On 07/26/2011 08:20 PM, Andy Lutomirski wrote:
> Three places in the kernel assume that the only long mode CPL 3
> selector is __USER_CS. This is not true on Xen -- Xen's sysretq
> changes cs to the magic value 0xe033.
>
> Two of the places are corner cases, but as of "x86-64: Improve
> vsyscall emulation CS and RIP handling"
> (c9712944b2a12373cb6ff8059afcfb7e826a6c54), vsyscalls will segfault
> if called with Xen's extra CS selector. This causes a panic when
> older init builds die.
>
> It seems impossible to make Xen use __USER_CS reliably without
> taking a performance hit on every system call, so this fixes the
> tests instead with a new paravirt op. It's a little ugly because
> ptrace.h can't include paravirt.h.
>
> Signed-off-by: Andy Lutomirski <luto@xxxxxxx>
> Reported-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> ---
> arch/x86/include/asm/desc.h | 4 ++--
> arch/x86/include/asm/paravirt_types.h | 6 ++++++
> arch/x86/include/asm/ptrace.h | 19 +++++++++++++++++++
> arch/x86/kernel/paravirt.c | 4 ++++
> arch/x86/kernel/step.c | 2 +-
> arch/x86/kernel/vsyscall_64.c | 6 +-----
> arch/x86/mm/fault.c | 2 +-
> arch/x86/xen/enlighten.c | 1 +
> 8 files changed, 35 insertions(+), 9 deletions(-)
>
> diff --git a/arch/x86/include/asm/desc.h b/arch/x86/include/asm/desc.h
> index 7b439d9..41935fa 100644
> --- a/arch/x86/include/asm/desc.h
> +++ b/arch/x86/include/asm/desc.h
> @@ -27,8 +27,8 @@ static inline void fill_ldt(struct desc_struct *desc, const
> struct user_desc *in
>
> desc->base2 = (info->base_addr & 0xff000000) >> 24;
> /*
> - * Don't allow setting of the lm bit. It is useless anyway
> - * because 64bit system calls require __USER_CS:
> + * Don't allow setting of the lm bit. It would confuse
> + * user_64bit_mode and would get overridden by sysret anyway.
> */
> desc->l = 0;
> }
> diff --git a/arch/x86/include/asm/paravirt_types.h
> b/arch/x86/include/asm/paravirt_types.h
> index 2c76521..8e8b9a4 100644
> --- a/arch/x86/include/asm/paravirt_types.h
> +++ b/arch/x86/include/asm/paravirt_types.h
> @@ -41,6 +41,7 @@
>
> #include <asm/desc_defs.h>
> #include <asm/kmap_types.h>
> +#include <asm/pgtable_types.h>
>
> struct page;
> struct thread_struct;
> @@ -63,6 +64,11 @@ struct paravirt_callee_save {
> struct pv_info {
> unsigned int kernel_rpl;
> int shared_kernel_pmd;
> +
> +#ifdef CONFIG_X86_64
> + u16 extra_user_64bit_cs; /* __USER_CS if none */
> +#endif
> +
> int paravirt_enabled;
> const char *name;
> };
> diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
> index 94e7618..3566454 100644
> --- a/arch/x86/include/asm/ptrace.h
> +++ b/arch/x86/include/asm/ptrace.h
> @@ -131,6 +131,9 @@ struct pt_regs {
> #ifdef __KERNEL__
>
> #include <linux/init.h>
> +#ifdef CONFIG_PARAVIRT
> +#include <asm/paravirt_types.h>
> +#endif
>
> struct cpuinfo_x86;
> struct task_struct;
> @@ -187,6 +190,22 @@ static inline int v8086_mode(struct pt_regs *regs)
> #endif
> }
>
> +#ifdef CONFIG_X86_64
> +static inline bool user_64bit_mode(struct pt_regs *regs)
> +{
> +#ifndef CONFIG_PARAVIRT
> + /*
> + * On non-paravirt systems, this is the only long mode CPL 3
> + * selector. We do not allow long mode selectors in the LDT.
> + */
> + return regs->cs == __USER_CS;
> +#else
> + /* Headers are too twisted for this to go in paravirt.h. */
> + return regs->cs == __USER_CS || regs->cs == pv_info.extra_user_64bit_cs;
Is this necessary because usermode may sometimes be on __USER_CS or
sometimes on Xen's? Could we just commit to one or the other and make
it a simple comparison?
What if __USER_CS were a variable?
J
> +#endif
> +}
> +#endif
> +
> /*
> * X86_32 CPUs don't save ss and esp if the CPU is already in kernel mode
> * when it traps. The previous stack will be directly underneath the saved
> diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
> index 613a793..d90272e 100644
> --- a/arch/x86/kernel/paravirt.c
> +++ b/arch/x86/kernel/paravirt.c
> @@ -307,6 +307,10 @@ struct pv_info pv_info = {
> .paravirt_enabled = 0,
> .kernel_rpl = 0,
> .shared_kernel_pmd = 1, /* Only used when CONFIG_X86_PAE is set */
> +
> +#ifdef CONFIG_X86_64
> + .extra_user_64bit_cs = __USER_CS,
> +#endif
> };
>
> struct pv_init_ops pv_init_ops = {
> diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c
> index 7977f0c..c346d11 100644
> --- a/arch/x86/kernel/step.c
> +++ b/arch/x86/kernel/step.c
> @@ -74,7 +74,7 @@ static int is_setting_trap_flag(struct task_struct *child,
> struct pt_regs *regs)
>
> #ifdef CONFIG_X86_64
> case 0x40 ... 0x4f:
> - if (regs->cs != __USER_CS)
> + if (!user_64bit_mode(regs))
> /* 32-bit mode: register increment */
> return 0;
> /* 64-bit mode: REX prefix */
> diff --git a/arch/x86/kernel/vsyscall_64.c b/arch/x86/kernel/vsyscall_64.c
> index dda7dff..1725930 100644
> --- a/arch/x86/kernel/vsyscall_64.c
> +++ b/arch/x86/kernel/vsyscall_64.c
> @@ -127,11 +127,7 @@ void dotraplinkage do_emulate_vsyscall(struct pt_regs
> *regs, long error_code)
>
> local_irq_enable();
>
> - /*
> - * Real 64-bit user mode code has cs == __USER_CS. Anything else
> - * is bogus.
> - */
> - if (regs->cs != __USER_CS) {
> + if (!user_64bit_mode(regs)) {
> /*
> * If we trapped from kernel mode, we might as well OOPS now
> * instead of returning to some random address and OOPSing
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 4d09df0..decd51a 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -105,7 +105,7 @@ check_prefetch_opcode(struct pt_regs *regs, unsigned char
> *instr,
> * but for now it's good enough to assume that long
> * mode only uses well known segments or kernel.
> */
> - return (!user_mode(regs)) || (regs->cs == __USER_CS);
> + return (!user_mode(regs) || user_64bit_mode(regs));
> #endif
> case 0x60:
> /* 0x64 thru 0x67 are valid prefixes in all modes. */
> diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
> index 974a528..a9c710a 100644
> --- a/arch/x86/xen/enlighten.c
> +++ b/arch/x86/xen/enlighten.c
> @@ -950,6 +950,7 @@ static unsigned xen_patch(u8 type, u16 clobbers, void
> *insnbuf,
> static const struct pv_info xen_info __initconst = {
> .paravirt_enabled = 1,
> .shared_kernel_pmd = 0,
> + .extra_user_64bit_cs = FLAT_USER_CS64,
>
> .name = "Xen",
> };
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