efi: cper: fix snprintf() use in cper_dimm_err_location()

snprintf() should be given the full buffer size, not one less. And it
guarantees nul-termination, so doing it manually afterwards is
pointless.

It's even potentially harmful (though probably not in practice because
CPER_REC_LEN is 256), due to the "return how much would have been
written had the buffer been big enough" semantics. I.e., if the bank
and/or device strings are long enough that the "DIMM location ..."
output gets truncated, writing to msg[n] is a buffer overflow.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Fixes: 3760cd2 ("CPER: Adjust code flow of some functions")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

Author: Villemoes

drivers/firmware/efi/cper.c

@@ -276,8 +276,7 @@ static int cper_dimm_err_location(struct cper_mem_err_compact *mem, char *msg)
 	if (!msg || !(mem->validation_bits & CPER_MEM_VALID_MODULE_HANDLE))
 		return 0;
 
-	n = 0;
-	len = CPER_REC_LEN - 1;
+	len = CPER_REC_LEN;
 	dmi_memdev_name(mem->mem_dev_handle, &bank, &device);
 	if (bank && device)
 		n = snprintf(msg, len, "DIMM location: %s %s ", bank, device);
@@ -286,7 +285,6 @@ static int cper_dimm_err_location(struct cper_mem_err_compact *mem, char *msg)
 			     "DIMM location: not present. DMI handle: 0x%.4x ",
 			     mem->mem_dev_handle);
 
-	msg[n] = '\0';
 	return n;
 }