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

[Jeff Cody]

On Wed, Jul 22, 2015 at 07:02:02PM +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.


> >          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]);
> >
>