Re: [PATCH] memory: Revert "memory: accept mismatching sizes in memory_region_access_valid"

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Aug 26, 2020 at 10:32:16PM -0700, Nathan Chancellor wrote:
> Hi all,
> 
> Sorry for the duplicate reply, my first one was rejected by a mailing
> list administrator for being too long so I resent it with the error logs
> as a link instead of inline.
> 
> On Wed, Jun 10, 2020 at 09:47:49AM -0400, Michael S. Tsirkin wrote:
> > Memory API documentation documents valid .min_access_size and .max_access_size
> > fields and explains that any access outside these boundaries is blocked.
> > 
> > This is what devices seem to assume.
> > 
> > However this is not what the implementation does: it simply
> > ignores the boundaries unless there's an "accepts" callback.
> > 
> > Naturally, this breaks a bunch of devices.
> > 
> > Revert to the documented behaviour.
> > 
> > Devices that want to allow any access can just drop the valid field,
> > or add the impl field to have accesses converted to appropriate
> > length.
> > 
> > Cc: qemu-stable@nongnu.org
> > Reviewed-by: Richard Henderson <rth@twiddle.net>
> > Fixes: CVE-2020-13754
> > Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1842363
> > Fixes: a014ed07bd5a ("memory: accept mismatching sizes in memory_region_access_valid")
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> >  memory.c | 29 +++++++++--------------------
> >  1 file changed, 9 insertions(+), 20 deletions(-)
> > 
> > diff --git a/memory.c b/memory.c
> > index 91ceaf9fcf..3e9388fb74 100644
> > --- a/memory.c
> > +++ b/memory.c
> > @@ -1352,35 +1352,24 @@ bool memory_region_access_valid(MemoryRegion *mr,
> >                                  bool is_write,
> >                                  MemTxAttrs attrs)
> >  {
> > -    int access_size_min, access_size_max;
> > -    int access_size, i;
> > +    if (mr->ops->valid.accepts
> > +        && !mr->ops->valid.accepts(mr->opaque, addr, size, is_write, attrs)) {
> > +        return false;
> > +    }
> >  
> >      if (!mr->ops->valid.unaligned && (addr & (size - 1))) {
> >          return false;
> >      }
> >  
> > -    if (!mr->ops->valid.accepts) {
> > +    /* Treat zero as compatibility all valid */
> > +    if (!mr->ops->valid.max_access_size) {
> >          return true;
> >      }
> >  
> > -    access_size_min = mr->ops->valid.min_access_size;
> > -    if (!mr->ops->valid.min_access_size) {
> > -        access_size_min = 1;
> > +    if (size > mr->ops->valid.max_access_size
> > +        || size < mr->ops->valid.min_access_size) {
> > +        return false;
> >      }
> > -
> > -    access_size_max = mr->ops->valid.max_access_size;
> > -    if (!mr->ops->valid.max_access_size) {
> > -        access_size_max = 4;
> > -    }
> > -
> > -    access_size = MAX(MIN(size, access_size_max), access_size_min);
> > -    for (i = 0; i < size; i += access_size) {
> > -        if (!mr->ops->valid.accepts(mr->opaque, addr + i, access_size,
> > -                                    is_write, attrs)) {
> > -            return false;
> > -        }
> > -    }
> > -
> >      return true;
> >  }
> >  
> > -- 
> > MST
> > 
> > 
> 
> I just ran into a regression with booting RISC-V kernels due to this
> commit. I can reproduce it with QEMU 5.1.0 and latest tip of tree
> (25f6dc28a3a8dd231c2c092a0e65bd796353c769 at the time of initially
> writing this).
> 
> The error message, commands, and bisect logs are available here:
> 
> https://gist.githubusercontent.com/nathanchance/c106dd22ec0c0e00f6a25daba106a1b9/raw/d929f2fff6da9126ded156affb0f19f359e9f693/qemu-5.1.0-issue-terminal-log.txt
> 
> I have attached the rootfs and kernel image used for these tests. If for
> some reason there is a problem receiving them, the kernel is just an
> arch/riscv/configs/defconfig kernel at Linux 5.9-rc2 and the rootfs is
> available here:
> 
> https://github.com/ClangBuiltLinux/boot-utils/blob/3b21a5b71451742866349ba4f18638c5a754e660/images/riscv/rootfs.cpio.zst
> 
> Please let me know if I can provide any follow up information or if I am
> doing something wrong.
> 
> Cheers,
> Nathan


So pls try this patch and use gdb backtrace to see access to which MR
triggers the assert.


diff --git a/softmmu/memory.c b/softmmu/memory.c
index 70b93104e8..dc8b7682aa 100644
--- a/softmmu/memory.c
+++ b/softmmu/memory.c
@@ -1368,6 +1368,7 @@ bool memory_region_access_valid(MemoryRegion *mr,
 
     if (size > mr->ops->valid.max_access_size
         || size < mr->ops->valid.min_access_size) {
+        assert(0);
         return false;
     }
     return true;