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SubjectRe: [PATCH v2] acpi: fix a potential inconsistency caused by double-fetch
On Wed, Jan 9, 2019 at 9:14 AM Kangjie Lu <kjlu@umn.edu> wrote:
>
> "user_buf->length" is in user space, and copied in twice. The second
> copy is after it passes the security check. If a user program races to
> change user_buf->length in user space, the data fetched in the second
> copy may invalidate the security check. The fix avoids the double-fetch
> issue by using the value passing the security check.

AFAICS the patch really does two things: it avoids the issue described
above and avoids using the (redundant) 'table' local variable on the
stack. Arguably, you could fix the issue without getting rid of the
redundant variable.

>
> Signed-off-by: Kangjie Lu <kjlu@umn.edu>
> ---
> drivers/acpi/custom_method.c | 10 ++++++----
> 1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/acpi/custom_method.c b/drivers/acpi/custom_method.c
> index 4451877f83b6..f10ee0519033 100644
> --- a/drivers/acpi/custom_method.c
> +++ b/drivers/acpi/custom_method.c
> @@ -26,17 +26,16 @@ static ssize_t cm_write(struct file *file, const char __user * user_buf,
> static u32 max_size;
> static u32 uncopied_bytes;
>
> - struct acpi_table_header table;
> acpi_status status;
>
> if (!(*ppos)) {
> /* parse the table header to get the table length */
> if (count <= sizeof(struct acpi_table_header))
> return -EINVAL;
> - if (copy_from_user(&table, user_buf,
> - sizeof(struct acpi_table_header)))
> + if (get_user(max_size,
> + &((struct acpi_table_header *)user_buf)->length))
> return -EFAULT;
> - uncopied_bytes = max_size = table.length;
> + uncopied_bytes = max_size;
> buf = kzalloc(max_size, GFP_KERNEL);
> if (!buf)
> return -ENOMEM;
> @@ -57,6 +56,9 @@ static ssize_t cm_write(struct file *file, const char __user * user_buf,
> return -EFAULT;
> }
>
> + /* Ensure table length is not changed in the second copy */
> + ((struct acpi_table_header *)(buf + (*ppos)))->length = max_size;

Why don't you return -EFAULT if max_size is different from ->length?
Surely, the table should not be used at all in that case.

Moreover, wouldn't it be even better to compare the entire header with
the one read previously and return -EFAULT if they don't match?

> +
> uncopied_bytes -= count;
> *ppos += count;
>
> --

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