Re: [PATCH] iommu/omap: fix buffer overflow in debugfs

[Robin Murphy <robin.murphy@arm.com>]

On 04/08/2022 3:32 pm, Dan Carpenter wrote:
> There are two issues here:
> 
> 1) The "len" variable needs to be checked before the very first write.
>     Otherwise if omap2_iommu_dump_ctx() with "bytes" less than 32 it is a
>     buffer overflow.
> 2) The snprintf() function returns the number of bytes that *would* have
>     been copied if there were enough space.  But we want to know the
>     number of bytes which were *actually* copied so use scnprintf()
>     instead.
> 
> Fixes: bd4396f09a4a ("iommu/omap: Consolidate OMAP IOMMU modules")

FWIW I think this has actually been broken since day one back in 
14e0e6796a0d, but I'm also not inclined to care that much - 2014 is 
already plenty long ago.

> Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
> ---
>   drivers/iommu/omap-iommu-debug.c | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/iommu/omap-iommu-debug.c b/drivers/iommu/omap-iommu-debug.c
> index a99afb5d9011..259f65291d90 100644
> --- a/drivers/iommu/omap-iommu-debug.c
> +++ b/drivers/iommu/omap-iommu-debug.c
> @@ -32,12 +32,12 @@ static inline bool is_omap_iommu_detached(struct omap_iommu *obj)
>   		ssize_t bytes;						\
>   		const char *str = "%20s: %08x\n";			\
>   		const int maxcol = 32;					\
> -		bytes = snprintf(p, maxcol, str, __stringify(name),	\
> +		if (len < maxcol)					\
> +			goto out;					\
> +		bytes = scnprintf(p, maxcol, str, __stringify(name),	\
>   				 iommu_read_reg(obj, MMU_##name));	\

I suppose snprintf is OK in practice since none of the names are 
anywhere near 20 characters long anyway, but I agree it's better to be 
obviously foolproof. Frankly this code is all kinds of horrible anyway, 
and deserves ripping out and replacing with a simple seq_file, but for 
this fix as a fix,

Reviewed-by: Robin Murphy <robin.murphy@arm.com>

Cheers,
Robin.

>   		p += bytes;						\
>   		len -= bytes;						\
> -		if (len < maxcol)					\
> -			goto out;					\
>   	} while (0)
>   
>   static ssize_t