fdt_support: bound serialN alias length before copying to stack

fdt_fixup_stdout() reads the path stored in /aliases/serialN with
fdt_getprop() and then memcpys it into a fixed 256-byte stack buffer.
The length returned by libfdt is the raw on-disk property size and is
not bounded by any console-path convention, so an oversized property
in a malformed or untrusted devicetree overflows the buffer with
attacker-controlled length and contents. The "/* long enough */"
comment next to tmp[] codifies an unchecked assumption.

Reject lengths that exceed sizeof(tmp) with a debug-only message and
return -FDT_ERR_NOSPACE. The fixup runs during fdt_chosen() on every
booted kernel when CONFIG_OF_STDOUT_VIA_ALIAS is enabled, and when
the OS devicetree is not signature-verified the property is reachable
from an attacker-influenced blob. Using debug() rather than printf()
keeps the rejection text out of production builds so there is no
.text or .rodata growth on space-constrained targets.

Signed-off-by: Aristo Chen <aristo.chen@canonical.com>

boot/fdt_support.c

@@ -160,6 +160,12 @@ static int fdt_fixup_stdout(void *fdt, int chosenoff)
 		goto noalias;
 	}
 
+	if (len > (int)sizeof(tmp)) {
+		debug("%s: %s alias path too long (%d bytes)\n",
+		      __func__, sername, len);
+		return -FDT_ERR_NOSPACE;
+	}
+
 	/* fdt_setprop may break "path" so we copy it to tmp buffer */
 	memcpy(tmp, path, len);