From: David Vrabel <[email protected]> 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 <[email protected]> Acked-by: Jan Beulich <[email protected]> Acked-by: Ian Campbell <[email protected]> Cc: [email protected] --- 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 */ 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 -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
