Re: [PATCH] tee: Prevent size calculation wraparound on 32-bit kernels

[David Laight <david.laight.linux@gmail.com>]

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On Mon, 28 Apr 2025 15:06:43 +0200
Jann Horn <jannh@google.com> wrote:

> The current code around TEE_IOCTL_PARAM_SIZE() is a bit wrong on
> 32-bit kernels: Multiplying a user-provided 32-bit value with the
> size of a structure can wrap around on such platforms.
> 
> Fix it by using saturating arithmetic for the size calculation.

Why not just add a sanity check on 'num_params' after it is read.
Max is 31 (1024-32)/32), but any sane limit will do because of
the buf.buf_len test.

	David

> 
> This has no security consequences because, in all users of
> TEE_IOCTL_PARAM_SIZE(), the subsequent kcalloc() implicitly checks
> for wrapping.
> 
> Signed-off-by: Jann Horn <jannh@google.com>
> ---
> Note that I don't have a test device with a TEE; I only compile-tested
> the change on an x86-64 build.
> ---
>  drivers/tee/tee_core.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/tee/tee_core.c b/drivers/tee/tee_core.c
> index d113679b1e2d..acc7998758ad 100644
> --- a/drivers/tee/tee_core.c
> +++ b/drivers/tee/tee_core.c
> @@ -10,6 +10,7 @@
>  #include <linux/fs.h>
>  #include <linux/idr.h>
>  #include <linux/module.h>
> +#include <linux/overflow.h>
>  #include <linux/slab.h>
>  #include <linux/tee_core.h>
>  #include <linux/uaccess.h>
> @@ -19,7 +20,7 @@
>  
>  #define TEE_NUM_DEVICES	32
>  
> -#define TEE_IOCTL_PARAM_SIZE(x) (sizeof(struct tee_param) * (x))
> +#define TEE_IOCTL_PARAM_SIZE(x) (size_mul(sizeof(struct tee_param), (x)))
>  
>  #define TEE_UUID_NS_NAME_SIZE	128
>  
> @@ -487,7 +488,7 @@ static int tee_ioctl_open_session(struct tee_context *ctx,
>  	if (copy_from_user(&arg, uarg, sizeof(arg)))
>  		return -EFAULT;
>  
> -	if (sizeof(arg) + TEE_IOCTL_PARAM_SIZE(arg.num_params) != buf.buf_len)
> +	if (size_add(sizeof(arg), TEE_IOCTL_PARAM_SIZE(arg.num_params)) != buf.buf_len)
>  		return -EINVAL;
>  
>  	if (arg.num_params) {
> @@ -565,7 +566,7 @@ static int tee_ioctl_invoke(struct tee_context *ctx,
>  	if (copy_from_user(&arg, uarg, sizeof(arg)))
>  		return -EFAULT;
>  
> -	if (sizeof(arg) + TEE_IOCTL_PARAM_SIZE(arg.num_params) != buf.buf_len)
> +	if (size_add(sizeof(arg), TEE_IOCTL_PARAM_SIZE(arg.num_params)) != buf.buf_len)
>  		return -EINVAL;
>  
>  	if (arg.num_params) {
> @@ -699,7 +700,7 @@ static int tee_ioctl_supp_recv(struct tee_context *ctx,
>  	if (get_user(num_params, &uarg->num_params))
>  		return -EFAULT;
>  
> -	if (sizeof(*uarg) + TEE_IOCTL_PARAM_SIZE(num_params) != buf.buf_len)
> +	if (size_add(sizeof(*uarg), TEE_IOCTL_PARAM_SIZE(num_params)) != buf.buf_len)
>  		return -EINVAL;
>  
>  	params = kcalloc(num_params, sizeof(struct tee_param), GFP_KERNEL);
> @@ -798,7 +799,7 @@ static int tee_ioctl_supp_send(struct tee_context *ctx,
>  	    get_user(num_params, &uarg->num_params))
>  		return -EFAULT;
>  
> -	if (sizeof(*uarg) + TEE_IOCTL_PARAM_SIZE(num_params) > buf.buf_len)
> +	if (size_add(sizeof(*uarg), TEE_IOCTL_PARAM_SIZE(num_params)) > buf.buf_len)
>  		return -EINVAL;
>  
>  	params = kcalloc(num_params, sizeof(struct tee_param), GFP_KERNEL);
> 
> ---
> base-commit: b4432656b36e5cc1d50a1f2dc15357543add530e
> change-id: 20250428-tee-sizecheck-299d5eff8fc7
>