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dm snapshot: fix header corruption race on invalidation

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If a persistent snapshot fills up, a race can corrupt the on-disk header
which causes a crash on any future attempt to activate the snapshot
(typically while booting).  This patch fixes the race.

When the snapshot overflows, __invalidate_snapshot is called, which calls
snapshot store method drop_snapshot. It goes to persistent_drop_snapshot that
calls write_header. write_header constructs the new header in the "area"
location.

Concurrently, an existing kcopyd job may finish, call copy_callback
and commit_exception method, that goes to persistent_commit_exception.
persistent_commit_exception doesn't do locking, relying on the fact that
callbacks are single-threaded, but it can race with snapshot invalidation and
overwrite the header that is just being written while the snapshot is being
invalidated.

The result of this race is a corrupted header being written that can
lead to a crash on further reactivation (if chunk_size is zero in the
corrupted header).

The fix is to use separate memory areas for each.

See the bug: https://bugzilla.redhat.com/show_bug.cgi?id=461506

Cc: stable@kernel.org
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
This commit is contained in:
Mikulas Patocka 2009-09-04 20:40:39 +01:00 committed by Alasdair G Kergon
parent 02d2fd31de
commit 61578dcd3f
1 changed files with 34 additions and 10 deletions
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44
drivers/md/dm-snap-persistent.c
View File

@ -105,6 +105,13 @@ struct pstore {
*/
void *zero_area;
/*
* An area used for header. The header can be written
* concurrently with metadata (when invalidating the snapshot),
* so it needs a separate buffer.
*/
void *header_area;
/*
* Used to keep track of which metadata area the data in
* 'chunk' refers to.
@ -148,16 +155,27 @@ static int alloc_area(struct pstore *ps)
*/
ps->area = vmalloc(len);
if (!ps->area)
return r;
goto err_area;
ps->zero_area = vmalloc(len);
if (!ps->zero_area) {
vfree(ps->area);
return r;
}
if (!ps->zero_area)
goto err_zero_area;
memset(ps->zero_area, 0, len);
ps->header_area = vmalloc(len);
if (!ps->header_area)
goto err_header_area;
return 0;
err_header_area:
vfree(ps->zero_area);
err_zero_area:
vfree(ps->area);
err_area:
return r;
}
static void free_area(struct pstore *ps)
@ -169,6 +187,10 @@ static void free_area(struct pstore *ps)
if (ps->zero_area)
vfree(ps->zero_area);
ps->zero_area = NULL;
if (ps->header_area)
vfree(ps->header_area);
ps->header_area = NULL;
}
struct mdata_req {
@ -285,11 +307,11 @@ static int read_header(struct pstore *ps, int *new_snapshot)
if (r)
return r;
r = chunk_io(ps, ps->area, 0, READ, 1);
r = chunk_io(ps, ps->header_area, 0, READ, 1);
if (r)
goto bad;
dh = (struct disk_header *) ps->area;
dh = ps->header_area;
if (le32_to_cpu(dh->magic) == 0) {
*new_snapshot = 1;
@ -339,15 +361,15 @@ static int write_header(struct pstore *ps)
{
struct disk_header *dh;
memset(ps->area, 0, ps->store->chunk_size << SECTOR_SHIFT);
memset(ps->header_area, 0, ps->store->chunk_size << SECTOR_SHIFT);
dh = (struct disk_header *) ps->area;
dh = ps->header_area;
dh->magic = cpu_to_le32(SNAP_MAGIC);
dh->valid = cpu_to_le32(ps->valid);
dh->version = cpu_to_le32(ps->version);
dh->chunk_size = cpu_to_le32(ps->store->chunk_size);
return chunk_io(ps, ps->area, 0, WRITE, 1);
return chunk_io(ps, ps->header_area, 0, WRITE, 1);
}
/*
@ -667,6 +689,8 @@ static int persistent_ctr(struct dm_exception_store *store,
ps->valid = 1;
ps->version = SNAPSHOT_DISK_VERSION;
ps->area = NULL;
ps->zero_area = NULL;
ps->header_area = NULL;
ps->next_free = 2; /* skipping the header and first area */
ps->current_committed = 0;