[xen staging] xen/arm: io: Harden sign extension check

[patchbot@xxxxxxx]

commit ec1635ed877783911f0eda1b0ab10f00474b3db2
Author:     Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
AuthorDate: Fri Jan 29 03:48:48 2021 +0200
Commit:     Julien Grall <jgrall@xxxxxxxxxx>
CommitDate: Fri Jan 29 16:55:41 2021 +0000

    xen/arm: io: Harden sign extension check
    
    In the ideal world we would never get an undefined behavior when
    propagating the sign bit since that bit can only be set for access
    size smaller than the register size (i.e byte/half-word for aarch32,
    byte/half-word/word for aarch64).
    
    In the real world we need to care for *possible* hardware bug such as
    advertising a sign extension for either 64-bit (or 32-bit) on Arm64
    (resp. Arm32).
    
    So harden a bit more the code to prevent undefined behavior when
    propagating the sign bit in case of buggy hardware.
    
    Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
    Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
    Reviewed-by: Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>
    CC: Julien Grall <julien.grall@xxxxxxx>
---
 xen/include/asm-arm/traps.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/xen/include/asm-arm/traps.h b/xen/include/asm-arm/traps.h
index c6b3cc75ca..2ed2b85c6f 100644
--- a/xen/include/asm-arm/traps.h
+++ b/xen/include/asm-arm/traps.h
@@ -94,7 +94,8 @@ static inline register_t sign_extend(const struct hsr_dabt 
dabt, register_t r)
      * Note that we expect the read handler to have zeroed the bits
      * outside the requested access size.
      */
-    if ( dabt.sign && (r & (1UL << (size - 1))) )
+    if ( dabt.sign && (size < sizeof(register_t) * 8) &&
+         (r & (1UL << (size - 1))) )
     {
         /*
          * We are relying on register_t using the same as
--
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