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Date:   Tue, 21 Sep 2021 09:42:16 +0200
From:   Juergen Gross <>
To:     Jan Beulich <>,
        Boris Ostrovsky <>
Cc:     Stefano Stabellini <>,
        lkml <>,
        "" <>
Subject: Re: [PATCH v2] xen/x86: fix PV trap handling on secondary processors

On 20.09.21 18:15, Jan Beulich wrote:
> The initial observation was that in PV mode under Xen 32-bit user space
> didn't work anymore. Attempts of system calls ended in #GP(0x402). All
> of the sudden the vector 0x80 handler was not in place anymore. As it
> turns out up to 5.13 redundant initialization did occur: Once from
> cpu_initialize_context() (through its VCPUOP_initialise hypercall) and a
> 2nd time while each CPU was brought fully up. This 2nd initialization is
> now gone, uncovering that the 1st one was flawed: Unlike for the
> set_trap_table hypercall, a full virtual IDT needs to be specified here;
> the "vector" fields of the individual entries are of no interest. With
> many (kernel) IDT entries still(?) (i.e. at that point at least) empty,
> the syscall vector 0x80 ended up in slot 0x20 of the virtual IDT, thus
> becoming the domain's handler for vector 0x20.
> Make xen_convert_trap_info() fit for either purpose, leveraging the fact
> that on the xen_copy_trap_info() path the table starts out zero-filled.
> This includes moving out the writing of the sentinel, which would also
> have lead to a buffer overrun in the xen_copy_trap_info() case if all
> (kernel) IDT entries were populated. Convert the writing of the sentinel
> to clearing of the entire table entry rather than just the address
> field.
> (I didn't bother trying to identify the commit which uncovered the issue
> in 5.14; the commit named below is the one which actually introduced the
> bad code.)
> Fixes: f87e4cac4f4e ("xen: SMP guest support")
> Cc:
> Signed-off-by: Jan Beulich <>

Pushed to xen/tip.git for-linus-5.15b


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