From a349e23d1cf746f8bdc603dcc61fae9ee4a695f6 Mon Sep 17 00:00:00 2001 From: David Vrabel <david.vrabel@citrix.com> Date: Fri, 19 Oct 2012 17:29:07 +0100 Subject: [PATCH] xen/x86: don't corrupt %eip when returning from a signal handler 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 and avoids some of the tests in handle_signal(). There were similar bugs in xen_failsafe_callback() of both 32 and 64-bit guests. If the fault was corrected and the normal return path was used then 0 was incorrectly pushed as the value for orig_ax. Signed-off-by: David Vrabel <david.vrabel@citrix.com> Acked-by: Jan Beulich <JBeulich@suse.com> Acked-by: Ian Campbell <ian.campbell@citrix.com> Cc: stable@vger.kernel.org Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> --- arch/x86/kernel/entry_32.S | 8 +++++--- arch/x86/kernel/entry_64.S | 2 +- 2 files changed, 6 insertions(+), 4 deletions(-) diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S index 623f288374763..8f8e8eea9085d 100644 --- a/arch/x86/kernel/entry_32.S +++ b/arch/x86/kernel/entry_32.S @@ -1016,7 +1016,7 @@ 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 @@ -1058,14 +1058,16 @@ ENTRY(xen_failsafe_callback) 2: mov 8(%esp),%es 3: mov 12(%esp),%fs 4: mov 16(%esp),%gs + /* EAX == 0 => Category 1 (Bad segment) + EAX != 0 => Category 2 (Bad IRET) */ testl %eax,%eax popl_cfi %eax lea 16(%esp),%esp CFI_ADJUST_CFA_OFFSET -16 jz 5f addl $16,%esp - jmp iret_exc # EAX != 0 => Category 2 (Bad IRET) -5: pushl_cfi $0 # EAX == 0 => Category 1 (Bad segment) + jmp iret_exc +5: pushl_cfi $-1 /* orig_ax = -1 => not a system call */ SAVE_ALL jmp ret_from_exception CFI_ENDPROC diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S index 69babd8c834f9..dcdd0ea33a324 100644 --- a/arch/x86/kernel/entry_64.S +++ b/arch/x86/kernel/entry_64.S @@ -1363,7 +1363,7 @@ 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 -- GitLab