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

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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
This commit is contained in:
Solomon Peachy 2020-07-11 16:22:21 -04:00
parent aa20b6af7a
commit 7249fabe63
1 changed files with 11 additions and 0 deletions
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11
firmware/drivers/fat.c
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@ -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);
}