[3.16.y-ckt stable] Patch "x86_64, traps: Fix the espfix64 #DF fixup and rewrite it in C" has been added to staging queue
Andy Lutomirski
luto at amacapital.net
Mon Dec 1 13:21:47 UTC 2014
On Mon, Dec 1, 2014 at 2:20 AM, Luis Henriques
<luis.henriques at canonical.com> wrote:
> This is a note to let you know that I have just added a patch titled
>
> x86_64, traps: Fix the espfix64 #DF fixup and rewrite it in C
I would suggest waiting a week or two before applying this and "Fix
bad_iret". AFAIK no one knows how to usefully exploit these two, and
letting them stew longer in Linus' tree is probably a good idea.
The #SS fix, on the other hand, has a straightforward public exploit.
--Andy
>
> to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree
> which can be found at:
>
> http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.16.y-queue
>
> This patch is scheduled to be released in version 3.16.7-ckt3.
>
> If you, or anyone else, feels it should not be added to this tree, please
> reply to this email.
>
> For more information about the 3.16.y-ckt tree, see
> https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable
>
> Thanks.
> -Luis
>
> ------
>
> From 87729e12d9bf72c8b5fa36e4718f06dff7a7d5e4 Mon Sep 17 00:00:00 2001
> From: Andy Lutomirski <luto at amacapital.net>
> Date: Sat, 22 Nov 2014 18:00:31 -0800
> Subject: x86_64, traps: Fix the espfix64 #DF fixup and rewrite it in C
>
> commit af726f21ed8af2cdaa4e93098dc211521218ae65 upstream.
>
> There's nothing special enough about the espfix64 double fault fixup to
> justify writing it in assembly. Move it to C.
>
> This also fixes a bug: if the double fault came from an IST stack, the
> old asm code would return to a partially uninitialized stack frame.
>
> Fixes: 3891a04aafd668686239349ea58f3314ea2af86b
> Signed-off-by: Andy Lutomirski <luto at amacapital.net>
> Reviewed-by: Thomas Gleixner <tglx at linutronix.de>
> Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
> Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
> ---
> arch/x86/kernel/entry_64.S | 34 ++--------------------------------
> arch/x86/kernel/traps.c | 24 ++++++++++++++++++++++++
> 2 files changed, 26 insertions(+), 32 deletions(-)
>
> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
> index c844f0816ab8..3bbf173ced93 100644
> --- a/arch/x86/kernel/entry_64.S
> +++ b/arch/x86/kernel/entry_64.S
> @@ -842,6 +842,7 @@ ENTRY(native_iret)
> jnz native_irq_return_ldt
> #endif
>
> +.global native_irq_return_iret
> native_irq_return_iret:
> iretq
> _ASM_EXTABLE(native_irq_return_iret, bad_iret)
> @@ -936,37 +937,6 @@ ENTRY(retint_kernel)
> CFI_ENDPROC
> END(common_interrupt)
>
> - /*
> - * If IRET takes a fault on the espfix stack, then we
> - * end up promoting it to a doublefault. In that case,
> - * modify the stack to make it look like we just entered
> - * the #GP handler from user space, similar to bad_iret.
> - */
> -#ifdef CONFIG_X86_ESPFIX64
> - ALIGN
> -__do_double_fault:
> - XCPT_FRAME 1 RDI+8
> - movq RSP(%rdi),%rax /* Trap on the espfix stack? */
> - sarq $PGDIR_SHIFT,%rax
> - cmpl $ESPFIX_PGD_ENTRY,%eax
> - jne do_double_fault /* No, just deliver the fault */
> - cmpl $__KERNEL_CS,CS(%rdi)
> - jne do_double_fault
> - movq RIP(%rdi),%rax
> - cmpq $native_irq_return_iret,%rax
> - jne do_double_fault /* This shouldn't happen... */
> - movq PER_CPU_VAR(kernel_stack),%rax
> - subq $(6*8-KERNEL_STACK_OFFSET),%rax /* Reset to original stack */
> - movq %rax,RSP(%rdi)
> - movq $0,(%rax) /* Missing (lost) #GP error code */
> - movq $general_protection,RIP(%rdi)
> - retq
> - CFI_ENDPROC
> -END(__do_double_fault)
> -#else
> -# define __do_double_fault do_double_fault
> -#endif
> -
> /*
> * APIC interrupts.
> */
> @@ -1138,7 +1108,7 @@ idtentry overflow do_overflow has_error_code=0
> idtentry bounds do_bounds has_error_code=0
> idtentry invalid_op do_invalid_op has_error_code=0
> idtentry device_not_available do_device_not_available has_error_code=0
> -idtentry double_fault __do_double_fault has_error_code=1 paranoid=1
> +idtentry double_fault do_double_fault has_error_code=1 paranoid=1
> idtentry coprocessor_segment_overrun do_coprocessor_segment_overrun has_error_code=0
> idtentry invalid_TSS do_invalid_TSS has_error_code=1
> idtentry segment_not_present do_segment_not_present has_error_code=1
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index 0d0e922fafc1..819662746e23 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -259,6 +259,30 @@ dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code)
> static const char str[] = "double fault";
> struct task_struct *tsk = current;
>
> +#ifdef CONFIG_X86_ESPFIX64
> + extern unsigned char native_irq_return_iret[];
> +
> + /*
> + * If IRET takes a non-IST fault on the espfix64 stack, then we
> + * end up promoting it to a doublefault. In that case, modify
> + * the stack to make it look like we just entered the #GP
> + * handler from user space, similar to bad_iret.
> + */
> + if (((long)regs->sp >> PGDIR_SHIFT) == ESPFIX_PGD_ENTRY &&
> + regs->cs == __KERNEL_CS &&
> + regs->ip == (unsigned long)native_irq_return_iret)
> + {
> + struct pt_regs *normal_regs = task_pt_regs(current);
> +
> + /* Fake a #GP(0) from userspace. */
> + memmove(&normal_regs->ip, (void *)regs->sp, 5*8);
> + normal_regs->orig_ax = 0; /* Missing (lost) #GP error code */
> + regs->ip = (unsigned long)general_protection;
> + regs->sp = (unsigned long)&normal_regs->orig_ax;
> + return;
> + }
> +#endif
> +
> exception_enter();
> /* Return not checked because double check cannot be ignored */
> notify_die(DIE_TRAP, str, regs, error_code, X86_TRAP_DF, SIGSEGV);
> --
> 2.1.0
>
--
Andy Lutomirski
AMA Capital Management, LLC
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