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+
+
+
+x86: retf or iret pagefault sets wrong error code
+
+With a x86_64 or i386 guest, non-KVM, when trying to execute a
+"iret/iretq/retf" instruction in userspace with invalid stack pointer
+(under a protected mode OS, like Linux), wrong bits are set in the
+pushed error code; bit 2 is not set, indicating the error comes from
+kernel space.
+
+If the guest OS is using this flag to decide whether this was a kernel
+or user page fault, it will mistakenly decide a kernel has irrecoverably
+faulted, possibly causing guest OS panic.
+
+
+How to reproduce the problem a guest (non-KVM) Linux:
+Note, on recent Linux kernel version, this needs a CPU with SMAP support
+(eg. -cpu max)
+
+$ cat tst.c
+int main()
+{
+__asm__ volatile (
+"mov $0,%esp\n"
+"retf"
+);
+return 0;
+}
+
+$ gcc tst.c
+$ ./a.out
+Killed
+
+
+"dmesg" shows the kernel has in fact triggered a "BUG: unable to handle
+kernel NULL pointer dereference...", but it has "recovered" by killing
+the faulting process (see attached screenshot).
+
+
+Using self-compiled qemu from git:
+commit 532cc6da74ec25b5ba6893b5757c977d54582949 (HEAD -> master, tag: v4.0.0-rc3, origin/master, origin/HEAD)
+Author: Peter Maydell <email address hidden>
+Date:   Wed Apr 10 15:38:59 2019 +0100
+
+    Update version for v4.0.0-rc3 release
+    
+    Signed-off-by: Peter Maydell <email address hidden>
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