Re: [Qemu-block] [PATCH v2 1/2] block: vpc - prevent overflow if max_table_entries >= 0x40000000

[Jeff Cody]

On Wed, Jul 22, 2015 at 07:29:47PM +0200, Max Reitz wrote:
> On 22.07.2015 19:26, Jeff Cody wrote:
> >On Wed, Jul 22, 2015 at 07:02:02PM +0200, Max Reitz wrote:
> >>On 21.07.2015 18:13, Jeff Cody wrote:
> >>>When we allocate the pagetable based on max_table_entries, we multiply
> >>>the max table entry value by 4 to accomodate a table of 32-bit integers.
> >>>However, max_table_entries is a uint32_t, and the VPC driver accepts
> >>>ranges for that entry over 0x40000000.  So during this allocation:
> >>>
> >>>s->pagetable = qemu_try_blockalign(bs->file, s->max_table_entries * 4);
> >>>
> >>>The size arg overflows, allocating significantly less memory than
> >>>expected.
> >>>
> >>>Since qemu_try_blockalign() size argument is size_t, cast the
> >>>multiplication correctly to prevent overflow.
> >>>
> >>>The value of "max_table_entries * 4" is used elsewhere in the code as
> >>>well, so store the correct value for use in all those cases.
> >>>
> >>>We also check the Max Tables Entries value, to make sure that it is <
> >>>SIZE_MAX / 4, so we know the pagetable size will fit in size_t.
> >>>
> >>>Reported-by: Richard W.M. Jones <address@hidden>
> >>>Signed-off-by: Jeff Cody <address@hidden>
> >>>---
> >>>  block/vpc.c | 17 +++++++++++++----
> >>>  1 file changed, 13 insertions(+), 4 deletions(-)
> >>>
> >>>diff --git a/block/vpc.c b/block/vpc.c
> >>>index 37572ba..6ef21e6 100644
> >>>--- a/block/vpc.c
> >>>+++ b/block/vpc.c
> >>>@@ -168,6 +168,7 @@ static int vpc_open(BlockDriverState *bs, QDict 
> >>>*options, int flags,
> >>>      uint8_t buf[HEADER_SIZE];
> >>>      uint32_t checksum;
> >>>      uint64_t computed_size;
> >>>+    uint64_t pagetable_size;
> >>>      int disk_type = VHD_DYNAMIC;
> >>>      int ret;
> >>>
> >>>@@ -269,7 +270,16 @@ static int vpc_open(BlockDriverState *bs, QDict 
> >>>*options, int flags,
> >>>              goto fail;
> >>>          }
> >>>
> >>>-        s->pagetable = qemu_try_blockalign(bs->file, s->max_table_entries 
> >>>* 4);
> >>>+        if (s->max_table_entries > SIZE_MAX / 4) {
> >>>+            error_setg(errp, "Max Table Entries too large (%" PRId32 ")",
> >>>+                        s->max_table_entries);
> >>>+            ret = -EINVAL;
> >>>+            goto fail;
> >>>+        }
> >>>+
> >>>+        pagetable_size = (uint64_t) s->max_table_entries * 4;
> >>>+
> >>>+        s->pagetable = qemu_try_blockalign(bs->file, pagetable_size);
> >>>          if (s->pagetable == NULL) {
> >>>              ret = -ENOMEM;
> >>>              goto fail;
> >>>@@ -277,14 +287,13 @@ static int vpc_open(BlockDriverState *bs, QDict 
> >>>*options, int flags,
> >>>
> >>>          s->bat_offset = be64_to_cpu(dyndisk_header->table_offset);
> >>>
> >>>-        ret = bdrv_pread(bs->file, s->bat_offset, s->pagetable,
> >>>-                         s->max_table_entries * 4);
> >>>+        ret = bdrv_pread(bs->file, s->bat_offset, s->pagetable, 
> >>>pagetable_size);
> >>
> >>The last parameter of bdrv_pread() is an int, so this will still
> >>overflow if s->max_table_entries > INT_MAX / 4.
> >>
> >
> >I thought about checking for INT_MAX overflow as well - I probably
> >should.  I wasn't too concerned about it, however, as bdrv_read() will
> >return -EINVAL for nb_sectors < 0.
> 
> Right, but SIZE_MAX may be larger than 2ull * INT_MAX, so on 64 bit
> systems, pagetable_size might become large enough for
> (int)pagetable_size to be positive, with (int)pagetable_size !=
> pagetable_size, which would make bdrv_pread() work just fine, but
> only read a part of the table.

The maximum size pagetable_size can be is UINT32_MAX * 4, because
max_table_entries is a 32-bit value from the VHD header.  But either
way, I'll add a check for INT_MAX, as that really is the correct (and
clearest) way to handle it.

> 
> Max
> 
> >>>          if (ret < 0) {
> >>>              goto fail;
> >>>          }
> >>>
> >>>          s->free_data_block_offset =
> >>>-            (s->bat_offset + (s->max_table_entries * 4) + 511) & ~511;
> >>>+            (s->bat_offset + pagetable_size + 511) & ~511;
> >>
> >>Not necessary, but perhaps we should be using ROUND_UP() here...
> >>
> >
> >Sure, why not :)
> >
> >>Max
> >>
> >>>          for (i = 0; i < s->max_table_entries; i++) {
> >>>              be32_to_cpus(&s->pagetable[i]);
> >>>
> >>
>