| Message ID | 20210222211528.848441-1-gmazyland@gmail.com (mailing list archive) |
|---|---|
| State | Superseded, archived |
| Delegated to: | Mike Snitzer |
| Headers | show |
| Series | dm-verity: Fix FEC for RS roots non-aligned to block size | expand |
Tested-by: Jérôme Carretero <cJ-ko@zougloub.eu>
I smoke-tested by fuzzing contiguous ranges of user data (not hash
tree) and correction works up to the amount of added correction (since
erasures are identified by hashes).
Since the fix looks obvious, this should probably CC to stable?
On 23/02/2021 04:00, Jérôme Carretero wrote: > Tested-by: Jérôme Carretero <cJ-ko@zougloub.eu> > > > I smoke-tested by fuzzing contiguous ranges of user data (not hash > tree) and correction works up to the amount of added correction (since > erasures are identified by hashes). Thanks for testing! > Since the fix looks obvious, this should probably CC to stable? Yes, I forgot to add the tag, I think this should go to all stable kernels. (It only needs dm_bufio_set_sector_offset() that was added later than FEC, IIRC.) Milan p.s. There are also bugs in userspace veritysetup if FEC has hash area is stored on the same device, then code miscalculates parity bytes then. This will be fixed soon in next stable cryptsetup release. -- dm-devel mailing list dm-devel@redhat.com https://listman.redhat.com/mailman/listinfo/dm-devel
Hi Milan, On Mon, Feb 22, 2021 at 1:15 PM Milan Broz <gmazyland@gmail.com> wrote: > > Optional Forward Error Correction (FEC) code in dm-verity uses > Reed-Solomon code and should support roots from 2 to 24. > > The error correction parity bytes (of roots lengths per RS block) are stored > on a separate device in sequence without any padding. > > Currently, to access FEC device, the dm-verity-fec code uses dm-bufio client > with block size set to verity data block (usually 4096 or 512 bytes). > > Because this block size is not divisible by some (most!) of the roots > supported lengths, data repair cannot work for partially stored > parity bytes. Ah, good catch! > This patch changes FEC device dm-bufio block size to "roots << SECTOR_SHIFT" > where we can be sure that the full parity data is always available. > (There cannot be partial FEC blocks because parity must cover whole sectors.) > > Because the optional FEC starting offset could be unaligned to this > new block size, we have to use dm_bufio_set_sector_offset() to configure it. > > The problem is easily reproducible using veritysetup, > here for example for roots=13: > > # create verity device with RS FEC > dd if=/dev/urandom of=data.img bs=4096 count=8 status=none > veritysetup format data.img hash.img --fec-device=fec.img --fec-roots=13 | awk '/^Root hash/{ print $3 }' >roothash > > # create an erasure that should be always repairable with this roots setting > dd if=/dev/zero of=data.img conv=notrunc bs=1 count=8 seek=4088 status=none > > # try to read it through dm-verity > veritysetup open data.img test hash.img --fec-device=fec.img --fec-roots=13 $(cat roothash) > dd if=/dev/mapper/test of=/dev/null bs=4096 status=noxfer > # wait for possible recursive recovery in kernel > udevadm settle > veritysetup close test > > Without it, FEC code usually ends on unrecoverable failure in RS decoder: > device-mapper: verity-fec: 7:1: FEC 0: failed to correct: -74 > ... > > With the patch, errors are properly repaired. > device-mapper: verity-fec: 7:1: FEC 0: corrected 8 errors > ... > > This problem is present in all kernels since the FEC code introduction (kernel 4.5). > > AFAIK the problem is not visible in Android ecosystem because it always > use default RS roots=2. That's correct, Android always uses the default value to minimize space overhead, which is why we never ran into this. > > Signed-off-by: Milan Broz <gmazyland@gmail.com> > --- > drivers/md/dm-verity-fec.c | 23 +++++++++++++---------- > 1 file changed, 13 insertions(+), 10 deletions(-) > > diff --git a/drivers/md/dm-verity-fec.c b/drivers/md/dm-verity-fec.c > index fb41b4f23c48..be170581eb69 100644 > --- a/drivers/md/dm-verity-fec.c > +++ b/drivers/md/dm-verity-fec.c > @@ -61,18 +61,18 @@ static int fec_decode_rs8(struct dm_verity *v, struct dm_verity_fec_io *fio, > static u8 *fec_read_parity(struct dm_verity *v, u64 rsb, int index, > unsigned *offset, struct dm_buffer **buf) > { > - u64 position, block; > + u64 position, block, rem; > u8 *res; > > position = (index + rsb) * v->fec->roots; > - block = position >> v->data_dev_block_bits; > - *offset = (unsigned)(position - (block << v->data_dev_block_bits)); > + block = div64_u64_rem(position, v->fec->roots << SECTOR_SHIFT, &rem); > + *offset = (unsigned)rem; > > - res = dm_bufio_read(v->fec->bufio, v->fec->start + block, buf); > + res = dm_bufio_read(v->fec->bufio, block, buf); > if (IS_ERR(res)) { > DMERR("%s: FEC %llu: parity read failed (block %llu): %ld", > v->data_dev->name, (unsigned long long)rsb, > - (unsigned long long)(v->fec->start + block), > + (unsigned long long)(block), Nit: parentheses not needed around block. > PTR_ERR(res)); > *buf = NULL; > } > @@ -155,7 +155,7 @@ static int fec_decode_bufs(struct dm_verity *v, struct dm_verity_fec_io *fio, > > /* read the next block when we run out of parity bytes */ > offset += v->fec->roots; > - if (offset >= 1 << v->data_dev_block_bits) { > + if (offset >= (v->fec->roots << SECTOR_SHIFT)) { Same here, but I suppose it might help with readability. > dm_bufio_release(buf); > > par = fec_read_parity(v, rsb, block_offset, &offset, &buf); > @@ -674,7 +674,7 @@ int verity_fec_ctr(struct dm_verity *v) > { > struct dm_verity_fec *f = v->fec; > struct dm_target *ti = v->ti; > - u64 hash_blocks; > + u64 hash_blocks, start_blocks, fec_blocks; > int ret; > > if (!verity_fec_is_enabled(v)) { > @@ -744,15 +744,18 @@ int verity_fec_ctr(struct dm_verity *v) > } > > f->bufio = dm_bufio_client_create(f->dev->bdev, > - 1 << v->data_dev_block_bits, > + f->roots << SECTOR_SHIFT, > 1, 0, NULL, NULL); > if (IS_ERR(f->bufio)) { > ti->error = "Cannot initialize FEC bufio client"; > return PTR_ERR(f->bufio); > } > > - if (dm_bufio_get_device_size(f->bufio) < > - ((f->start + f->rounds * f->roots) >> v->data_dev_block_bits)) { > + dm_bufio_set_sector_offset(f->bufio, f->start << v->data_dev_block_bits >> SECTOR_SHIFT); > + > + start_blocks = div64_u64(f->start << v->data_dev_block_bits, v->fec->roots << SECTOR_SHIFT); > + fec_blocks = div64_u64(f->rounds * f->roots, v->fec->roots << SECTOR_SHIFT); > + if (dm_bufio_get_device_size(f->bufio) < (start_blocks + fec_blocks)) { > ti->error = "FEC device is too small"; > return -E2BIG; > } I haven't tested the code, but it looks correct to me. Thanks for fixing this! Reviewed-by: Sami Tolvanen <samitolvanen@google.com> Sami -- dm-devel mailing list dm-devel@redhat.com https://listman.redhat.com/mailman/listinfo/dm-devel
diff --git a/drivers/md/dm-verity-fec.c b/drivers/md/dm-verity-fec.c index fb41b4f23c48..be170581eb69 100644 --- a/drivers/md/dm-verity-fec.c +++ b/drivers/md/dm-verity-fec.c @@ -61,18 +61,18 @@ static int fec_decode_rs8(struct dm_verity *v, struct dm_verity_fec_io *fio, static u8 *fec_read_parity(struct dm_verity *v, u64 rsb, int index, unsigned *offset, struct dm_buffer **buf) { - u64 position, block; + u64 position, block, rem; u8 *res; position = (index + rsb) * v->fec->roots; - block = position >> v->data_dev_block_bits; - *offset = (unsigned)(position - (block << v->data_dev_block_bits)); + block = div64_u64_rem(position, v->fec->roots << SECTOR_SHIFT, &rem); + *offset = (unsigned)rem; - res = dm_bufio_read(v->fec->bufio, v->fec->start + block, buf); + res = dm_bufio_read(v->fec->bufio, block, buf); if (IS_ERR(res)) { DMERR("%s: FEC %llu: parity read failed (block %llu): %ld", v->data_dev->name, (unsigned long long)rsb, - (unsigned long long)(v->fec->start + block), + (unsigned long long)(block), PTR_ERR(res)); *buf = NULL; } @@ -155,7 +155,7 @@ static int fec_decode_bufs(struct dm_verity *v, struct dm_verity_fec_io *fio, /* read the next block when we run out of parity bytes */ offset += v->fec->roots; - if (offset >= 1 << v->data_dev_block_bits) { + if (offset >= (v->fec->roots << SECTOR_SHIFT)) { dm_bufio_release(buf); par = fec_read_parity(v, rsb, block_offset, &offset, &buf); @@ -674,7 +674,7 @@ int verity_fec_ctr(struct dm_verity *v) { struct dm_verity_fec *f = v->fec; struct dm_target *ti = v->ti; - u64 hash_blocks; + u64 hash_blocks, start_blocks, fec_blocks; int ret; if (!verity_fec_is_enabled(v)) { @@ -744,15 +744,18 @@ int verity_fec_ctr(struct dm_verity *v) } f->bufio = dm_bufio_client_create(f->dev->bdev, - 1 << v->data_dev_block_bits, + f->roots << SECTOR_SHIFT, 1, 0, NULL, NULL); if (IS_ERR(f->bufio)) { ti->error = "Cannot initialize FEC bufio client"; return PTR_ERR(f->bufio); } - if (dm_bufio_get_device_size(f->bufio) < - ((f->start + f->rounds * f->roots) >> v->data_dev_block_bits)) { + dm_bufio_set_sector_offset(f->bufio, f->start << v->data_dev_block_bits >> SECTOR_SHIFT); + + start_blocks = div64_u64(f->start << v->data_dev_block_bits, v->fec->roots << SECTOR_SHIFT); + fec_blocks = div64_u64(f->rounds * f->roots, v->fec->roots << SECTOR_SHIFT); + if (dm_bufio_get_device_size(f->bufio) < (start_blocks + fec_blocks)) { ti->error = "FEC device is too small"; return -E2BIG; }
Optional Forward Error Correction (FEC) code in dm-verity uses Reed-Solomon code and should support roots from 2 to 24. The error correction parity bytes (of roots lengths per RS block) are stored on a separate device in sequence without any padding. Currently, to access FEC device, the dm-verity-fec code uses dm-bufio client with block size set to verity data block (usually 4096 or 512 bytes). Because this block size is not divisible by some (most!) of the roots supported lengths, data repair cannot work for partially stored parity bytes. This patch changes FEC device dm-bufio block size to "roots << SECTOR_SHIFT" where we can be sure that the full parity data is always available. (There cannot be partial FEC blocks because parity must cover whole sectors.) Because the optional FEC starting offset could be unaligned to this new block size, we have to use dm_bufio_set_sector_offset() to configure it. The problem is easily reproducible using veritysetup, here for example for roots=13: # create verity device with RS FEC dd if=/dev/urandom of=data.img bs=4096 count=8 status=none veritysetup format data.img hash.img --fec-device=fec.img --fec-roots=13 | awk '/^Root hash/{ print $3 }' >roothash # create an erasure that should be always repairable with this roots setting dd if=/dev/zero of=data.img conv=notrunc bs=1 count=8 seek=4088 status=none # try to read it through dm-verity veritysetup open data.img test hash.img --fec-device=fec.img --fec-roots=13 $(cat roothash) dd if=/dev/mapper/test of=/dev/null bs=4096 status=noxfer # wait for possible recursive recovery in kernel udevadm settle veritysetup close test Without it, FEC code usually ends on unrecoverable failure in RS decoder: device-mapper: verity-fec: 7:1: FEC 0: failed to correct: -74 ... With the patch, errors are properly repaired. device-mapper: verity-fec: 7:1: FEC 0: corrected 8 errors ... This problem is present in all kernels since the FEC code introduction (kernel 4.5). AFAIK the problem is not visible in Android ecosystem because it always use default RS roots=2. Signed-off-by: Milan Broz <gmazyland@gmail.com> --- drivers/md/dm-verity-fec.c | 23 +++++++++++++---------- 1 file changed, 13 insertions(+), 10 deletions(-)