Re: [PATCH v3 02/11] gpu: nova-core: vbios: limit `BitToken` entry reads

["Alexandre Courbot" <acourbot@nvidia.com>]

On Tue Apr 21, 2026 at 5:20 PM JST, Eliot Courtney wrote:
> If `header.token_size` is smaller than `BitToken`, then we currently can
> read past the end of `image.base.data`. Check that the token size is at
> least as big as `BitToken`.
>
> Fixes: dc70c6ae2441 ("gpu: nova-core: vbios: Add support to look up PMU table in FWSEC")
> Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>
> Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
> ---
>  drivers/gpu/nova-core/vbios.rs | 34 +++++++++++++++++-----------------
>  1 file changed, 17 insertions(+), 17 deletions(-)
>
> diff --git a/drivers/gpu/nova-core/vbios.rs b/drivers/gpu/nova-core/vbios.rs
> index 6de7e58e0da0..de856000de23 100644
> --- a/drivers/gpu/nova-core/vbios.rs
> +++ b/drivers/gpu/nova-core/vbios.rs
> @@ -423,31 +423,31 @@ impl BitToken {
>      /// Find a BIT token entry by BIT ID in a PciAtBiosImage
>      fn from_id(image: &PciAtBiosImage, token_id: u8) -> Result<Self> {
>          let header = &image.bit_header;
> +        let entry_size = usize::from(header.token_size);
> +
> +        if entry_size < size_of::<BitToken>() {
> +            return Err(EINVAL);
> +        }

You can get rid of this check if you convert the code as suggested
below.

>  
>          // Offset to the first token entry
>          let tokens_start = image.bit_offset + usize::from(header.header_size);
>  
>          for i in 0..usize::from(header.token_entries) {
> -            let entry_offset = tokens_start + (i * usize::from(header.token_size));
> -
> -            // Make sure we don't go out of bounds
> -            if entry_offset + usize::from(header.token_size) > image.base.data.len() {
> -                return Err(EINVAL);
> -            }
> +            let entry_offset = tokens_start + (i * entry_size);

Should we use checked arithmetic here?

> +            let entry = image
> +                .base
> +                .data
> +                .get(entry_offset..)
> +                .and_then(|data| data.get(..entry_size))
> +                .ok_or(EINVAL)?;
>  
>              // Check if this token has the requested ID
> -            if image.base.data[entry_offset] == token_id {
> +            if entry[0] == token_id {
>                  return Ok(BitToken {
> -                    id: image.base.data[entry_offset],
> -                    data_version: image.base.data[entry_offset + 1],
> -                    data_size: u16::from_le_bytes([
> -                        image.base.data[entry_offset + 2],
> -                        image.base.data[entry_offset + 3],
> -                    ]),
> -                    data_offset: u16::from_le_bytes([
> -                        image.base.data[entry_offset + 4],
> -                        image.base.data[entry_offset + 5],
> -                    ]),
> +                    id: entry[0],
> +                    data_version: entry[1],
> +                    data_size: u16::from_le_bytes([entry[2], entry[3]]),
> +                    data_offset: u16::from_le_bytes([entry[4], entry[5]]),

A common pattern in this file (with several such sites still to fix), is
that since Nova only supports little-endian we can leverage `FromBytes`
in order to avoid all these `from_le_bytes` call. Here this would look
as follows:

    for i in 0..usize::from(header.token_entries) {
        let entry_offset = i
            .checked_mul(entry_size)
            .and_then(|off| tokens_start.checked_add(off))
            .ok_or(EINVAL)?;

        let entry = image
            .base
            .data
            .get(entry_offset..entry_offset + entry_size)
            .and_then(|data| data.get(..entry_size))
            .ok_or(EINVAL)?;

        let (token, _) = BitToken::from_bytes_copy_prefix(entry).ok_or(EINVAL)?;

        if token.id == token_id {
            return Ok(token);
        }
    }

which has several benefits:

- No error-prone `entry[index]` accesses,
- The size check on `entry_size` is done for free by
  `from_bytes_copy_prefix`, and the slice bounds cannot be wrong,
- Shorter, more readable code overall.

Unfortunately we cannot just use `from_bytes_prefix` because we don't
have any alignment guarantee, but this is still an improvement IMHO.

If you go that way and derive `FromBytes` on `BitToken`, don't forget to
also make it `#[repr(C)]`. :)