Re: [x86/asm/entry] BUG: unable to handle kernel paging request

[Andy Lutomirski]

On Mon, Mar 9, 2015 at 5:55 AM, Denys Vlasenko <vda.linux@xxxxxxxxxxxxxx> wrote:
> On Sun, Mar 8, 2015 at 8:41 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>> It seems to be true that 32-bit x86 always has two unused words
>>> beyond "struct pt_regs" on stack during syscalls.
>>>
>>> IOW: tss.sp0 is *not* set at the very end of the stack on 32-bit.
>>
>> So maybe the idea is that, if we end up using sp0 for an entry *from*
>> ring 0, then the struct pt_regs won't dangle off the top of the stack.
>> But this still doesn't make sense to me: if we enter from ring 0, then
>> the CPU won't look at sp0 in the first place.
>>
>> Linus, if you're paying attention here, I suspect that you wrote this
>> code.  Do you remember what the point is?
>
> It was added in 2005:
>
> commit 5df240826c90afdc7956f55a004ea6b702df9203
> Author: Stas Sergeev <stsp@xxxxxxxx>
> Date:   Sat Apr 16 15:24:01 2005 -0700
>
>     [PATCH] fix crash in entry.S restore_all
>
>     Fix the access-above-bottom-of-stack crash.
>
>     1. Allows to preserve the valueable optimization
>
>     2. Works for NMIs
>
>     3.  Doesn't care whether or not there are more of the like instances
>        where the stack is left empty.
>
>     4. Seems to work for me without the crashes:)
>
>     (akpm: this is still under discussion, although I _think_ it's OK.
> You might
>     want to hold off)
>
>     Signed-off-by: Stas Sergeev <stsp@xxxxxxxx>
>     Signed-off-by: Andrew Morton <akpm@xxxxxxxx>
>     Signed-off-by: Linus Torvalds <torvalds@xxxxxxxx>
>
> diff --git a/arch/i386/kernel/entry.S b/arch/i386/kernel/entry.S
> index 1e45ff2..3c73dc8 100644
> --- a/arch/i386/kernel/entry.S
> +++ b/arch/i386/kernel/entry.S
> @@ -245,6 +245,9 @@ syscall_exit:
>
>  restore_all:
>      movl EFLAGS(%esp), %eax        # mix EFLAGS, SS and CS
> +    # Warning: OLDSS(%esp) contains the wrong/random values if we
> +    # are returning to the kernel.
> +    # See comments in process.c:copy_thread() for details.
>      movb OLDSS(%esp), %ah
>      movb CS(%esp), %al
>      andl $(VM_MASK | (4 << 8) | 3), %eax
> diff --git a/arch/i386/kernel/process.c b/arch/i386/kernel/process.c
> index c36fedf..36145ef 100644
> --- a/arch/i386/kernel/process.c
> +++ b/arch/i386/kernel/process.c
> @@ -405,7 +405,17 @@ int copy_thread(int nr, unsigned long
> clone_flags, unsigned long esp,
>      childregs->esp = esp;
>
>      p->thread.esp = (unsigned long) childregs;
> -    p->thread.esp0 = (unsigned long) (childregs+1);
> +    /*
> +     * The below -8 is to reserve 8 bytes on top of the ring0 stack.
> +     * This is necessary to guarantee that the entire "struct pt_regs"
> +     * is accessable even if the CPU haven't stored the SS/ESP registers
> +     * on the stack (interrupt gate does not save these registers
> +     * when switching to the same priv ring).
> +     * Therefore beware: accessing the xss/esp fields of the
> +     * "struct pt_regs" is possible, but they may contain the
> +     * completely wrong values.
> +     */
> +    p->thread.esp0 = (unsigned long) (childregs+1) - 8;
>
>      p->thread.eip = (unsigned long) ret_from_fork;
>
>
>
>
> Apparently, code which uses this still exists, here
> (current git):
>
> restore_all:
>         TRACE_IRQS_IRET
> restore_all_notrace:
> #ifdef CONFIG_X86_ESPFIX32
>         movl PT_EFLAGS(%esp), %eax      # mix EFLAGS, SS and CS
>         # Warning: PT_OLDSS(%esp) contains the wrong/random values if we
>         # are returning to the kernel.
>         # See comments in process.c:copy_thread() for details.
>         movb PT_OLDSS(%esp), %ah
>         movb PT_CS(%esp), %al
>         andl $(X86_EFLAGS_VM | (SEGMENT_TI_MASK << 8) | SEGMENT_RPL_MASK), %eax
>         cmpl $((SEGMENT_LDT << 8) | USER_RPL), %eax
>         CFI_REMEMBER_STATE
>         je ldt_ss                       # returning to user-space with LDT SS
> #endif
>
> "restore_all_notrace" is used by nmi handler too.
> If nmi happened right after sysenter, the stack will indeed
> only have three words, SS,ESP,EFLAGS, PT_OLDSS(%esp)
> would refer to a word above them.
>

But if we're returning from nmi to userspace, then that nmi didn't hit
early in sysenter.

Wouldn't it make more sense to change the espfix code to check CS
before reading SS so that, if we aren't returning to userspace, we
don't read past the top of the stack?

--Andy

> This needs better comments...



-- 
Andy Lutomirski
AMA Capital Management, LLC
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