[PATCH] drm/amdgpu/atom: do not dereference NULL vbios string pointer

[Noah Weinberger]

Hello,

In atom_get_vbios_pn(), when the part-number string is empty and
atom_find_str_in_rom() cannot find the ATOMBIOS anchor, the fallback
computes NULL + sizeof(BIOS_ATOM_PREFIX) - 1, i.e. (unsigned char *)8:

    vbios_str = atom_find_str_in_rom(ctx, BIOS_ATOM_PREFIX, 3, 1024, 64);
    if (vbios_str == NULL)
            vbios_str += sizeof(BIOS_ATOM_PREFIX) - 1;

The subsequent range check is false for a near-NULL pointer, so the
value survives and *vbios_str dereferences address 8 - a NULL-page oops
during probe with a malformed VBIOS that passes the magic checks. The
recent hardening of the neighboring date/version/build helpers closed
their OOB reads; this fallback path was left intact.

Simply leave the part number empty when the anchor is not found.

Affected version/commit: mainline fa4ad798f6f681031775266044ba1b0cf3c670bd (v7.4-rc1, 2026-11-07), drivers/gpu/drm/amd/amdgpu/atom.c:1491-1503 (atom_get_vbios_pn).
Pointer-arithmetic-on-NULL is pre-existing; this window's hardening series made it the sole fallback outcome.

This was found during an assisted (AI-supported) static source review of
mainline fa4ad798f6f681031775266044ba1b0cf3c670bd (v7.4-rc1, 2026-11-07); every code path cited below was re-verified by the
reporter against that tree. The patch below has NOT been compiled and has
NOT been runtime tested, and no reproducer was produced - the triggering
conditions are described above instead. Please treat this as a report for
verification rather than a battle-tested fix.

Reported-by: Noah Weinberger <noahweinberger2003@xxxxxxxxx>
---
diff --git a/drivers/gpu/drm/amd/amdgpu/atom.c b/drivers/gpu/drm/amd/amdgpu/atom.c
index 1a599d5..7598606 100644
--- a/drivers/gpu/drm/amd/amdgpu/atom.c
+++ b/drivers/gpu/drm/amd/amdgpu/atom.c
@@ -1493,8 +1493,6 @@ static void atom_get_vbios_pn(struct atom_context *ctx)
 
 	if (vbios_str == NULL || *vbios_str == 0) {
 		vbios_str = atom_find_str_in_rom(ctx, BIOS_ATOM_PREFIX, 3, 1024, 64);
-		if (vbios_str == NULL)
-			vbios_str += sizeof(BIOS_ATOM_PREFIX) - 1;
 		if (vbios_str >= p_rom + ctx->bios_size)
 			vbios_str = NULL;
 	}