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Re: [PATCH] x86emul: avoid UB shifts in FLDENV/FRSTOR handling
- To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Jason Andryuk <jason.andryuk@xxxxxxx>
- Date: Mon, 28 Apr 2025 17:24:16 -0400
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Fabian Specht <f.specht@xxxxxx>
- Delivery-date: Mon, 28 Apr 2025 21:24:36 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 2025-04-28 07:29, Jan Beulich wrote:
16-bit quantities, no matter whether expressed as uint16_t or as
bitfield, will be promoted to plain int before doing any arithmetic on
them. Shifting such values by 16 will therefore shift into the sign bit,
which is UB if that bit becomes set. To account for all reads and all
writes accessing opposite members of the same union, introduce yet more
local variables to reduce the shift counts to 12.
Fixes: be55ed744ed8 ("x86emul: support FLDENV and FRSTOR")
Reported-by: Fabian Specht<f.specht@xxxxxx>
Signed-off-by: Jan Beulich<jbeulich@xxxxxxxx>
Reviewed-by: Jason Andryuk <jason.andryuk@xxxxxxx>
Thanks,
Jason
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