[xen stable-4.20] x86/emul: Fix emulation of RDSEED with older toolchains

[patchbot@xxxxxxx]

commit ddec00769cd80a44b412281d8b52482e4e20c2e5
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Thu May 15 20:09:35 2025 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Fri May 16 21:34:19 2025 +0100

    x86/emul: Fix emulation of RDSEED with older toolchains
    
    This is reported as a MISRA R16.3 (missing break) violation, but turns out 
to
    be substantially more complicated than expected.
    
    In commit a8fe4ec5320a ("x86emul: support RDRAND/RDSEED"), the switch()
    statement had a default case going to cannot_emulate, with both the case 6 
and
    case 7 labels being fully within #ifdef HAVE_GAS_RD{RAND,SEED}.
    
    Therefore, when the toolchain didn't understand the RDRAND/RDSEED
    instructions, attempts to emulate them suffered #UD.  (In principle, this 
is a
    problem as there's no interlock to prevent RDRAND/RDSEED being advertised to
    the guest, but as instructions with only register encodings, they can only
    legitimately be emulated when VM Introspection is in use.)
    
    In commit 58f1bba44033 ("x86emul: support RDPID"), case 7 was taken outside 
of
    HAVE_GAS_RDSEED, meaning that emulating an RDSEED instruction no longer hit
    the default case when the toolchain was too old.
    
    Instead, it would fall out of the switch statement and be completed 
normally,
    behaving as a NOP to the guest.
    
    Retrofit a "return X86EMUL_UNIMPLEMENTED" in the case that the toolchain
    doesn't know the RDRAND instruction, matching how RDRAND work.
    
    Note that this has been fixed differently in Xen 4.21.  Commit
    05bf9f1f0f52 ("x86/emulate: Remove HAVE_AS_RDRAND and HAVE_AS_RDSEED") has
    removed the problematic condition due to the toolchain baseline upgrade.
    
    Fixes: 58f1bba44033 ("x86emul: support RDPID")
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/x86_emulate/0fc7.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/xen/arch/x86/x86_emulate/0fc7.c b/xen/arch/x86/x86_emulate/0fc7.c
index 5268d5cafd..2b6b444bab 100644
--- a/xen/arch/x86/x86_emulate/0fc7.c
+++ b/xen/arch/x86/x86_emulate/0fc7.c
@@ -102,6 +102,8 @@ int x86emul_0fc7(struct x86_emulate_state *s,
             if ( carry )
                 regs->eflags |= X86_EFLAGS_CF;
             break;
+#else
+            return X86EMUL_UNIMPLEMENTED;
 #endif
         }
     }
--
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