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Re: [Qemu-devel] [PATCH 2/4] exec: add wrapper for host pointer access
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] [PATCH 2/4] exec: add wrapper for host pointer access |
Date: |
Mon, 17 Nov 2014 12:59:44 +0000 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
* Michael S. Tsirkin (address@hidden) wrote:
> On Mon, Nov 17, 2014 at 10:58:53AM +0000, Dr. David Alan Gilbert wrote:
> > * Michael S. Tsirkin (address@hidden) wrote:
> > > host pointer accesses force pointer math, let's
> > > add a wrapper to make them safer.
> > >
> > > Signed-off-by: Michael S. Tsirkin <address@hidden>
> > > ---
> > > include/exec/cpu-all.h | 5 +++++
> > > exec.c | 10 +++++-----
> > > 2 files changed, 10 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/include/exec/cpu-all.h b/include/exec/cpu-all.h
> > > index c085804..9d8d408 100644
> > > --- a/include/exec/cpu-all.h
> > > +++ b/include/exec/cpu-all.h
> > > @@ -313,6 +313,11 @@ typedef struct RAMBlock {
> > > int fd;
> > > } RAMBlock;
> > >
> > > +static inline void *ramblock_ptr(RAMBlock *block, ram_addr_t offset)
> > > +{
> > > + return (char *)block->host + offset;
> > > +}
> >
> > I'm a bit surprised you don't need to pass a length to this to be able
> > to tell how much you can access.
>
> This is because at the moment all accesses only touch a single page.
> Said assumption seems to be made all over the code, and won't
> be easy to remove.
>
> > > typedef struct RAMList {
> > > QemuMutex mutex;
> > > /* Protected by the iothread lock. */
> > > diff --git a/exec.c b/exec.c
> > > index ad5cf12..9648669 100644
> > > --- a/exec.c
> > > +++ b/exec.c
> > > @@ -840,7 +840,7 @@ static void tlb_reset_dirty_range_all(ram_addr_t
> > > start, ram_addr_t length)
> > >
> > > block = qemu_get_ram_block(start);
> > > assert(block == qemu_get_ram_block(end - 1));
> > > - start1 = (uintptr_t)block->host + (start - block->offset);
> > > + start1 = (uintptr_t)ramblock_ptr(block, start - block->offset);
> > > cpu_tlb_reset_dirty_all(start1, length);
> > > }
> > >
> > > @@ -1500,7 +1500,7 @@ void qemu_ram_remap(ram_addr_t addr, ram_addr_t
> > > length)
> > > QTAILQ_FOREACH(block, &ram_list.blocks, next) {
> > > offset = addr - block->offset;
> > > if (offset < block->length) {
> > > - vaddr = block->host + offset;
> > > + vaddr = ramblock_ptr(block, offset);
> > > if (block->flags & RAM_PREALLOC) {
> > > ;
> > > } else if (xen_enabled()) {
> > > @@ -1551,7 +1551,7 @@ void *qemu_get_ram_block_host_ptr(ram_addr_t addr)
> > > {
> > > RAMBlock *block = qemu_get_ram_block(addr);
> > >
> > > - return block->host;
> > > + return ramblock_ptr(block, 0);
> > > }
> > >
> > > /* Return a host pointer to ram allocated with qemu_ram_alloc.
> > > @@ -1578,7 +1578,7 @@ void *qemu_get_ram_ptr(ram_addr_t addr)
> > > xen_map_cache(block->offset, block->length, 1);
> > > }
> > > }
> > > - return block->host + (addr - block->offset);
> > > + return ramblock_ptr(block, addr - block->offset);
> > > }
> >
> > which then makes me wonder if all the uses of this are safe near the
> > end of the block.
> >
> > > /* Return a host pointer to guest's ram. Similar to qemu_get_ram_ptr
> > > @@ -1597,7 +1597,7 @@ static void *qemu_ram_ptr_length(ram_addr_t addr,
> > > hwaddr *size)
> > > if (addr - block->offset < block->length) {
> > > if (addr - block->offset + *size > block->length)
> > > *size = block->length - addr + block->offset;
> > > - return block->host + (addr - block->offset);
> > > + return ramblock_ptr(block, addr - block->offset);
> > > }
> >
> > but then this sounds like it's going to have partial duplication, it
> > already looks
> > like it's only going to succeed if it finds itself a block that the access
> > fits
> > in.
> >
> >
> > Dave
>
> Sorry, I don't really understand what you are saying here.
qemu_ram_ptr_length already does some checks, so using ramblock_ptr is
duplicating
some of that; not a big issue.
Dave
>
> > > }
> > >
> > > --
> > > MST
> > >
> > --
> > Dr. David Alan Gilbert / address@hidden / Manchester, UK
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK
[Qemu-devel] [PATCH 3/4] cpu: assert host pointer offset within block, Michael S. Tsirkin, 2014/11/12
[Qemu-devel] [PATCH 4/4] cpu: verify that block->host is set, Michael S. Tsirkin, 2014/11/12
Re: [Qemu-devel] [PATCH 0/4] migration: fix CVE-2014-7840, Amit Shah, 2014/11/17
- Re: [Qemu-devel] [PATCH 0/4] migration: fix CVE-2014-7840, Michael S. Tsirkin, 2014/11/17
- Re: [Qemu-devel] [PATCH 0/4] migration: fix CVE-2014-7840, Amit Shah, 2014/11/17
- Re: [Qemu-devel] [PATCH 0/4] migration: fix CVE-2014-7840, Michael S. Tsirkin, 2014/11/17
- Re: [Qemu-devel] [PATCH 0/4] migration: fix CVE-2014-7840, Amit Shah, 2014/11/17
- Re: [Qemu-devel] [PATCH 0/4] migration: fix CVE-2014-7840, Michael S. Tsirkin, 2014/11/17
- Re: [Qemu-devel] [PATCH 0/4] migration: fix CVE-2014-7840, Amit Shah, 2014/11/17