[Xen-changelog] [xen stable-4.12] x86/pv: Fix undefined behaviour in check_descriptor()

[patchbot@xxxxxxx]

commit ec3d131d9d64f46bb2d6ab9a16074a15c309f6d8
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Fri Jul 5 10:21:44 2019 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Jul 5 10:21:44 2019 +0200

    x86/pv: Fix undefined behaviour in check_descriptor()
    
    UBSAN reports:
    
      (XEN) 
================================================================================
      (XEN) UBSAN: Undefined behaviour in x86_64/mm.c:1108:31
      (XEN) left shift of 255 by 24 places cannot be represented in type 'int'
      (XEN) ----[ Xen-4.13-unstable  x86_64  debug=y   Tainted:    H ]----
      (XEN) CPU:    60
      (XEN) RIP:    e008:[<ffff82d0802a54ce>] ubsan.c#ubsan_epilogue+0xa/0xc2
      <snip>
      (XEN) Xen call trace:
      (XEN)    [<ffff82d0802a54ce>] ubsan.c#ubsan_epilogue+0xa/0xc2
      (XEN)    [<ffff82d0802a6009>] 
__ubsan_handle_shift_out_of_bounds+0x15d/0x16c
      (XEN)    [<ffff82d08033abd7>] check_descriptor+0x191/0x3dd
      (XEN)    [<ffff82d0804ef920>] do_update_descriptor+0x7f/0x2b6
      (XEN)    [<ffff82d0804efb75>] compat_update_descriptor+0x1e/0x20
      (XEN)    [<ffff82d0804fa1cc>] pv_hypercall+0x87f/0xa6f
      (XEN)    [<ffff82d080501acb>] do_entry_int82+0x53/0x58
      (XEN)    [<ffff82d08050702b>] entry_int82+0xbb/0xc0
      (XEN)
      (XEN) 
================================================================================
    
    As this is a constant, express it in longhand for correctness, and 
consistency
    with the surrounding code.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
    master commit: bd5be40ce2307ea5e8f52e3103d1b48ca9dfdce9
    master date: 2019-06-06 20:04:33 +0100
---
 xen/arch/x86/x86_64/mm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xen/arch/x86/x86_64/mm.c b/xen/arch/x86/x86_64/mm.c
index d8f558bc3a..899b883b2d 100644
--- a/xen/arch/x86/x86_64/mm.c
+++ b/xen/arch/x86/x86_64/mm.c
@@ -1105,7 +1105,7 @@ int check_descriptor(const struct domain *dom, seg_desc_t 
*d)
              * 0xf6800000. Extend these to allow access to the larger read-only
              * M2P table available in 32on64 mode.
              */
-            base = (b & (0xff << 24)) | ((b & 0xff) << 16) | (a >> 16);
+            base = (b & 0xff000000) | ((b & 0xff) << 16) | (a >> 16);
 
             limit = (b & 0xf0000) | (a & 0xffff);
             limit++; /* We add one because limit is inclusive. */
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.12

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