firmware: dmi_scan: Prevent dmi_num integer overflow

dmi_num is a u16, dmi_len is a u32, so this construct:

	dmi_num = dmi_len / 4;

would result in an integer overflow for a DMI table larger than
256 kB. I've never see such a large table so far, but SMBIOS 3.0
makes it possible so maybe we'll see such tables in the future.

So instead of faking a structure count when the entry point does
not provide it, adjust the loop condition in dmi_table() to properly
deal with the case where dmi_num is not set.

This bug was introduced with the initial SMBIOS 3.0 support in commit
fc43026278b2 ("dmi: add support for SMBIOS 3.0 64-bit entry point").

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Matt Fleming <matt.fleming@intel.com>
Cc: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Cc: <stable@vger.kernel.org>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>

1 files changed, 7 insertions, 15 deletions

diff --git a/drivers/firmware/dmi_scan.c b/drivers/firmware/dmi_scan.c
index 69fac068669f..2eebd28b4c40 100644
--- a/drivers/firmware/dmi_scan.c
+++ b/drivers/firmware/dmi_scan.c
@@ -86,10 +86,13 @@ static void dmi_table(u8 *buf, u32 len, int num,
         int i = 0;
 
         /*
-         *      Stop when we see all the items the table claimed to have
-         *      OR we run off the end of the table (also happens)
+         * Stop when we have seen all the items the table claimed to have
+         * (SMBIOS < 3.0 only) OR we reach an end-of-table marker OR we run
+         * off the end of the table (should never happen but sometimes does
+         * on bogus implementations.)
          */
-        while ((i < num) && (data - buf + sizeof(struct dmi_header)) <= len) {
+        while ((!num || i < num) &&
+               (data - buf + sizeof(struct dmi_header)) <= len) {
                 const struct dmi_header *dm = (const struct dmi_header *)data;
 
                 /*
@@ -529,21 +532,10 @@ static int __init dmi_smbios3_present(const u8 *buf)
         if (memcmp(buf, "_SM3_", 5) == 0 &&
             buf[6] < 32 && dmi_checksum(buf, buf[6])) {
                 dmi_ver = get_unaligned_be16(buf + 7);
+                dmi_num = 0;                    /* No longer specified */
                 dmi_len = get_unaligned_le32(buf + 12);
                 dmi_base = get_unaligned_le64(buf + 16);
 
-                /*
-                 * The 64-bit SMBIOS 3.0 entry point no longer has a field
-                 * containing the number of structures present in the table.
-                 * Instead, it defines the table size as a maximum size, and
-                 * relies on the end-of-table structure type (#127) to be used
-                 * to signal the end of the table.
-                 * So let's define dmi_num as an upper bound as well: each
-                 * structure has a 4 byte header, so dmi_len / 4 is an upper
-                 * bound for the number of structures in the table.
-                 */
-                dmi_num = dmi_len / 4;
-
                 if (dmi_walk_early(dmi_decode) == 0) {
                         pr_info("SMBIOS %d.%d present.\n",
                                 dmi_ver >> 8, dmi_ver & 0xFF);