* David Hildenbrand (da...@redhat.com) wrote:
> Resizing while migrating is dangerous and does not work as expected.
> The whole migration code works on the usable_length of ram blocks and does
> not expect this to change at random points in time.
>
> In the case of postcopy, relying on used_length is racy as soon as the
> guest is running. Also, when used_length changes we might leave the
> uffd handler registered for some memory regions, reject valid pages
> when migrating and fail when sending the recv bitmap to the source.
>
> Resizing can be trigger *after* (but not during) a reset in
> ACPI code by the guest
> - hw/arm/virt-acpi-build.c:acpi_ram_update()
> - hw/i386/acpi-build.c:acpi_ram_update()
>
> Let's remember the original used_length in a separate variable and
> use it in relevant postcopy code. Make sure to update it when we resize
> during precopy, when synchronizing the RAM block sizes with the source.
>
> Reviewed-by: Peter Xu <pet...@redhat.com>
> Cc: "Dr. David Alan Gilbert" <dgilb...@redhat.com>
> Cc: Juan Quintela <quint...@redhat.com>
> Cc: Eduardo Habkost <ehabk...@redhat.com>
> Cc: Paolo Bonzini <pbonz...@redhat.com>
> Cc: Igor Mammedov <imamm...@redhat.com>
> Cc: "Michael S. Tsirkin" <m...@redhat.com>
> Cc: Richard Henderson <richard.hender...@linaro.org>
> Cc: Shannon Zhao <shannon.z...@linaro.org>
> Cc: Alex Bennée <alex.ben...@linaro.org>
> Cc: Peter Xu <pet...@redhat.com>
> Signed-off-by: David Hildenbrand <da...@redhat.com>
> ---
> include/exec/ramblock.h | 10 ++++++++++
> migration/postcopy-ram.c | 15 ++++++++++++---
> migration/ram.c | 11 +++++++++--
> 3 files changed, 31 insertions(+), 5 deletions(-)
>
> diff --git a/include/exec/ramblock.h b/include/exec/ramblock.h
> index 07d50864d8..664701b759 100644
> --- a/include/exec/ramblock.h
> +++ b/include/exec/ramblock.h
> @@ -59,6 +59,16 @@ struct RAMBlock {
> */
> unsigned long *clear_bmap;
> uint8_t clear_bmap_shift;
> +
> + /*
> + * RAM block length that corresponds to the used_length on the migration
> + * source (after RAM block sizes were synchronized). Especially, after
> + * starting to run the guest, used_length and postcopy_length can differ.
> + * Used to register/unregister uffd handlers and as the size of the
> received
> + * bitmap. Receiving any page beyond this length will bail out, as it
> + * could not have been valid on the source.
> + */
> + ram_addr_t postcopy_length;
> };
> #endif
> #endif
> diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> index a36402722b..c68caf4e42 100644
> --- a/migration/postcopy-ram.c
> +++ b/migration/postcopy-ram.c
> @@ -17,6 +17,7 @@
> */
>
> #include "qemu/osdep.h"
> +#include "qemu/rcu.h"
> #include "exec/target_page.h"
> #include "migration.h"
> #include "qemu-file.h"
> @@ -31,6 +32,7 @@
> #include "qemu/error-report.h"
> #include "trace.h"
> #include "hw/boards.h"
> +#include "exec/ramblock.h"
>
> /* Arbitrary limit on size of each discard command,
> * keeps them around ~200 bytes
> @@ -456,6 +458,13 @@ static int init_range(RAMBlock *rb, void *opaque)
> ram_addr_t length = qemu_ram_get_used_length(rb);
> trace_postcopy_init_range(block_name, host_addr, offset, length);
>
> + /*
> + * Save the used_length before running the guest. In case we have to
> + * resize RAM blocks when syncing RAM block sizes from the source during
> + * precopy, we'll update it manually via the ram block notifier.
> + */
> + rb->postcopy_length = length;
> +
> /*
> * We need the whole of RAM to be truly empty for postcopy, so things
> * like ROMs and any data tables built during init must be zero'd
> @@ -478,7 +487,7 @@ static int cleanup_range(RAMBlock *rb, void *opaque)
> const char *block_name = qemu_ram_get_idstr(rb);
> void *host_addr = qemu_ram_get_host_addr(rb);
> ram_addr_t offset = qemu_ram_get_offset(rb);
> - ram_addr_t length = qemu_ram_get_used_length(rb);
> + ram_addr_t length = rb->postcopy_length;
> MigrationIncomingState *mis = opaque;
> struct uffdio_range range_struct;
> trace_postcopy_cleanup_range(block_name, host_addr, offset, length);
> @@ -600,7 +609,7 @@ static int nhp_range(RAMBlock *rb, void *opaque)
> const char *block_name = qemu_ram_get_idstr(rb);
> void *host_addr = qemu_ram_get_host_addr(rb);
> ram_addr_t offset = qemu_ram_get_offset(rb);
> - ram_addr_t length = qemu_ram_get_used_length(rb);
> + ram_addr_t length = rb->postcopy_length;
> trace_postcopy_nhp_range(block_name, host_addr, offset, length);
>
> /*
> @@ -644,7 +653,7 @@ static int ram_block_enable_notify(RAMBlock *rb, void
> *opaque)
> struct uffdio_register reg_struct;
>
> reg_struct.range.start = (uintptr_t)qemu_ram_get_host_addr(rb);
> - reg_struct.range.len = qemu_ram_get_used_length(rb);
> + reg_struct.range.len = rb->postcopy_length;
> reg_struct.mode = UFFDIO_REGISTER_MODE_MISSING;
>
> /* Now tell our userfault_fd that it's responsible for this area */
> diff --git a/migration/ram.c b/migration/ram.c
> index 1a5ff07997..ee5c3d5784 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -244,7 +244,7 @@ int64_t ramblock_recv_bitmap_send(QEMUFile *file,
> return -1;
> }
>
> - nbits = block->used_length >> TARGET_PAGE_BITS;
> + nbits = block->postcopy_length >> TARGET_PAGE_BITS;
>
> /*
> * Make sure the tmp bitmap buffer is big enough, e.g., on 32bit
> @@ -3160,7 +3160,13 @@ static int ram_load_postcopy(QEMUFile *f)
> break;
> }
>
> - if (!offset_in_ramblock(block, addr)) {
> + /*
> + * Relying on used_length is racy and can result in false
> positives.
> + * We might place pages beyond used_length in case RAM was shrunk
> + * while in postcopy, which is fine - trying to place via
> + * UFFDIO_COPY/UFFDIO_ZEROPAGE will never segfault.
> + */
Is this actually safe? Imagine that the region had got shrunk, would it
still be mmap'd in there - or could there now be a space where something
else might have landed in?
> + if (!block->host || addr >= block->postcopy_length) {
> error_report("Illegal RAM offset " RAM_ADDR_FMT, addr);
> ret = -EINVAL;
> break;
> @@ -3757,6 +3763,7 @@ static void ram_mig_ram_block_resized(RAMBlockNotifier
> *n, void *host,
> rb->idstr);
> }
> }
> + rb->postcopy_length = new_size;
> break;
> case POSTCOPY_INCOMING_NONE:
> case POSTCOPY_INCOMING_RUNNING:
> --
> 2.24.1
>
--
Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
