Hi Michael,
On Sat, Oct 13, 2018 at 4:23 AM Michael Schmitz <[email protected]> wrote:
> Am 12.10.2018 um 21:54 schrieb Geert Uytterhoeven:
> > Thanks for being persistent!
> >
> > BTW, there's another possible overflow in "blk *= blksize", but that one
> > is very unlikely to happen, as most (all?) partitioners store partition
> > blocks close to the beginning of the disk.
>
> Thanks - we can at least change the type of blk to sector_t to limit the
> potential for multiplication overflow, but with RDB_ALLOCATION_LIMIT at
> 16, we would miss RDB partition blocks not at the very beginning of a
> disk anyway.
I thought so, too, but then realized that RDB_ALLOCATION_LIMIT applies to
the RDB block itself, not to the partition blocks.
> >> --- a/block/partitions/amiga.c
> >> +++ b/block/partitions/amiga.c
> >
> >> @@ -32,9 +40,12 @@ int amiga_partition(struct parsed_partitions *state)
> >> unsigned char *data;
> >> struct RigidDiskBlock *rdb;
> >> struct PartitionBlock *pb;
> >> - sector_t start_sect, nr_sects;
> >> + u64 start_sect, nr_sects;
> >> + sector_t end_sect;
> >> + u32 cylblk; /* rdb_CylBlocks = nr_heads*sect_per_track
> >> */
> >> + u32 nr_hd, nr_sect, lo_cyl, hi_cyl;
> >> int blk, part, res = 0;
> >> - int blksize = 1; /* Multiplier for disk block size */
> >> + unsigned int blksize = 1; /* Multiplier for disk block size
> >> */
> >> int slot = 1;
> >> char b[BDEVNAME_SIZE];
> >>
> >> @@ -99,19 +110,70 @@ int amiga_partition(struct parsed_partitions *state)
> >> if (checksum_block((__be32 *)pb,
> >> be32_to_cpu(pb->pb_SummedLongs) & 0x7F) != 0 )
> >> continue;
> >>
> >> - /* Tell Kernel about it */
> >> + /* RDB gives us more than enough rope to hang ourselves
> >> with,
> >> + * many times over (2^128 bytes if all fields max out).
> >> + * Some careful checks are in order, so check for potential
> >> + * overflows.
> >> + * We are multiplying four 32 bit numbers to one sector_t!
> >> + */
> >> +
> >> + nr_hd = be32_to_cpu(pb->pb_Environment[NR_HD]);
> >> + nr_sect = be32_to_cpu(pb->pb_Environment[NR_SECT]);
> >> +
> >> + /* CylBlocks is total number of blocks per cylinder */
> >> + if (check_mul_overflow(nr_hd, nr_sect, &cylblk)) {
> >> + pr_err("Dev %s: heads*sects %u overflows u32,
> >> skipping partition!\n",
> >> + bdevname(state->bdev, b), cylblk);
> >> + continue;
> >> + }
> >> +
> >> + /* check for consistency with RDB defined CylBlocks */
> >> + if (cylblk > be32_to_cpu(rdb->rdb_CylBlocks)) {
> >> + pr_warn("Dev %s: cylblk %u > rdb_CylBlocks %u!\n",
> >> + bdevname(state->bdev, b), cylblk,
> >> + be32_to_cpu(rdb->rdb_CylBlocks));
> >> + }
> >> +
> >> + /* RDB allows for variable logical block size -
> >> + * normalize to 512 byte blocks and check result.
> >> + */
> >> +
> >> + if (check_mul_overflow(cylblk, blksize, &cylblk)) {
> >> + pr_err("Dev %s: partition %u bytes per cyl.
> >> overflows u32, skipping partition!\n",
> >> + bdevname(state->bdev, b), part);
> >
> > Unlike the comparison with 32-bit rdb_CylBlocks above, this is an
> > artificial limitation, right?
> > You can relax it by using 64-bit arithmetic, but that would complicate the
> > calculation of start_sect and nr_sects below, as they may overflow 64-bit.
>
> Correct - this allows me to skip the overflow check on the final result
> (see comment below). But making cylblk a 32 bit type for the purpose of
> this overflow check has tripped me up below.
>
> I'd still like to retain this check - it is highly unlikely to ever
> trigger with RDB blocks in current use, and should disks ever get so
> large as to require the total number of 512 byte blocks per cylinder to
> exceed the 32 bit limit, I'd certainly hope other partition table
> options get chosen.
Fair enough.
> >> + if (check_add_overflow(start_sect, nr_sects, &end_sect)) {
> >> + pr_err("Dev %s: partition %u (%llu-%llu) needs LBD
> >> device support, skipping partition!\n",
> >> + bdevname(state->bdev, b), part,
> >> + start_sect, (u64) end_sect);
> >
> > The cast to u64 is not needed.
>
> It is, in case the kernel is compiled without LBD support (making
> end_sect a 32 bit sector_t). I haven't found a better way to check
> whether the partition exceeds what's possible to represent without LBD
> support.
You're right. I missed that end_sect was not converted to u64.
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- [email protected]
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
