptr to 'char *' and increment it by 1,
>> then it will point to address 101.
> 
> Yep, I got confused with the proper addition compared to dereferencing.

No problem.

> Patch looks legitimate.

Great. :)

Thanks
--
Gustavo

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Re: [PATCH] mtd: physmap: physmap-bt1-rom: Fix unintentional stack access

[Serge Semin]

On Fri, Feb 12, 2021 at 04:40:22AM -0600, Gustavo A. R. Silva wrote:
> Cast &data to (char *) in order to avoid unintentionally accessing
> the stack.
> 
> Notice that data is of type u32, so any increment to &data
> will be in the order of 4-byte chunks, and this piece of code
> is actually intended to be a byte offset.

Thanks, one more time.)
Acked-by: Serge Semin <fancer.lancer@gmail.com>

> 
> Fixes: b3e79e7682e0 ("mtd: physmap: Add Baikal-T1 physically mapped ROM support")
> Addresses-Coverity-ID: 1497765 ("Out-of-bounds access")
> Cc: stable@vger.kernel.org
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> ---
>  drivers/mtd/maps/physmap-bt1-rom.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/mtd/maps/physmap-bt1-rom.c b/drivers/mtd/maps/physmap-bt1-rom.c
> index a35450002284..58782cfaf71c 100644
> --- a/drivers/mtd/maps/physmap-bt1-rom.c
> +++ b/drivers/mtd/maps/physmap-bt1-rom.c
> @@ -79,7 +79,7 @@ static void __xipram bt1_rom_map_copy_from(struct map_info *map,
>  	if (shift) {
>  		chunk = min_t(ssize_t, 4 - shift, len);
>  		data = readl_relaxed(src - shift);
> -		memcpy(to, &data + shift, chunk);
> +		memcpy(to, (char *)&data + shift, chunk);
>  		src += chunk;
>  		to += chunk;
>  		len -= chunk;
> -- 
> 2.27.0
> 

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Re: [PATCH] mtd: physmap: physmap-bt1-rom: Fix unintentional stack access

[Miquel Raynal]

On Fri, 2021-02-12 at 10:40:22 UTC, "Gustavo A. R. Silva" wrote:
> Cast &data to (char *) in order to avoid unintentionally accessing
> the stack.
> 
> Notice that data is of type u32, so any increment to &data
> will be in the order of 4-byte chunks, and this piece of code
> is actually intended to be a byte offset.
> 
> Fixes: b3e79e7682e0 ("mtd: physmap: Add Baikal-T1 physically mapped ROM support")
> Addresses-Coverity-ID: 1497765 ("Out-of-bounds access")
> Cc: stable@vger.kernel.org
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> Acked-by: Serge Semin <fancer.lancer@gmail.com>

Applied to https://git.kernel.org/pub/scm/linux/kernel/git/mtd/linux.git mtd/next, thanks.

Miquel

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