[PATCH v2 05/11] gpu: nova-core: vbios: use checked accesses in `setup_falcon_data`

[Eliot Courtney <ecourtney@nvidia.com>]

Use checked arithmetic for `ucode_offset` in `setup_falcon_data`. This
prevents a malformed firmware from causing a panic.

Fixes: dc70c6ae2441 ("gpu: nova-core: vbios: Add support to look up PMU table in FWSEC")
Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
---
 drivers/gpu/nova-core/vbios.rs | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/nova-core/vbios.rs b/drivers/gpu/nova-core/vbios.rs
index bc752d135cbf..d8633e61178b 100644
--- a/drivers/gpu/nova-core/vbios.rs
+++ b/drivers/gpu/nova-core/vbios.rs
@@ -955,14 +955,15 @@ fn setup_falcon_data(
             .find_entry_by_type(FALCON_UCODE_ENTRY_APPID_FWSEC_PROD)
         {
             Ok(entry) => {
-                let mut ucode_offset = usize::from_safe_cast(entry.data);
-                ucode_offset -= pci_at_image.base.data.len();
-                if ucode_offset < first_fwsec.base.data.len() {
-                    dev_err!(self.base.dev, "Falcon Ucode offset not in second Fwsec.\n");
-                    return Err(EINVAL);
-                }
-                ucode_offset -= first_fwsec.base.data.len();
-                self.falcon_ucode_offset = Some(ucode_offset);
+                self.falcon_ucode_offset = Some(
+                    usize::from_safe_cast(entry.data)
+                        .checked_sub(pci_at_image.base.data.len())
+                        .and_then(|o| o.checked_sub(first_fwsec.base.data.len()))
+                        .ok_or(EINVAL)
+                        .inspect_err(|_| {
+                            dev_err!(self.base.dev, "Falcon Ucode offset not in second Fwsec.\n");
+                        })?,
+                );
             }
             Err(e) => {
                 dev_err!(

-- 
2.53.0