fat: Validate FS Info Sector signature when attempting to mount volume

The "try to mount as superfloppy" fails with some partitioning layouts
because sector 0 can have a mostly-valid FAT32 signature.  However, in
all dumps I've looked at, sector 0's fsinfo offset value points at a place
which lacks the fsinfo signature.

Resolves FS#13213, no known regressions.

Change-Id: Ib323d35cca6ca54e11aca6ba77041bf33a05a277

1 files changed, 11 insertions, 0 deletions

diff --git a/firmware/drivers/fat.c b/firmware/drivers/fat.c
index b14e461a65..cc6dbf517c 100644
--- a/firmware/drivers/fat.c
+++ b/firmware/drivers/fat.c
@@ -178,9 +178,12 @@ struct fsinfo
                                 clusters, or 0xffffffff for no hint */
 };
 /* fsinfo offsets */
+#define FSINFO_SIGNATURE 0
 #define FSINFO_FREECOUNT 488
 #define FSINFO_NEXTFREE  492
 
+#define FSINFO_SIGNATURE_VAL 0x41615252
+
 #ifdef HAVE_FAT16SUPPORT
 #define BPB_FN_SET16(bpb, fn)      (bpb)->fn##__ = fn##16
 #define BPB_FN_SET32(bpb, fn)      (bpb)->fn##__ = fn##32
@@ -1251,6 +1254,14 @@ static int fat_mount_internal(struct bpb *fat_bpb)
             FAT_ERROR(rc * 10 - 8);
         }
 
+        /* Sanity check FS info */
+        long info = BYTES2INT32(buf, FSINFO_SIGNATURE);
+        if (info != FSINFO_SIGNATURE_VAL) {
+                DEBUGF("%S() FSInfo signature mismatch (%x)\n",
+                       __func__, info);
+                FAT_ERROR(-9);
+        }
+
         fat_bpb->fsinfo.freecount = BYTES2INT32(buf, FSINFO_FREECOUNT);
         fat_bpb->fsinfo.nextfree = BYTES2INT32(buf, FSINFO_NEXTFREE);
     }