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fs/msdosfs: Fix potential panic and size calculations

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Some combinations of FAT12 file system parameters could cause a kernel
panic due to an unmapped access if the size of the FAT was larger than
the CPU page size. The reason is that FAT12 uses 3 bytes to store
2 FAT pointers, leading to partial FAT pointers at the end of buffers
of a size that is not a multiple of 3.

With a typical page size of 4 KB, this caused the FAT entry at byte
offsets 4095 and 4096 to cross the page boundary, with only the first
page mapped. This was fixed by adjusting the mapping to always cover
both bytes of each FAT entry.

Testing revealed 2 other inconsistencies that are fixed by this commit:

1) The calculation of the size of the data area did not take into
   account the fact that the first two data block numbers are reserved
   and that the data area starts with block 2. This could cause a
   FAT12 file system created with the maximum supported number of
   blocks to be incorrectly identified as FAT16.

2) The root directory does not take up space in the data area of a
   FAT12 or FAT16 file system, since it is placed into a reserved
   area outside of that data area. This commits makes stat() report
   the logical size of the root directory, but with 0 blocks allocated
   from the data area.

PR:		270587
Reviewed by:	mckusick
Differential Revision:	https://reviews.freebsd.org/D39386
This commit is contained in:
Stefan Eßer 2023-04-25 08:35:16 +02:00
parent 4b932c2c9e
commit 0728695c63
3 changed files with 23 additions and 11 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

12
sys/fs/msdosfs/msdosfs_fat.c
View File

@ -89,11 +89,17 @@ static void
fatblock(struct msdosfsmount *pmp, u_long ofs, u_long *bnp, u_long *sizep,
u_long *bop)
{
u_long bn, size;
u_long bn, size, fatblocksec;
fatblocksec = pmp->pm_fatblocksec;
if (FAT12(pmp) && fatblocksec % 3 != 0) {
fatblocksec *= 3;
if (fatblocksec % 6 == 0)
fatblocksec /= 2;
}
bn = ofs / pmp->pm_fatblocksize * pmp->pm_fatblocksec;
size = min(pmp->pm_fatblocksec, pmp->pm_FATsecs - bn)
* DEV_BSIZE;
size = roundup(min(fatblocksec, pmp->pm_FATsecs - bn) * DEV_BSIZE,
pmp->pm_BlkPerSec * DEV_BSIZE);
bn += pmp->pm_fatblk + pmp->pm_curfat * pmp->pm_FATsecs;
if (bnp)

15
sys/fs/msdosfs/msdosfs_vfsops.c
View File

@ -700,11 +700,14 @@ mountmsdosfs(struct vnode *odevvp, struct mount *mp)
}
pmp->pm_maxcluster = (pmp->pm_HugeSectors - pmp->pm_firstcluster) /
SecPerClust + 1;
pmp->pm_fatsize = pmp->pm_FATsecs * DEV_BSIZE; /* XXX not used? */
pmp->pm_fatsize = pmp->pm_FATsecs * DEV_BSIZE;
if (pmp->pm_fatmask == 0) {
if (pmp->pm_maxcluster <= ((CLUST_RSRVD - CLUST_FIRST) &
FAT12_MASK)) {
/*
* The last 10 (or 16?) clusters are reserved and must not
* be allocated for data.
*/
if (pmp->pm_maxcluster < (CLUST_RSRVD & FAT12_MASK)) {
/*
* This will usually be a floppy disk. This size makes
* sure that one FAT entry will not be split across
@ -720,11 +723,11 @@ mountmsdosfs(struct vnode *odevvp, struct mount *mp)
}
}
clusters = (pmp->pm_fatsize / pmp->pm_fatmult) * pmp->pm_fatdiv;
clusters = (pmp->pm_fatsize / pmp->pm_fatmult) * pmp->pm_fatdiv ;
if (pmp->pm_maxcluster >= clusters) {
#ifdef MSDOSFS_DEBUG
printf("Warning: number of clusters (%ld) exceeds FAT "
"capacity (%ld)\n", pmp->pm_maxcluster + 1, clusters);
"capacity (%ld)\n", pmp->pm_maxcluster - 1, clusters);
#endif
pmp->pm_maxcluster = clusters - 1;
}
@ -1045,7 +1048,7 @@ msdosfs_statfs(struct mount *mp, struct statfs *sbp)
pmp = VFSTOMSDOSFS(mp);
sbp->f_bsize = pmp->pm_bpcluster;
sbp->f_iosize = pmp->pm_bpcluster;
sbp->f_blocks = pmp->pm_maxcluster + 1;
sbp->f_blocks = pmp->pm_maxcluster - CLUST_FIRST + 1;
sbp->f_bfree = pmp->pm_freeclustercount;
sbp->f_bavail = pmp->pm_freeclustercount;
sbp->f_files = howmany(pmp->pm_rootdirsize * DEV_BSIZE,

7
sys/fs/msdosfs/msdosfs_vnops.c
View File

@ -313,8 +313,11 @@ msdosfs_getattr(struct vop_getattr_args *ap)
vap->va_flags |= UF_SYSTEM;
vap->va_gen = 0;
vap->va_blocksize = pmp->pm_bpcluster;
vap->va_bytes =
(dep->de_FileSize + pmp->pm_crbomask) & ~pmp->pm_crbomask;
if (dep->de_StartCluster != MSDOSFSROOT)
vap->va_bytes =
(dep->de_FileSize + pmp->pm_crbomask) & ~pmp->pm_crbomask;
else
vap->va_bytes = 0; /* FAT12/FAT16 root dir in reserved area */
vap->va_type = ap->a_vp->v_type;
vap->va_filerev = dep->de_modrev;
return (0);