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Re: [Xen-devel] [PATCH] xen: hypercall: fix out-of-bounds memcpy

On 02/03/2018 10:12 AM, Arnd Bergmann wrote:
On Sat, Feb 3, 2018 at 12:33 AM, Boris Ostrovsky
<boris.ostrovsky@xxxxxxxxxx> wrote:
On 02/02/2018 10:32 AM, Arnd Bergmann wrote:
The legacy hypercall handlers were originally added with
a comment explaining that "copying the argument structures in
HYPERVISOR_event_channel_op() and HYPERVISOR_physdev_op() into the local
variable is sufficiently safe" and only made sure to not write
past the end of the argument structure, the checks in linux/string.h
disagree with that, when link-time optimizations are used:

In function 'memcpy',
     inlined from 'pirq_query_unmask' at drivers/xen/fallback.c:53:2,
     inlined from '__startup_pirq' at drivers/xen/events/events_base.c:529:2,
     inlined from 'restore_pirqs' at drivers/xen/events/events_base.c:1439:3,
     inlined from 'xen_irq_resume' at drivers/xen/events/events_base.c:1581:2:
include/linux/string.h:350:3: error: call to '__read_overflow2' declared with 
attribute error: detected read beyond size of object passed as 2nd parameter
make[3]: *** [ccLujFNx.ltrans15.ltrans.o] Error 1
make[3]: Target 'all' not remade because of errors.
lto-wrapper: fatal error: make returned 2 exit status
compilation terminated.
ld: error: lto-wrapper failed

This changes the functions so that each argument is accessed with
exactly the correct length based on the command code.

Fixes: cf47a83fb06e ("xen/hypercall: fix hypercall fallback code for very old 
Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
  drivers/xen/fallback.c | 94 ++++++++++++++++++++++++++++----------------------
  1 file changed, 53 insertions(+), 41 deletions(-)

-             WARN_ON(rc != -ENOSYS);
-             break;
+             return -ENOSYS;

+     memcpy(&op.u, arg, len);
+     rc = _hypercall1(int, event_channel_op_compat, &op);
+     memcpy(arg, &op.u, len);

We don't copy back for all commands, only those that are COPY_BACK.

Not sure what you mean. Is it harmful to copy back the data for the others
in any way? Otherwise I wouldn't micro-optimize this.

I should have checked the original commit for fallback.c --- the code that it replaced was doing copybacks for all hypercalls and selective copybacks is an optimization introduced in that commit.


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