[Xen-changelog] [xen stable-4.8] xen/cmpxchg: Provide helper to safely modify guest memory atomically

[patchbot@xxxxxxx]

commit 181ed91826a4bdc0219534cb96821d1dec8dd942
Author:     Julien Grall <julien.grall@xxxxxxx>
AuthorDate: Mon Apr 29 15:05:27 2019 +0100
Commit:     Julien Grall <julien.grall@xxxxxxx>
CommitDate: Fri Jun 14 15:46:00 2019 +0100

    xen/cmpxchg: Provide helper to safely modify guest memory atomically
    
    On Arm, exclusive load-store atomics should only be used between trusted
    thread. As not all the guests are trusted, it may be possible to DoS Xen
    when updating shared memory with guest atomically.
    
    This patch adds a new helper that will update the guest memory safely.
    For x86, it is already possible to use the current helper safely. So
    just wrap it.
    
    For Arm, we will first attempt to update the guest memory with the
    loop bounded by a maximum number of iterations. If it fails, we will
    pause the domain and try again.
    
    Note that this heuristics assumes that a page can only
    be shared between Xen and one domain. Not Xen and multiple domain.
    
    The maximum number of iterations is based on how many times atomic_inc()
    can be executed in 1uS. The maximum value is per-CPU to cater big.LITTLE
    and calculated when the CPU is booting.
    
    The maximum number of iterations is based on how many times a simple
    load-store atomic operation can be executed in 1uS. The maximum
    value is per-CPU to cater big.LITTLE and calculated when the CPU is
    booting. The heuristic was randomly chosen and can be modified if
    impact too much good-behaving guest.
    
    This is part of XSA-295.
    
    Signed-of-by: Julien Grall <julien.grall@xxxxxxx>
    Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
    Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/include/asm-arm/guest_atomics.h | 25 +++++++++++++++++++++++++
 xen/include/asm-x86/guest_atomics.h |  2 ++
 2 files changed, 27 insertions(+)

diff --git a/xen/include/asm-arm/guest_atomics.h 
b/xen/include/asm-arm/guest_atomics.h
index 4f127fda41..61925d313d 100644
--- a/xen/include/asm-arm/guest_atomics.h
+++ b/xen/include/asm-arm/guest_atomics.h
@@ -65,6 +65,31 @@ guest_testop(test_and_change_bit)
 
 #undef guest_testop
 
+static inline unsigned long __guest_cmpxchg(struct domain *d,
+                                            volatile void *ptr,
+                                            unsigned long old,
+                                            unsigned long new,
+                                            unsigned int size)
+{
+    unsigned long oldval = old;
+
+    if ( __cmpxchg_mb_timeout(ptr, &oldval, new, size,
+                              this_cpu(guest_safe_atomic_max)) )
+        return oldval;
+
+    domain_pause_nosync(d);
+    oldval = __cmpxchg_mb(ptr, old, new, size);
+    domain_unpause(d);
+
+    return oldval;
+}
+
+#define guest_cmpxchg(d, ptr, o, n)                         \
+    ((__typeof__(*(ptr)))__guest_cmpxchg(d, ptr,            \
+                                         (unsigned long)(o),\
+                                         (unsigned long)(n),\
+                                         sizeof (*(ptr))))
+
 #endif /* _ARM_GUEST_ATOMICS_H */
 /*
  * Local variables:
diff --git a/xen/include/asm-x86/guest_atomics.h 
b/xen/include/asm-x86/guest_atomics.h
index 0c71d2d278..029417c8ff 100644
--- a/xen/include/asm-x86/guest_atomics.h
+++ b/xen/include/asm-x86/guest_atomics.h
@@ -19,6 +19,8 @@
 #define guest_test_and_change_bit(d, nr, p) \
     ((void)(d), test_and_change_bit(nr, p))
 
+#define guest_cmpxchg(d, ptr, o, n) ((void)(d), cmpxchg(ptr, o, n))
+
 #endif /* _X86_GUEST_ATOMICS_H */
 /*
  * Local variables:
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.8

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