Re: [Qemu-devel] [PATCH 2/5] qcow2: Metadata overlap checks

[Max Reitz]

Am 27.08.2013 12:17, schrieb Kevin Wolf:
> Am 26.08.2013 um 15:04 hat Max Reitz geschrieben:
> > Two new functions are added; the first one checks a given range in the
> > image file for overlaps with metadata (main header, L1 tables, L2
> > tables, refcount table and blocks).
> >
> > The second one should be used immediately before writing to the image
> > file as it calls the first function and, upon collision, marks the
> > image as corrupt and makes the BDS unusable, thereby preventing
> > further access.
> >
> > Both functions take a bitmask argument specifying the structures which
> > should be checked for overlaps, making it possible to also check
> > metadata writes against colliding with other structures.
> >
> > Signed-off-by: Max Reitz <address@hidden>
> > ---
> >   block/qcow2-refcount.c | 142 +++++++++++++++++++++++++++++++++++++++++++++++++
> >   block/qcow2.h          |  28 ++++++++++
> >   2 files changed, 170 insertions(+)
> >
> > diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
> > index 1244693..c8141c8 100644
> > --- a/block/qcow2-refcount.c
> > +++ b/block/qcow2-refcount.c
> > @@ -25,6 +25,7 @@
> >   #include "qemu-common.h"
> >   #include "block/block_int.h"
> >   #include "block/qcow2.h"
> > +#include "qemu/range.h"
> >   
> >   static int64_t alloc_clusters_noref(BlockDriverState *bs, int64_t size);
> >   static int QEMU_WARN_UNUSED_RESULT update_refcount(BlockDriverState *bs,
> > @@ -1372,3 +1373,144 @@ fail:
> >       return ret;
> >   }
> >   
> > +/*
> > + * Checks if the given offset into the image file is actually free to use by
> > + * looking for overlaps with important metadata sections (L1/L2 tables etc.),
> > + * i.e. a sanity check without relying on the refcount tables.
> > + *
> > + * The chk parameter specifies exactly what checks to perform.
> > + *
> > + * Returns:
> > + * - 0 if writing to this offset will not affect the mentioned metadata
> > + * - a positive QCow2MetadataOverlap value indicating one overlapping section
> > + * - a negative value (-errno) indicating an error while performing a check,
> > + *   e.g. when bdrv_read failed on QCOW2_OL_INACTIVE_L2
> > + */
> > +int qcow2_check_metadata_overlap(BlockDriverState *bs, QCow2MetadataOverlap 
> > chk,
> chk is really just an int, because you don't pass a single enum value but
> a bit mask consisting of multiple enum values ored together.
Okay, I justed wanted to make clear what values make up that mask, but 
yes, you're right, more often than not chk will not assume a value from 
the enum itself.

> > +                                 int64_t offset, int64_t size)
> > +{
> > +    BDRVQcowState *s = bs->opaque;
> > +    int i, j;
> > +
> > +    if (!size) {
> > +        return 0;
> > +    }
> > +
> > +    if (chk & QCOW2_OL_MAIN_HEADER) {
> > +        if (offset < s->cluster_size) {
> > +            return QCOW2_OL_MAIN_HEADER;
> > +        }
> > +    }
> > +
> > +    if ((chk & QCOW2_OL_ACTIVE_L1) && s->l1_size) {
> > +        if (ranges_overlap(offset, size, s->l1_table_offset,
> > +            s->l1_size * sizeof(uint64_t))) {
> The size could be rounded up to the next cluster boundary (same thing
> for other metadata types).
Would this actually change anything?

> > +            return QCOW2_OL_ACTIVE_L1;
> > +        }
> > +    }
> > +
> > +    if ((chk & QCOW2_OL_REFCOUNT_TABLE) && s->refcount_table_size) {
> > +        if (ranges_overlap(offset, size, s->refcount_table_offset,
> > +            s->refcount_table_size * sizeof(uint64_t))) {
> > +            return QCOW2_OL_REFCOUNT_TABLE;
> > +        }
> > +    }
> > +
> > +    if ((chk & QCOW2_OL_SNAPSHOT_TABLE) && s->snapshots_size) {
> > +        if (ranges_overlap(offset, size, s->snapshots_offset,
> > +            s->snapshots_size)) {
> > +            return QCOW2_OL_SNAPSHOT_TABLE;
> > +        }
> > +    }
> > +
> > +    if ((chk & QCOW2_OL_INACTIVE_L1) && s->snapshots) {
> > +        for (i = 0; i < s->nb_snapshots; i++) {
> > +            if (s->snapshots[i].l1_size &&
> > +                ranges_overlap(offset, size, s->snapshots[i].l1_table_offset,
> > +                s->snapshots[i].l1_size * sizeof(uint64_t))) {
> > +                return QCOW2_OL_INACTIVE_L1;
> > +            }
> > +        }
> > +    }
> > +
> > +    if ((chk & QCOW2_OL_ACTIVE_L2) && s->l1_table) {
> > +        for (i = 0; i < s->l1_size; i++) {
> > +            if ((s->l1_table[i] & L1E_OFFSET_MASK) &&
> > +                ranges_overlap(offset, size, s->l1_table[i] & L1E_OFFSET_MASK,
> > +                s->cluster_size)) {
> > +                return QCOW2_OL_ACTIVE_L2;
> > +            }
> > +        }
> > +    }
> > +
> > +    if ((chk & QCOW2_OL_REFCOUNT_BLOCK) && s->refcount_table) {
> > +        for (i = 0; i < s->refcount_table_size; i++) {
> > +            if ((s->refcount_table[i] & REFT_OFFSET_MASK) &&
> > +                ranges_overlap(offset, size, s->refcount_table[i] &
> > +                REFT_OFFSET_MASK, s->cluster_size)) {
> > +                return QCOW2_OL_REFCOUNT_BLOCK;
> > +            }
> > +        }
> > +    }
> > +
> > +    if ((chk & QCOW2_OL_INACTIVE_L2) && s->snapshots) {
> > +        for (i = 0; i < s->nb_snapshots; i++) {
> > +            uint64_t l1_ofs = s->snapshots[i].l1_table_offset;
> > +            uint32_t l1_sz  = s->snapshots[i].l1_size;
> > +            uint64_t *l1 = g_malloc(l1_sz * sizeof(uint64_t));
> > +            int ret;
> > +
> > +            ret = bdrv_read(bs->file, l1_ofs / BDRV_SECTOR_SIZE, (uint8_t *)l1,
> > +                            l1_sz * sizeof(uint64_t) / BDRV_SECTOR_SIZE);
> > +
> > +            if (ret < 0) {
> > +                g_free(l1);
> > +                return ret;
> > +            }
> > +
> > +            for (j = 0; j < l1_sz; j++) {
> > +                if ((l1[j] & L1E_OFFSET_MASK) &&
> > +                    ranges_overlap(offset, size, l1[j] & L1E_OFFSET_MASK,
> > +                    s->cluster_size)) {
> > +                    g_free(l1);
> > +                    return QCOW2_OL_INACTIVE_L2;
> > +                }
> > +            }
> > +
> > +            g_free(l1);
> > +        }
> > +    }
> > +
> > +    return 0;
> > +}
> > +
> > +/*
> > + * First performs a check for metadata overlaps (through
> > + * qcow2_check_metadata_overlap); if that fails with a negative value (error
> > + * while performing a check), it will print a message but otherwise ignore that
> > + * error. If an impending overlap is detected, the BDS will be made unusable 
> > and
> > + * the qcow2 file marked corrupt.
> > + *
> > + * Returns 0 if there were no overlaps (or an error occured while checking for
> > + * overlaps) or a positive QCow2MetadataOverlap value on overlap (then, the BDS
> > + * will be unusable and the qcow2 file marked corrupt).
> > + */
> > +int qcow2_pre_write_overlap_check(BlockDriverState *bs, QCow2MetadataOverlap 
> > chk,
> > +                                  int64_t offset, int64_t size)
> > +{
> > +    int ret = qcow2_check_metadata_overlap(bs, chk, offset, size);
> > +
> > +    if (ret < 0) {
> > +        fprintf(stderr, "qcow2: Error while checking for metadata overlaps: "
> > +                "%s\n", strerror(-ret));
> Leftover debug code?
Oh, yes, kind of. At one point in time this code didn't pass the error 
code but simply ignored it. Therefore it made sense to at least give an 
information to the user. When changing this behavior, I thought it 
wouldn't be too bad to leave that message in.

> > +        return ret;
> > +    } else if (ret > 0) {
> > +        fprintf(stderr, "qcow2: Preventing invalid write on metadata; "
> > +                "image marked as corrupt.\n");
> This one makes actually sense to keep even for production as it is a
> condition that we want to make sure to appear in log files.
>
> Another thing to consider would be to send out a QMP event when this
> happens.
Okay.

> > +        qcow2_mark_corrupt(bs);
> > +        bs->drv = NULL; /* make BDS unusable */
> > +        return ret;
> > +    }
> > +
> > +    return 0;
> > +}
> Kevin
Thanks for reviewing and your comments,

Max