staging: media: atomisp: Fix stack buffer overflow in gmin_get_var_int()

When gmin_get_config_var() calls efi.get_variable() and the EFI variable
is larger than the expected buffer size, two behaviors combine to create
a stack buffer overflow:

1. gmin_get_config_var() does not return the proper error code when
   efi.get_variable() fails. It returns the stale 'ret' value from
   earlier operations instead of indicating the EFI failure.

2. When efi.get_variable() returns EFI_BUFFER_TOO_SMALL, it updates
   *out_len to the required buffer size but writes no data to the output
   buffer. However, due to bug #1, gmin_get_var_int() believes the call
   succeeded.

The caller gmin_get_var_int() then performs:
- Allocates val[CFG_VAR_NAME_MAX + 1] (65 bytes) on stack
- Calls gmin_get_config_var(dev, is_gmin, var, val, &len) with len=64
- If EFI variable is >64 bytes, efi.get_variable() sets len=required_size
- Due to bug #1, thinks call succeeded with len=required_size
- Executes val[len] = 0, writing past end of 65-byte stack buffer

This creates a stack buffer overflow when EFI variables are larger than
64 bytes. Since EFI variables can be controlled by firmware or system
configuration, this could potentially be exploited for code execution.

Fix the bug by returning proper error codes from gmin_get_config_var()
based on EFI status instead of stale 'ret' value.

The gmin_get_var_int() function is called during device initialization
for camera sensor configuration on Intel Bay Trail and Cherry Trail
platforms using the atomisp camera stack.

Reported-by: zepta <z3ptaa@gmail.com>
Closes: https://lore.kernel.org/all/CAPBS6KoQyM7FMdPwOuXteXsOe44X4H3F8Fw+y_qWq6E+OdmxQA@mail.gmail.com


Fixes: 38d4f74b ("media: atomisp_gmin_platform: stop abusing efivar API")
Reviewed-by: Hans de Goede <hansg@kernel.org>
Link: https://lore.kernel.org/r/20250724080756.work.741-kees@kernel.org


Signed-off-by: Kees Cook <kees@kernel.org>