[PATCH] drm/amdgpu: prevent parameter-space underflow in nested ATOM table calls

[Aldo Ariel Panzardo]

atom_op_calltable() invokes a child ATOM table, forwarding the
parent's parameter space with an offset:

    amdgpu_atom_execute_table_locked(ctx->ctx, idx,
        ctx->ps + ctx->ps_shift,
        ctx->ps_size - ctx->ps_shift);

ctx->ps_shift is derived from the child table's PS byte count
(ps / 4) in amdgpu_atom_execute_table_locked(), while ctx->ps_size
carries the remaining capacity from the parent. A malformed ATOM
table chain in the VBIOS (or a GPU that reports corrupted table
headers) can produce ps_shift > ps_size, underflowing the
subtraction to a huge positive value passed as params_size to the
recursive call. The child table then reads and writes far beyond
the stack-allocated parameter buffer.

Reject the call when the shift exceeds the available size.

Fixes: d38ceaf99ed0 ("drm/amdgpu: add coordinate ATOMBIOS table support")
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
Signed-off-by: Aldo Ariel Panzardo <qwe.aldo@xxxxxxxxx>
---
diff --git a/drivers/gpu/drm/amd/amdgpu/atom.c b/drivers/gpu/drm/amd/amdgpu/atom.c
diff --git a/drivers/gpu/drm/amd/amdgpu/atom.c b/drivers/gpu/drm/amd/amdgpu/atom.c
index e0e585f..638bd18 100644
--- a/drivers/gpu/drm/amd/amdgpu/atom.c
+++ b/drivers/gpu/drm/amd/amdgpu/atom.c
@@ -646,8 +646,13 @@ static void atom_op_calltable(atom_exec_context *ctx, int *ptr, int arg)
 		SDEBUG("   table: %d (%s)\n", idx, atom_table_names[idx]);
 	else
 		SDEBUG("   table: %d\n", idx);
-	if (U16(ctx->ctx->cmd_table + 4 + 2 * idx))
+	if (U16(ctx->ctx->cmd_table + 4 + 2 * idx)) {
+		if (ctx->ps_shift > ctx->ps_size) {
+			ctx->abort = true;
+			return;
+		}
 		r = amdgpu_atom_execute_table_locked(ctx->ctx, idx, ctx->ps + ctx->ps_shift, ctx->ps_size - ctx->ps_shift);
+	}
 	if (r) {
 		ctx->abort = true;
 	}
-- 
2.43.0