Re: [Xen-devel] [PATCHv2] xen/x86: don't corrupt %eip when returning from a signal handler

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 17.10.12 at 15:29, David Vrabel <david.vrabel@xxxxxxxxxx> wrote:
> From: David Vrabel <david.vrabel@xxxxxxxxxx>
> 
> In 32 bit guests, if a userspace process has %eax == -ERESTARTSYS
> (-512) or -ERESTARTNOINTR (-513) when it is interrupted by an event
> /and/ the process has a pending signal then %eip (and %eax) are
> corrupted when returning to the main process after handling the
> signal.  The application may then crash with SIGSEGV or a SIGILL or it
> may have subtly incorrect behaviour (depending on what instruction it
> returned to).
> 
> The occurs because handle_signal() is incorrectly thinking that there
> is a system call that needs to restarted so it adjusts %eip and %eax
> to re-execute the system call instruction (even though user space had
> not done a system call).
> 
> If %eax == -514 (-ERESTARTNOHAND (-514) or -ERESTART_RESTARTBLOCK
> (-516) then handle_signal() only corrupted %eax (by setting it to
> -EINTR).  This may cause the application to crash or have incorrect
> behaviour.
> 
> handle_signal() assumes that regs->orig_ax >= 0 means a system call so
> any kernel entry point that is not for a system call must push a
> negative value for orig_ax.  For example, for physical interrupts on
> bare metal the inverse of the vector is pushed and page_fault() sets
> regs->orig_ax to -1, overwriting the hardware provided error code.
> 
> xen_hypervisor_callback() was incorrectly pushing 0 for orig_ax
> instead of -1.
> 
> Classic Xen kernels pushed %eax which works as %eax cannot be both
> non-negative and -RESTARTSYS (etc.), but using -1 is consistent with
> other non-system call entry points.
> 
> There were similar bugs in xen_failsafe_callback(), if the fault was
> corrected and normal return path was used.  64 bit guests would push 0
> which is broken.  32 bit guests would push %eax which is safe (see
> previous paragraph), but for consistency this is also changed to -1.
> 
> Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
> Acked-by: Jan Beulich <JBeulich@xxxxxxxx>
> Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx 
> ---
>  arch/x86/kernel/entry_32.S |    4 ++--
>  arch/x86/kernel/entry_64.S |    2 +-
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
> index 2c63407..6a19e66 100644
> --- a/arch/x86/kernel/entry_32.S
> +++ b/arch/x86/kernel/entry_32.S
> @@ -1042,7 +1042,7 @@ ENTRY(xen_sysenter_target)
>  
>  ENTRY(xen_hypervisor_callback)
>       CFI_STARTPROC
> -     pushl_cfi $0
> +     pushl_cfi $-1 /* orig_ax = -1 => not a system call */
>       SAVE_ALL
>       TRACE_IRQS_OFF
>  
> @@ -1078,7 +1078,7 @@ ENDPROC(xen_hypervisor_callback)
>  # We distinguish between categories by maintaining a status value in EAX.
>  ENTRY(xen_failsafe_callback)
>       CFI_STARTPROC
> -     pushl_cfi %eax
> +     pushl_cfi $-1  /* orig_ax = -1 => not a system call */

While making this apply to the 2.6.18 tree, I noticed that you
replaced the wrong push here, thus causing register corruption.
Just like on the 64-bit side, the one that needs fixing is the one
right before the SAVE_ALL (and hence it's again not just for
consistency, as zero is being pushed there too).

Jan

>       movl $1,%eax
>  1:   mov 4(%esp),%ds
>  2:   mov 8(%esp),%es
> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
> index cdc790c..430b1fc 100644
> --- a/arch/x86/kernel/entry_64.S
> +++ b/arch/x86/kernel/entry_64.S
> @@ -1451,7 +1451,7 @@ ENTRY(xen_failsafe_callback)
>       CFI_RESTORE r11
>       addq $0x30,%rsp
>       CFI_ADJUST_CFA_OFFSET -0x30
> -     pushq_cfi $0
> +     pushq_cfi $-1 /* orig_ax = -1 => not a system call */
>       SAVE_ALL
>       jmp error_exit
>       CFI_ENDPROC
> -- 
> 1.7.2.5
> 
> 
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